UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9599 REPORT OF INVESTIGATIONS/1996

Environmental Impacts of Cemented Mine

Waste Backfill
U.S. Department of the Interior

Mission Statement

As the Nation’s principal conservation agency, the Department of the

Interior has responsibility for most of our nationally-owned public

lands and natural resources. This includes fostering sound use of our

land and water resources; protecting our fish, wildlife, and biological

diversity; preserving the environmental and cultural values of our

national parks and historical places; and providing for the enjoyment

of life through outdoor recreation. The Department assesses our

energy and mineral resources and works to ensure that their

development is in the best interests of all our people by encouraging

stewardship and citizen participation in their care. The Department

also has a major responsibility for American Indian reservation

communities and for people who live in island territories under U.S.

administration.
Report of Investigations 9599

Environmental Impacts of Cemented Mine Waste

Backfill

By R. L. Levens, A. D. Marcy, and C. M. K. Boldt 

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES

Rhea Lydia Graham, Director
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ........................................................................................... 1

Introduction ........................................................................................ 2

Background ..................................................................................... 2

Purpose and objectives ............................................................................ 2

Hydrochemistry ..................................................................................... 2

Air permeability and water saturation of backfill .......................................................... 3

Related research .................................................................................... 4

Methodology ....................................................................................... 5

Laboratory data collection .......................................................................... 5

Field data collection ............................................................................... 6

Data presentation and analysis ......................................................................... 7

Assay and leachability of sample CN ................................................................. 7

Particle-size distribution ........................................................................... 8

Neutralizing capacity of cemented backfill ............................................................. 8

Deionized water wash ............................................................................. 9

Sulfuric acid wash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Field water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

ILLUSTRATIONS

1. Cross section through Tempe pressure cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2. Schematic of mine workings and test stope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3. Particle-size distribution of tailings samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4. pH of H SO wash leachate, sample CN 2 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5. pH of H SO wash leachate, sample SO 2 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

6. pH of H SO wash leachate, sample SN 2 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

TABLES

1. Mineralogical analyses of tailings used in backfill ..................................................... 6

2. Chemical assay of backfill materials ................................................................ 7

3. ICP-AES data from analysis of strong acid extraction of backfill materials ................................. 8

4. Acid-neutralizing capacity of portland cement in backfill samples ........................................ 9

5. Acid-neutralizing capacity of tailings in backfill samples ............................................... 9

6. Acid-neutralizing capacity of cemented backfill samples ................................................ 9

7. Element concentrations in samples containing cement after washing with deionized water for 7 days . . . . . . . . . . . . 10

8. Maximum neutralizing potential of cemented backfill samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

9. Neutralizing potential of cemented backfill samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

10. Element concentrations in cemented backfill samples after washing in H SO for 227 days 2 4 . . . . . . . . . . . . . . . . . . . . 13

11. Average and range of element concentrations in water samples from cemented backfill from test mine . . . . . . . . . . 14

12. Saturation indices of water samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter mm millimeter

cm cubic centimeter mmho/cm millimho per centimeter 3

g gram mol mole

h hour Mpa megapascal

kg kilogram m/s meter per second

L liter mV millivolt

m meter pct percent

mequi. Miliequivalent ppm part per million

mg milligram S siemens

mg/g milligram per gram t metric ton

mg/L milligram per liter EC degree Celsius

mL milliliter μm micrometer

Reference to specific products does not imply endorsement by the U.S. Bureau of Mines.
Environmental Impacts of Cemented Mine Waste Backfill

By R. L. Levens, A. D. Marcy, and C. M. K. Boldt 1 2 3

ABSTRACT

Researchers at the U. S. Bureau of Mines conducted investigations to evaluate the potential for ground water

contamination by mine waste used as backfill. Samples of cemented waste backfill and water discharging from

drillholes and seeps were collected to use in chemical analyses and laboratory tests to determine the physical and

chemical factors that control release of heavy metals to ground water.

Greater water retention by cemented backfill (as compared to uncemented sandfill) reduces the surface area

exposed to oxidation, which in turn reduces the amount of acid produced. The acid is neutralized by the cement

and minerals contained in the backfill. The grain-size distribution of tailings used for backfill affects the

structural integrity of cemented backfill under attack by acidic water; breakdown of the backfill structure releases

neutralizing materials faster. Backfilled stopes in rock with low hydraulic conductivities will constitute

preferential flow paths after mine flooding; however, the rate of flow through backfill will be much slower than

when the stope is partially saturated during mine operation. Considering all factors, acid generation and release

of metal ions from cemented backfill should be less than in uncemented sandfill.

Hydrologist. 1

Research chemist. 2

Civil engineer. 3

Spokane Research Center, U.S. Bureau of Mines, Spokane, WA.
2

INTRODUCTION

BACKGROUND

The Safe Drinking Water Act was enacted in 1974 by mobilization of metals. More important, changes in the physical

Congress to develop regulations to protect the Nation's structure of the backfill resulting from the addition of cement

underground sources of drinking water. One result was the may increase the capacity of the backfill to retain water prior to

Underground Injection Control program (UIC), promulgated in mine closure. Greater water retention will reduce the perme1981 by the U.S. Environmental Protection Agency (EPA). ability of the backfill to air and decrease the total surface area of

Under the provisions of the UIC, a Class V well includes "Sand sulfides available for direct oxidation. Furthermore, the addition

backfill and other backfill wells used to inject a mixture of water of cement reduces the hydraulic conductivity of backfill

and sand, mill tailings or other solids into mined-out portions of significantly, which acts to reduce transport of metals by

subsurface mines whether what is injected is a radioactive waste reducing the groundwater flow rate through the backfill once it's

or not" [40 CFR 146.5(e)(8)]. A well is broadly defined as flooded.

either a dug hole or a bored, drilled, or driven shaft whose depth The mechanisms controlling dissolution of minerals in

is greater than its largest surface dimension. cemented backfill and the mobility of heavy metals change

Because placement of mine waste backfill in underground when a mine becomes flooded after mine closure. Reduced

openings is considered to be underground injection under UIC amounts of oxygen will limit the oxidation of sulfides as well as

regulations, the effect on mine backfilling practices could be the precipitation of metal hydroxides. The nature of the changes

extensive. Currently, over 20 metal mines in the United States likely to occur after mine flooding must be considered in order

use backfill materials ranging from quarried rock to cemented to estimate the impacts of placed mine waste backfill on ground

mill tailings. The major purpose of the backfill is to support the water.

ground during mining; however, backfill is increasingly

considered a means of minimizing surface disposal of mine PURPOSE AND OBJECTIVES

waste.

Water affected by backfill during active mining is usually The purpose of this research by the U.S. Bureau of Mines

pumped to the surface, where its disposal is regulated as surface (USBM) was to study the possible impacts of cemented mine

discharge. Backfilled areas may become flooded after a mine is waste backfill on the quality of ground water in the rock

closed, possibly resulting in contamination of ground water. surrounding a stope. Both field and laboratory studies were

Therefore, in contrast to regulations addressing surface disposal, conducted on the mobilization and transport of metals from

regulations concerning the contamination of ground water after cemented backfill. The objectives of the field work were to

mine closure will be a primary focus under the UIC program. (1) document the impacts of cemented backfill on water being

Returning mine waste to the original excavation is an discharged from a selected stope, (2) investigate how the

attractive option because the original minerals are stable in that mobility of metals retained in cemented backfill differed from

environment. However, oxidation of the backfill after it is their mobility in uncemented mine waste sandfill, and

placed underground and prior to flooding after mine closure (3) discuss the long-term impacts of cemented backfill after

may release metals and produce secondary mineral precipitates mine closure and subsequent mine flooding. Laboratory studies

that are more soluble than the original minerals. Because many were conducted to (1) measure changes in acid-neutralization

mines remain open for decades, dissolution of secondary capacity that result from adding cement to backfill, (2) measure

minerals stored in partially saturated backfill can be a significant the rate of neutralization processes, (3) evaluate the impact of

source of metals contamination of ground water after mine differences in grain-size distribution on the strength of the

closure. cemented backfill and the rate of chemical attack, and

Addition of cement to backfill changes both the chemical and (4) investigate the effect of adding cement to backfill on

physical characteristics of the backfill. The addition of cement hydraulic conductivity and water saturation.

will buffer acid-producing oxidation reactions, resulting in less

HYDROCHEMISTRY

Oxidation of acid-producing sulfides, such as pyrite (FeS ), Initially, pyrite is oxidized by molecular 2 oxygen (O ) in the

and associated dissolution of other minerals control the chemical presence of water, yielding sulfate ion (SO ), ferrous ion (Fe ),

composition of water draining from sulfide mines. and hydrogen ion (H ). Next, Fe is oxidized by O to

2

4

2! 2+

+ 2+

2
3

ferric ion (Fe ). This second step is catalyzed by the bacteria magnesium (Mg ), sodium (Na ), potassium (K ), bicarbonate 3+

Thiobacillus ferrooxidans. Pyrite is oxidized by Fe , yielding (HCO ), and silicic acid (H SiO ). Typical reactions involving 3+

additional Fe , SO , and H in the final step (Lowson, 1982). 2+ 2! +

4

The three steps of the oxidation of pyrite are given below.

FeS (s) + 7/2 O + H O <=> Fe 2 2 2

2+

+ 2 SO + 2 H . (A) 4

2& +

Fe +1/4 O + H <=> Fe +1/2 H O. (B) 2+ + 3+

2 2

FeS (s) + 14 Fe + 8 H O <=> 15 Fe 2 2

3+ 2+

+ 2 SO + 16 H . (C) 4

2& +

Fe , SO , and H are released into solution through pyrite 2+ 2! +

4

oxidation. Fe may precipitate as ferrihydrite [Fe(OH) ], goethite 3+

3

[FeO(OH)], or jarosite [KFe (SO ) (OH) ], 3 42 6 which are collectively

known as "yellow boy." The solubility of Fe increases at low 3+

pH values (<4.5) and is reduced during oxidation of additional

pyrite. Fe becomes the dominant oxidizing agent below pH 3.0. 3+

The presence of T. ferrooxidans greatly increases the rate at

which Fe is oxidized, thereby perpetuating the oxidation of 2+

pyrite by Fe . 3+

The H produced during oxidation of pyrite may be con- +

sumed in reactions involving carbonate and silicate minerals, releasing major ions, including calcium (Ca ), 2+

2+ + +

3 4 4

!

calcite (CaCO ) and olivine (Mg SiO ) are given below. 3 2 4

CaCO + H <=> Ca + HCO . (D) 3 3

+ 2+ &

Mg SiO + 4 H <=> 2 Mg + H SiO . (E) 2 4 4 4

+ 2+

Metals other than Fe also may be leached from ore, backfill,

or country rock by acidic water created by oxidation. The

resulting concentrations of heavy metals are controlled by the

buffering capacity of the geochemical system, oxidationprecipitation reactions, dissolution-precipitation reactions, or

desorption-adsorption reactions. The buffering capacity of the

system is determined to a great extent by the geochemistry of the

major ions. Therefore, knowledge of the dissolution and

precipitation reactions involving the major ions is a key to

understanding the mobility of metals.

Oxidation of sulfide minerals contained in backfill that has

been partially saturated prior to mine closure and flooding

produces secondary minerals that are retained in the backfill.

The solubility of these minerals will increase after the backfill

becomes saturated, leading to the possible release of metals.

Dissolution of secondary minerals containing Fe may release 3+

this oxidizing agent, resulting in a temporary continuation of

sulfide oxidation after the backfill has been flooded.

AIR PERMEABILITY AND WATER SATURATION OF BACKFILL

The permeability of porous materials such as backfill to air Equations by Corey (1986) for S and k , as well as the

(k for permeability of nonwetting phase) is a function of pore generalized Kozeny-Karmen equation of air flow (q ), are nw

geometry and the amount of water in pores. Geometric presented below.

properties of pores include porosity, shape, and dimensions of

pore spaces, and tortuosity of flow pathways through S (P ) = (P /P ) , (1)

interconnected pore spaces; these properties are usually

combined into a single coefficient, the pore size distribution k (S ) = k (1-S ) (1-S ), (2)

index (8).

Water saturation is usually expressed as effective saturation and q = k /F (-Mp /Mx + D g), (3)

(S ) when considering air or water flow; S is defined as the e e

fraction of the voids that water actually flows through (0 # S # where ( = (2+8)/8, e

1.0). Effective saturation is a function of capillary pressure (P ), c

defined as the difference between air and water pressures in pore F = dynamic viscosity of air,

spaces, which in turn is a function of pore geometry. Displacement pressure (P ) is the P where S drops below 1.0 during p = pressure of air, d c e

drainage of pore water. The fraction of the total pore volume

that does not contribute to flow (S = 0) is defined as the residual D = mass density of air, e

saturation (S ). Residual saturation ranges from 0.05 to 0.40 and r

is reached when water is held in pores that are smaller than the and g = acceleration of gravity.

majority of pores. Larger values of S and smaller values of r 8 are

usually associated with finer materials.

e nw

nw

e c dc

8

nw e e e

2 (

nw nw nw nw nw

nw

nw

nw
4

When backfill drains after placement, saturation will be Water flow through cemented backfill that has shrinkage cracks

approximately equal to S . Effective saturation will increase and r

k will decrease if water nw continues to flow into the backfill from

the country rock. Air permeability decreases rapidly as Se

increases, decreases for common materials by a factor of 10 for

S = 0.6, and reaches 0 when S = 1.0. Oxidation of sulfides is e e

effectively shut off because the amount of O dissolved in the 2

incoming water is very low.

Many porous materials have distinctively different pore sizes

that exhibit different saturation, permeability, and flow

characteristics. Fractured rock in which water flows through

primary pores and/or secondary fractures is one example. Under

saturated conditions, water flows preferentially through the

fracture network; however, when S drops below 1.0, the larger e

fractures are the first pores to drain. At some intermediate Se

value, flow through the primary pores becomes dominant, and

when S approaches 0, water will be held in primary pore spaces e

while the fractures will be dry.

or gaps at rock-backfill interfaces will behave in a similar manner to

fractured porous rock. The intact backfill and the secondary cracks

and openings will have separate S (P ) and k (S ) relationships, and e c nw e

the importance of each will depend on the rate of water inflow from

the country rock. Under dry conditions, water will be held in the

intact backfill and secondary openings will be dry, while under

wetter conditions, flow through the larger openings will increase.

When backfill is flooded after mine closure, flow will be primarily

through the secondary openings.

The S, as well as the S (P ) and k (S ) relationships, for a given r e c nw e

backfill will affect the oxidation of sulfide minerals during active

mining while the backfill is partially saturated. The larger the S , the r

more pores will remain saturated, resulting in less mineral surface

area available to be oxidized. Surface area available for oxidation

will be less under wetter conditions (S > 0). The formation of e

shrinkage cracks and gaps will increase oxidation when they are

drained under dry conditions.

RELATED RESEARCH

Several studies have addressed the effects of coal and Studies of the impacts of uranium mill tailings backfill have been

uranium backfill materials on ground water. The contamination

potential of coal waste backfill in surface and underground mines

has been studied by Henderson and Norton (1984), Geidel and

Caruccio (1982), McCurry and Rauch (1986), and Senyur

(1989). The important considerations in these studies were the

potential for acid production and the amount of water flow

through the wastes. Snow concluded that flooding mine 4

workings to control acid mine drainage is difficult when the

workings are located above the mine level where drainage

occurs. Sealing mines with fly ash was proposed to facilitate

flooding above the drainage level under these circumstances.

Senyur (1989) observed decreases in the permeability of backfill

and the rate of water flow through the backfill over time.

The applicability of coal mine case studies in an investigation

of the environmental impacts of backfill in metal mines is

limited, however. The primary difference between the two types

of mines is that heavy metals are released in metal mines as a

result of oxidation of sulfides, whereas in coal mines, relatively

smaller amounts of heavy metals are contained in wastes,

resulting in less metals release.

R. E. Snow, ""Estimation and Control of Ground Water Inflow and 4

Discharge from Underground Mines." Paper presented at the annual meeting

of the Society for Mining Engineers, Salt Lake City, UT, Feb. 26-Mar. 1, 1990.

9 pp.

extensive (Longmire and others, 1981; Brookins and others,

1982; Thomson and Heggen, 1982; Thomson and others, 1986).

Field samples of tailings and backfill, and studies of water

discharging from backfill were used to interpret geochemical

transformations in backfill. The fate of heavy metals contained

in the uranium tailings was considered as well as the fate of

radioactive constituents. The authors concluded that the shortand long-term environmental impacts of backfill associated with

active mining and postclosure activities were negligible.

Factors controlling metals release from surface tailings

impoundments have been investigated through the use of

leaching columns and simulated rain water to analyze tailings

(Doepker and O'Conner, 1990a, 1990b; Doepker, 1991a, 1991b).

The factors investigated included degree of saturation;

availability of O ; wet-dry cycles; leachate residence time; 2

buffering capacity of tailings; evaporation of pore waters;

composition of host rock; composition, amount, and rate of

formation of secondary minerals; and leachant pH, ionic

strength, and types of ions present.

The major cause of metal dissolution was found to be

oxidation of sulfide minerals by atmospheric O under partially 2

saturated conditions. Enhanced metal dissolution was observed

following alternating wet and dry periods or after extended

drying periods; the degree of enhanced metal dissolution was

affected by the percentage of water removed during a dry period
5

(Doepker and O'Conner, 1990a). Furthermore, reduced partial from secondary minerals may occur as a backfilled stope

pressure of O was determined to be the only way to minimize becomes submerged. However, metals release would be 2

metals release from sulfidic tailings. Concentrations in leachates expected to decrease as secondary minerals were depleted from

from well-oxidized tailings differed from concentrations from the tailings.

fresh tailings because secondary minerals were more or less The impacts of mine waste sandfill on groundwater quality

soluble than primary minerals. Secondary minerals formed in surrounding a stope was investigated in a study parallel to the

backfill under partially saturated conditions before mine flooding one being presented here (Levens and Boldt, 1992).

may dissolve after flooding, which may cause a temporary Concentrations of metals in water collected as it left a sandfilled

increase in metal mobility. The oxidation of cemented backfill stope were below or slightly above the maximum concentration

and subsequent release of metals and formation of secondary levels (MCL's) allowed under the Safe Drinking Water Act. The

minerals are expected to be less than what would occur in lack of metals release from the sandfill was attributed to the

uncemented sandfill because of increased residual water buffering of acid by dissolution of carbonate minerals.

saturation and decreased surface area resulting from the addition The primary issues identified in earlier research were a

of cement. reduction in O and oxidation reactions resulting from flooding

Several factors affecting metal dissolution from submerged of placed backfill, the detrimental effects of allowing tailings to

tailings were investigated using batch tests (Doepker and Drake, dry out or of mechanically dewatering backfill prior to

1990, 1991). Oxidation of sulfide minerals in submerged backfilling, the buffering capacities of backfill, and the extent of

tailings was much slower than oxidation in nonsubmerged backfill oxidation prior to flooding (Brookins and others, 1982;

tailings because O diffuses slowly in water. However, metals Thomson and others, 1986; Doepker and Drake, 1990; Levens 2

were released from submerged, oxidized tailings even when they and Boldt, 1994).

were neutralized because of the dissolution of secondary

minerals. Thus a short-term release of metals 

2

METHODOLOGY

This research is part of a larger effort in which the impacts of comparison mine (samples SO and SN) (Levens and Boldt,

mine waste backfill on water quality are being investigated in 1994) were obtained for chemical analyses and laboratory

four underground sulfide mines (Levens and Boldt, 1992). tests. Two separate samples were collected at the comparison

Backfill materials used in the four mines included cemented, mine, one of oxidized backfill from a 10-year-old stope

total-gradation tailings; uncemented, classified sand; and (sample SO) and the other of new tailings from the secondary

uncemented, low-grade gob. Important consequences of cyclone (sample SN). These samples were similar in particleadding cement to backfill include (1) reduced mineral size distributions and mineralogy, differing only in age.

dissolution resulting from enhanced buffering capacity, (2) The three samples were sent to the USBM's Reno Research

reduced oxidation of sulfide minerals resulting from greater Center for chemical assay and mineral identification by X-ray

water retention, (3) reduced migration of metals from backfill diffraction. The chemical assay method used was lithium

into ground water because of lower hydraulic conductivity, borate fusion followed by acid digestion using hydrochloric

and (4) increased strength and reduced chemical attack acid (HCl) and inductively coupled plasma (ICP) analysis. A

attributable to differences in backfill grain-size distributions. maximum leachability assay of 1 g of each backfill sample was

Backfill samples were analyzed to identify mineralogy and also conducted using a mixture of HCl (2 cm ), nitric acid

to determine the maximum amounts of metals that could be (HNO ) (4 cm ), and water (20 cm ) at the USBM's Spokane

leached. Leachability tests using deionized water and sulfuric Research Center (SRC). The samples were placed on a shaker

acid were conducted to characterize neutralization capacities overnight and the supernatant liquid was filtered and analyzed

and rates. Grain-size distributions were plotted from sieve by inductively coupled plasma-atomic emission spectroscopy

analyses, and capillary pressure-saturation relationships were (ICP-AES). The maximum leachability extraction procedure

determined using Tempe pressure cells. can be assumed to dissolve 100 pct of the water-soluble salts

LABORATORY DATA COLLECTION salts (sulfides and carbonates). The metal oxides will also be

A sample of dewatered, freshly milled tailings used in X-ray diffraction data (table 1), provide the information

backfilling in the test mine (sample CN), as well as samples of necessary to determine the source of metals that can be

sandfill from a released in an acidic environment.

3

3 3

3

(chlorides, sulfates, and nitrates), as well as the water-insoluble

solubilized. Maximum leachability data, in combination with
6

Table 1.—Mineralogical analyses of tailings used in backfill1

Mineral CN sample SO sample SN sample

Quartz ........ Major. Major. Major.

Muscovite ..... Trace. Trace. Trace.

Pyrite ......... None. Trace. Trace.

Calcite ........ None. None. None.

Siderite ....... Trace. Trace. Trace.

Galena ....... Trace. Possible trace. None.

Unidentified .... Trace. Trace. Trace.

 to 30 pct. Trace = 1 to 10 pct. Major = 30 to 100 pct. Minor = 10 1

Accuracy was approximately ±25 pct of amount present.

Leaching investigations were conducted in a controlled (FeCO ) gangue compose the ore mineralogy. The stope

environmental chamber to ascertain the neutralization extends approximately 120 m along strike of the vein and is

capacities of the three backfill samples and to measure the rate mined by underhand longwall cut-and-fill with backfill. The

of neutralization. Each sample was mixed with 4-pct-by- backfill consists of cemented tailings in which most of the

weight Type I, II portland cement at 70-pct slurry density fines have been retained. The underhand longwall cut-and-fill

(weight of solids ÷ weight of slurry). The mixes were poured method differs from overhand cut-and-fill by progressing

into three replicate 5-cm cube molds and allowed to cure for downward in successive slices instead of overhead. Mining

28 days in a fogroom maintained at 100-pct humidity. Two of proceeds after the previous cut has been backfilled with

the three cubes of each cemented backfill sample were placed cemented tailings and is allowed to set; consequently, the back,

into glass jars and placed in the environmental chamber. The or roof, of the stope consists of reinforced cemented backfill.

third cube of each sample was kept in the fogroom to be used Groundwater recharge to the Precambrian metasediments

as a control sample in strength tests. near the test mine is primarily the result of infiltration from

Each of the jars in the environmental chamber was filled overlying surface drainage basins. The drainage basin above

with 2,500 mL of distilled water. Distilled water was the mine is underlain by variably fractured bedrock except

circulated over the samples for the first 7 days using small where narrow strips of alluvium are found along stream

pumps. The electrical conductivity (EC), pH, and temperature channels.

of the distilled water leachate was measured daily; after 7 days, The unfractured rock surrounding the mine has very low

water samples were collected for anion and cation analyses. hydraulic conductivity; consequently, water percolates along

At the end of 7 days, 1.4 mL of concentrated H SO was fractures and faults. Similarly, water from the bedrock enters 2 4

added to the leachate circulating over the sample. The pH, EC, the mine where fractures and faults are intersected by

and temperature of the leachate were measured daily. As backfilled or open mine workings or by boreholes. The

evaporation occurred in the chambers, distilled water was hydraulic conductivity of similarly fractured metasediments

added to maintain the total volume of the leachate at was estimated at 1 × 10 m/s by Lachmar (1988). Water

approximately 2,500 mL. When the pH of the leachate on any entering the mine drains to collection sumps and is pumped to

of the samples increased to about 6, 1.4 mL of concentrated the surface for discharge. For comparison, the hydraulic

H SO was added to lower the pH. Samples were collected at conductivity of intact backfill with 6 pct cement added was 2 4

2- to 4-week intervals for analysis of cations and anions. calculated to be less than 1 × 10 m/s in a laboratory

Relationships between capillary pressure and backfill permeameter. Placed backfill will probably have shrinkage

saturation were determined for uncemented tailings and for cracks that may increase the bulk hydraulic conductivity of the

tailings mixed with 6 pct cement by weight using Tempe backfill two or more orders of magnitude.

pressure cells equipped with 0.1 MPa, ceramic, porous plates Water sampling was initiated in November 1990 and

(figure 1). Four replicate cemented samples were cast in the continued on a monthly basis through April 1993; the stope

test cylinders used in the Tempe cells and cured for 4 weeks in advanced 32 m, and 53,500 t of ore were extracted during this

the fogroom. Samples were submerged in a water-lime time period. Samples were collected both before and after the

solution to maintain saturation and to prevent dissolution of water was in contact with the cemented backfill in order to

the cement while curing. The bottom port of the Tempe cell determine the impacts of placed mine waste backfill on water

was connected to an adjustable burret to maintain the outflow quality. Water was collected from exploratory boreholes that

water head at the level of the ceramic plate. Capillary pressure intersected accessible drifts on three levels, and additional

was increased samples were taken from seeps in the advancing stope

by increasing nitrogen (N ) pressure at the top port. Saturation 2

changes were determined from the volume of water displaced

into the burret and from sample porosity.

FIELD DATA COLLECTION

Samples of tailings (sample CN) and water were collected

from a test stope in an underground sulfide mine for analysis.

The rock types in the test stope consisted of Precambrian

metasediments, including argillite, sericitic quartzite, and

vitreous quartzite. Massive tetrahedrite (Cu Sb S ), galena 12 4 13

(PbS), and sphalerite (ZnS) with quartz (SiO ) and siderite 2

3

!7

!9
7

and from a sump that received commingled drainage on filtered through 0.45-Fm filters and placed in flexible 50-mL

sampling level 2 (figure 2). Data from the boreholes represent polyethylene bottles. Separate bottles were filled for cation

water quality within the country rock, whereas the commingled analysis using the Perkin-Elmer Plasma II ICP and for anion

water and seeps that contact the backfill represent water quality analysis using a Dionex Series 4000i gradient ion chromatograph

affected by backfilling. The advancing underhand cut-and-fill (IC). Samples for cation analysis were preserved with HNO

stope cut through a sampling borehole on level 2; probably as a according to EPA procedures (1983) for stabilizing ions in

consequence, the borehole dried up after the seventh sampling solution. Samples were then transported to the chemistry

visit. laboratory at SRC and stored in a refrigerator until being tested.

The pH, EC, redox potential (Eh), temperature, and alkalinity

of samples were measured in the field. Samples were vacuum

3

DATA PRESENTATION AND ANALYSIS

Mineral assays of backfill prior to the addition of cement, Knowledge of the ranges of reaction kinetics and mineral

published mineralogic descriptions, and information from mine stability is necessary to interpret the validity of the

records, in combination with the X-ray diffraction data, provide thermodynamic approach used by WATEQ4F. Also, chemical

the information necessary to determine the sources of metals that reactionsmay be slowed because tailings particles become coated

can be expected to be released in an acidic environment. The with cement and because of increased water saturation; the first

differences between the results of the mineral assay and data factor limits diffusion of water to reactive minerals while the

from the maximum leachability tests using tailings from the test second factor limits diffusion of O into small pores.

mine are compared to similar data from a sandfilled test mine

discussed by Levens and Boldt (1994). This comparison ASSAY AND LEACHABILITY OF SAMPLE CN

provides a baseline for determining the role of cement in

controlling leaching of metals from backfill. Analysis by X-ray diffraction indicated sample CN consisted

Particle size and capillary pressure-water saturation of quartz with traces of muscovite [KAl (AlSi O ) (OH) ],

measurements provide information about the susceptibility of a siderite, and galena. Chemical assays showed that, prior to the

backfill to oxidation prior to flooding. Particle-size analysis also addition of cement, approximately one-third of the backfill used

may explain variations in cemented backfill strengths that, in in the test stope consisted of silica (Si) (table 2). Significant

turn, lead to variations in the degree of chemical attack by acidic quantities of aluminum (Al), Ca, Mg, K, chromium (Cr), copper

solutions. Hydraulic conductivity measurements indicate (Cu), nickel (Ni), lead (Pb), and zinc (Zn) were also detected.

whether water will flow preferentially through the cemented

backfill after flooding.

Leaching experiments conducted in an environmental

chamber shed light on the susceptibility of cemented backfill to

leaching by external sources of acidic water. Calculated

neutralization capacities based on the amount of cement added

and mineral assay data are calculated first. The pH and ion

concentrations of leachates from the deionized water wash and

the H SO wash demonstrate the effects of different chemical and 2 4

physical factors on dissolution of minerals in backfill by acidic

water.

The ultimate use of the laboratory data is to determine factors

that explain the chemical makeup of water collected in a test

mine. Cation and anion levels in water samples were compared

to determine if significant differences could be detected and

attributed to the backfill material, in this case, cemented tailings.

A thermodynamically based geochemical computer model,

WATEQ4F (Ball and others, 1987), was used to investigate

element speciation and mineral saturation in water samples.

Saturation indices calculated by WATEQ4F provide a basis for

evaluating the potential for dissolution and precipitation reactions among minerals in backfill and infiltrating water.

2

2 3 10 2

Table 2.—Chemical assay of backfill materials, parts per million

Analyte CN sample SO sample SN sample

Al . . . . . . . . . . . . . . . . 19,000 15,000 14,000 

As . . . . . . . . . . . . . . . . <300 500 1,500 

Ba . . . . . . . . . . . . . . . . 200 80 44 

Ca . . . . . . . . . . . . . . . . 2,300 3,300 3,700 

Cd . . . . . . . . . . . . . . . . 16 8 13 

Co . . . . . . . . . . . . . . . . 10 20 30 

Cr . . . . . . . . . . . . . . . . 90 80 100 

Cu . . . . . . . . . . . . . . . . 210 250 620 

Fe . . . . . . . . . . . . . . . . 53,000 57,000 54,000 

K ................. 5,500 5,800 5,100 

Mg . . . . . . . . . . . . . . . . 1,900 2,400 1,400 

Mn . . . . . . . . . . . . . . . . 5,600 5,000 1,600 

Mo . . . . . . . . . . . . . . . . <50 <50 <50 

Na . . . . . . . . . . . . . . . . 400 600 500 

Ni . . . . . . . . . . . . . . . . 200 330 240 

P ................. <1,000 <1,000 <1,000 

Pb . . . . . . . . . . . . . . . . 1,800 1,500 1,500 

Si . . . . . . . . . . . . . . . . 334,000 348,000 358,000 

Zn . . . . . . . . . . . . . . . . 4,300 2,500 5,800 
' '

8

Table 3.—ICP-AES data from analysis of strong acid extraction

of backfill materials, milligrams per liter

Analyte CN sample SO sample SN sample

Ag . . . . . . . . . . . . . . . 0.04 0.02 0.03

Al . . . . . . . . . . . . . . . 2.1 2.0 1.9

As . . . . . . . . . . . . . . . 1.2 6.0 10.4

Ba . . . . . . . . . . . . . . . 0.3 0.03 0.02

Ca . . . . . . . . . . . . . . . 23.0 35.4 38.8

Cd . . . . . . . . . . . . . . . 0.17 0.21 0.34

Cu . . . . . . . . . . . . . . . 0.69 0.56 0.69

Fe . . . . . . . . . . . . . . . 306 312 186

K . . . . . . . . . . . . . . . . 0.9 0.9 0.8

Mg . . . . . . . . . . . . . . . 11.2 12.4 6.4

Mn . . . . . . . . . . . . . . . 32.4 29.4 9.5

Na . . . . . . . . . . . . . . . 0.52 0.12 0.41

Pb . . . . . . . . . . . . . . . 32.0 21.6 27.8

S . . . . . . . . . . . . . . . . 7.8 37 86

Si . . . . . . . . . . . . . . . 2.2 2.2 1.7

Zn . . . . . . . . . . . . . . . 21.9 14.9 35.4

The maximum leachability assay using a mixture of HCl and PARTICLE-SIZE DISTRIBUTION

HNO yielded significant concentrations of Ca and Mg as well as 3

the metals Fe, manganese (Mn), Pb, and Zn (table 3). Assuming

that the Ca extracted by the acid exists as CaCO , the CN sample 3

contains 0.58 pct CaCO . Lesser amounts of silver (Ag), arsenic 3

(As), cadmium (Cd), Cr, Cu, and Ni were also leached. The Ca

concentration in the acid leachate indicates a low acidneutralization capacity in the tailings while low Si and Al

concentrations indicate the tailings have very little reactive

aluminosilicate minerals that might contain trace metals.

The Pb and Zn contained in the leachate from the CN sample

were 32.0 and 21.9 ppm, respectively. Both these elements

should exist as sulfides because the sample was collected from

fresh tailings; however, there is about half the necessary sulfur

(S) to account for dissolution of galena and sphalerite. Fe

probably occurs primarily as siderite because the sample was

fresh and therefore contained little iron oxide from oxidation.

The mass of the CN sample dissolved during the maximum

leachability assay was estimated by converting concentrations to

oxides of Si, Na, K, and Al; carbonates of Ca, Mg, and Fe; and

sulfides of Pb, As, and Zn for all element concentrations above

1 mg/L. When these values were converted to milligrams per

gram, the amount of sample dissolved was 111 mg/g, or about 11

pct. Less than this quantity of tailings will react, leaving at least

89 pct of the sample to be bound by cement.

Particle-size distributions relate to the structural integrity of the

tailings-cement matrix and therefore to the susceptibility of

acemented backfill to chemical attack. Particle-size distributions of

samples CN, SO, and SN are shown in figure 3. A comparison of

the curves shows that samples SN and SO have similar narrow

ranges of particle sizes while sample CN has a wider range with

significantly finer particles. Sample CN had a coefficient of

uniformity (a measure of the gradation range of the particle-size

curve) of 14 and a coefficient of curvature (a measure of the shape

of the particle-size curve) of 1.28; on the basis of these values,

sample CN is classified as a well-graded sand. The coefficients of

uniformity for samples SN and SO were 3 and 6, respectively, and

the coefficients of curvature were 0.69 and 0.85, respectively.

Samples SN and SO are classified as uniform, poorly graded sands.

Differences in particle-size distributions are expected to affect the

strength and structural integrity of cemented backfill and, as a

consequence, the degree of degradation when the material is attacked

by acid solutions. Therefore, the results of particle-size analyses are

considered in the following discussion of the laboratory leaching

experiments.

NEUTRALIZING CAPACITY OF CEMENTED BACKFILL

Water retention characteristics of cemented backfill affect the access of air to sulfide minerals and therefore oxidation rates. Leachate analyses from both the deionized

water and acid wash experiments indicate the contribution

of cement to acid neutralization relative to the contribution

from minerals contained in the tailings.

The resistance of the cemented backfill samples to attack by acidic water depends on the neutralizing capacity

of the backfill and the rate at which acidic water diffuses

into the samples. Neutralizing capacity is calculated from

the ratio of the weight of the cement to the weight of the

backfill and the composition of each as determined from

published values and chemical analyses.

The average composition of compounds in Type I, II

portland cement that will contribute to acid-neutralization

capacity are 65.9 pct calcium oxide (CaO), 2.3 pct magnesium oxide (MgO), and 0.72 pct potassium oxide (K O). 2

Based on the quantity of cement used to make the samples, the milliequivalents (mequi.) of neutralizing capacity

from each component were calculated and then totaled to

obtain the total neutralizing capacity of the cement in each

sample (table 4).

For example, the contribution from CaO in the CN

sample would be
' '

9

Table 4.—Acid-neutralizing capacity of portland cement in

backfill samples, milliequivalents

Sample CaO MgO K O2 Total

CN . . . . . . . . . 314.0 25.2 2.O 341.2

SN . . . . . . . . . 264.4 21.2 1.6 287.2

SO . . . . . . . . . 344.6 27.6 2.2 374.4

Table 5.—Acid-neutralizing capacity of tailings

in backfill samples, milliequivalents

Sample Sum of Ca, Mg, 

Mn, K, and Na 

Fe Total

CN . . . . . . . . . 109.7 366.6 476.3

SN . . . . . . . . . 80.2 124.4 204.6

SO . . . . . . . . . 142.2 403.2 545.4

Table 6.—Acid-neutralizing capacity of cemented

backfill samples, milliequivalents

Sample Sum of Ca, Mg,

Mn, K, and Na

Fe Total

CN . . . . . . . 450.9 366.6 817.5

SN . . . . . . . 367.4 124.4 491.8

SO . . . . . . . 516.6 403.2 919.8

The neutralizing capacities of the uncemented tailings The neutralizing capacities of the uncemented tailings

samples were calculated on the basis of maximum leachability samples were calculated on the basis of maximum leachability

data and estimated mineralogy (table 5). Ca, Mg, and Mn were data and estimated mineralogy (table 5). Ca, Mg, and Mn were

assigned to carbonate minerals whereas Na assigned to carbonate minerals whereas Na 

If Pb and Zn were present as sulfides and sulfates, then the

remaining small amount of S was assigned to Fe in a molar

ratio for pyrite; the remainder of the Fe was assigned to siderite.

The reaction between siderite and H SO is— 2 4

FeCO (s) + 3 2 H 6 Fe q) + 2+ (aq) (a

 CO (F) + H O + (g). 2 2

Oxidation of Fe is— 2+

Fe (aq) + (g) 1/4 O + H 6 Fe q) 2+ + 3+

2 (aq) (a

(G) + 1/2 H O. 2

Hydrolysis of Fe is— 3+

Fe (aq) O + 3 H e(OH) 3 H (H) 6 F (s) + 3+ +

2 3 (aq).

Reactions F and G consume 3 mol of H for each mol of +

siderite. However, depending on the extent of hydrolysis of

Fe (equation H), 3 mol of H may be produced. Therefore, 3+ +

dissolution of siderite may result in net acid consumption

ranging from 0 to 3 mol of H per mol of siderite. +

Sample CN had all the measured S required for Pb and Zn

compounds. Therefore, in agreement with the mineralogical

analyses, all Fe in the sample was assigned to siderite in

calculations of neutralization capacity. Samples SO and SN

had small amounts of Fe assigned to pyrite with the rest

considered to be siderite. The value for siderite in sample SO

is probably too high because some Fe probably has oxidized to

goethite; however, an approximation can be completed using

lesser amounts of Fe and taking this first calculation as an

extreme value.

The total neutralization capacity of the cemented backfill

calculated as the sum of the capacities of the cement and the

backfill is given in table 6. The neutralizing capacity of the

cemented tailings compared to tailings alone, and not including

contributions from siderite, was approximately 3.6 times

greater in the SO sample, 4.1 times greater in the CN sample,

and 4.6 times greater in the SN sample. The quantity of siderite

dissolved is important from the perspective of the physical

strength of the sample even if no acid is consumed. Because

siderite is part of the particles being bound together by cement,

its destruction will weaken the sample.

DEIONIZED WATER WASH 

High concentrations of Na, K, and Ca were detected in the

deionized water used to flush the surfaces of the cemented

backfill cubes prior to washing with H SO (table 7). 2 4

Hydroxides and sulfates of these elements derived from the

cement were probably flushed from the surfaces of the samples.

Also, the amounts of Al and Si detected were low, which would
10

Table 7.—Element concentrations in samples containing cement

after washing with deionized water

for 7 days, milligrams per liter 

Analyte CN sample SO sample SN sample

Al . . . . . . . . . . 0.2 2.2 0.2

Ba . . . . . . . . . . 0.17 0.08 0.06

Ca . . . . . . . . . . 54.2 96.1 183

Cu . . . . . . . . . . 0.04 0.09 ND 

Fe . . . . . . . . . . 0.2 0.2 0.1

K . . . . . . . . . . . 7.3 15.5 26.6

Mn . . . . . . . . . . ND 0.1 ND 

Na . . . . . . . . . . 7.9 14.7 14.0

Pb . . . . . . . . . . 0.06 ND 0.02

Si . . . . . . . . . . 6.6 6.7 8.5

Zn . . . . . . . . . . 0.06 ND 0.05

pH . . . . . . . . . . 8.6 8.6 8.0

SO . . . . . . . . . 4 86.1 130 366

ND No data.

be expected because minerals containing these elements have penetration of the acid. Degradation of the structural integrity

low solubilities. No measurable amounts of trace metals were of the SO and SN samples apparently allowed penetration of

dissolved. the acid, leading to the rapid reactions observed.

SULFURIC ACID WASH either of the two other samples. Spikes of increased pH similar

Tables 8 and 9 show the amount of H SO added and the 2 4

remaining acid-neutralization capacities of the three samples of

cemented backfill during the H SO wash 2 4 test. The values in

table 8 include the potential contribution of siderite, whereas

the values presented in table 9 include only the contributions of

Ca, Mg, and Mn carbonates and K and Na oxides. The values

in table 9 are based on the assumption that acid consumption

(equations F and G) is balanced by acid production by

hydrolysis (equation H). Large amounts of oxyhydroxides

precipitated on the surface of the three samples is evidence that

siderite was reacting and that the hydrolysis reaction occurred

to a limited extent. Therefore, the values in tables 8 and 9 are

reasonable bounds for actual remaining neutralization

capacities.

Figures 4, 5, and 6 show the pH of the fluid washing the

samples as a function of time after H SO was added. The pH 2 4

of the wash solution dropped from about 8 to less than 3 for all

samples after the H SO wash began. The pH of the wash from 2 4

samples SO and SN increased to the basic range after 72 h,

indicating an aggressive attack of the samples by the acid,

resulting in neutralization of the H SO . Sample CN had a 2 4

slower reaction rate as indicated by an increase in pH in the

wash to only 4.93. Samples SO and SN had similar particlesize distributions, whereas sample CN had a higher percentage

of both large and fine particles. The broader particle-size

distribution of sample CN may have promoted the formation of

a more resistant cement-tailings matrix, which would limit

The pH of the wash solution from sample CN showed a

slow rise after each addition of H SO , becoming significantly 2 4

slower as the test progressed. Early in the test, acid

neutralization took place more rapidly as reactions occurred on

the surface of the cubes, but over time, the surface of the

sample accumulated reaction products (such as gypsum and

iron oxyhydroxides) that slowed penetration of additional acid

into the cube. Also, the acid had to diffuse into the sample to

greater depths each time the acid was added, slowing the rate of

neutralization. As shown in figure 4, the sample was still

neutralizing acid at the end of the experiment.

Sample SO rapidly neutralized each addition of acid, and pH

recovered more slowly as the test progressed. After 70 days,

pH remained nearly constant, gaining only 0.2 pH units over

the period of the rest of the experiment. It appears that most of

the material available for acid neutralization had been

consumed by that time.

Sample SN responded differently to the acid wash than

to those shown by the SO sample occurred during the first few

days. The acid appeared to penetrate the sample rapidly, so that

all the easily accessible basic material was consumed in about

10 days. Then pH decreased for the rest of the experiment

while no more acid was added. The continuing decrease in pH

can be attributed to the presence of reduced iron sulfide compounds, such as pyrite, in the sample, which would oxidize

over the course of the experiment and produce acid, which

would lower the pH. The pyrite in the sample was calculated

as 0.112 mol, which would produce 450 mequi. of acidity if

oxidized and precipitated as ferrihydrite; this amount was

greater than the amount of basic material in the sample.

Oxidation of sulfides at a faster rate than acid neutralization

would also explain the continued decline in pH, even though

calculations showed that significant quantities of basic material

remained in the SN sample.

The results of the chemical analyses of the acid wash

solutions are given in table 10. All washes showed increased

concentrations of metals at the end of the experiment as

compared to the beginning. WATEQ4F was used to evaluate

solubility controls of the Pb and Zn ions in the SN sample wash

because concentrations of these elements were high in the

tailings. Zn concentrations continued to increase throughout

the experiment, but concentrations were probably limited by

diffusion of the acid into the tailings to oxidize and dissolve the

sphalerite present. Pb concentrations did not increase above 4

mg/L.
11

Table 8.—Maximum neutralizing potential of cemented backfill materials

Elapsed time,

days

CN SN SO

mL of acid

 added remaining added remaining added remaining

mequi. pH mL of acid mequi. pH mL of acid mequi. pH 

0.0 818 8.01 0.0 492 8.05 0.0 920 8.65

0 . . . . . . . . . . . . . 1.4 768 1.4 442 1.4 870

3 . . . . . . . . . . . . . 1.4 718 4.93 1.4 392 7.43 1.4 920 8.18 1 11

5 . . . . . . . . . . . . . 718 2.59 1.4 342 7.44 1.4 770 7.92 1 1

6 . . . . . . . . . . . . . 718 2.90 342 5.27 1.4 720 6.73 1

7 . . . . . . . . . . . . . 718 2.85 1.4 292 6.28 1.4 670 7.54 1 1

8 . . . . . . . . . . . . . 718 292 670

9 . . . . . . . . . . . . . 718 292 670

10 . . . . . . . . . . . . 718 3.20 1.4 242 6.69 1.4 620 6.94 1 1

11 . . . . . . . . . . . . 718 3.57 242 3.85 620 3.35

12 . . . . . . . . . . . . 718 242 620

13 . . . . . . . . . . . . 718 4.19 242 4.92 620 5.46

14 . . . . . . . . . . . . 718 4.31 242 4.96 1.4 570 7.01 1

15 . . . . . . . . . . . . 718 242 570

16 . . . . . . . . . . . . 718 242 570

17 . . . . . . . . . . . . 718 4.73 242 4.48 570 5.09

18 . . . . . . . . . . . . 718 4.91 242 4.65 570 5.79

19 . . . . . . . . . . . . 718 5.44 242 5.04 1.4 520 6.59 1

20 . . . . . . . . . . . . 718 5.12 242 4.81 520 3.04

21 . . . . . . . . . . . . 718 242 520

22 . . . . . . . . . . . . 718 242 520

28 . . . . . . . . . . . . 718 4.76 242 5.92 520 5.70

31 . . . . . . . . . . . . 1.4 668 6.18 242 4.82 520 5.36 1

32 . . . . . . . . . . . . 668 2.27 242 4.67 1.4 470 6.24 1

33 . . . . . . . . . . . . 668 2.42 242 4.66 470 2.47

42 . . . . . . . . . . . . 668 3.07 242 4.40 470 3.93

60 . . . . . . . . . . . . 668 5.16 242 4.12 470 4.73

668 242 470

81 . . . . . . . . . . . . 1.4 618 6.10 242 3.64 470 5.06 1

82 . . . . . . . . . . . . 618 2.24 242 3.63 470 5.04

618 242 470

109 . . . . . . . . . . . 618 4.06 242 3.52 470 5.06

618 242 470

136 . . . . . . . . . . . 618 5.28 242 3.31 470 4.97

618 242 470

158 . . . . . . . . . . . 1.4 568 6.49 242 3.02 470 4.82 1

159 . . . . . . . . . . . 568 2.31 242 3.09 470 4.85

568 470

223 . . . . . . . . . . . 568 4.72 242 2.28 470 4.83

568 One cube of SN disintegrated. 470

568 470

238 . . . . . . . . . . . 568 5.19 2.17 470 4.84

Total mL of acid

added: .........

7.0 7.0 12.6

Total mequi. of 250 250 450

acid added: .....

ition of acid. pH prior to add 1

NOTE.—Blank cells indicate no data.
12

Table 9.—Neutralizing potential of cemented backfill materials, excluding FcCO3

Elapsed time,

days

CN SN SO

mL of acid

 added remaining added remaining added remaining

mequi. pH mL of acid mequi. pH mL of acid mequi. pH 

0.0 451 8.01 0.0 367 8.05 0.0 517 8.65

0 . . . . . . . . . . . . . . . . . . . . . . . 1.4 401 1.87 1.4 317 2.57 1.4 467 2.17 2 22

3 . . . . . . . . . . . . . . . . . . . . . . . 1.4 351 4.93 1.4 267 7.43 1.4 417 8.18 1 11

5 . . . . . . . . . . . . . . . . . . . . . . . 351 2.59 1.4 217 7.44 1.4 367 7.92 1 1

6 . . . . . . . . . . . . . . . . . . . . . . . 351 2.90 217 5.27 1.4 317 6.73 1

7 . . . . . . . . . . . . . . . . . . . . . . . 351 2.85 1.4 167 6.28 1.4 267 7.54 1 1

8 . . . . . . . . . . . . . . . . . . . . . . . 351 167 267

9 . . . . . . . . . . . . . . . . . . . . . . . 351 167 267

10 . . . . . . . . . . . . . . . . . . . . . . 351 3.20 1.4 117 6.69 1.4 217 6.94 1 1

11 . . . . . . . . . . . . . . . . . . . . . . 351 3.57 117 3.85 217 3.35

12 . . . . . . . . . . . . . . . . . . . . . . 351 117 217

13 . . . . . . . . . . . . . . . . . . . . . . 351 4.19 117 4.92 217 5.46

14 . . . . . . . . . . . . . . . . . . . . . . 351 4.31 117 4.96 1.4 167 7.01 1

15 . . . . . . . . . . . . . . . . . . . . . . 351 117 167

16 . . . . . . . . . . . . . . . . . . . . . . 351 117 167

17 . . . . . . . . . . . . . . . . . . . . . . 351 4.73 117 4.48 167 5.09

18 . . . . . . . . . . . . . . . . . . . . . . 351 4.91 117 4.65 167 5.79

19 . . . . . . . . . . . . . . . . . . . . . . 351 5.44 117 5.04 1.4 117 6.59 1

20 . . . . . . . . . . . . . . . . . . . . . . 351 5.12 117 4.81 117 3.04

21 . . . . . . . . . . . . . . . . . . . . . . 351 117 117

22 . . . . . . . . . . . . . . . . . . . . . . 351 117 117

28 . . . . . . . . . . . . . . . . . . . . . . 351 4.76 117 5.92 117 5.70

31 . . . . . . . . . . . . . . . . . . . . . . 1.4 301 6.18 117 4.82 117 5.36 1

32 . . . . . . . . . . . . . . . . . . . . . . 301 2.27 117 4.67 1.4 67 6.24 1

33 . . . . . . . . . . . . . . . . . . . . . . 301 2.42 117 4.66 67 2.47

42 . . . . . . . . . . . . . . . . . . . . . . 301 3.07 117 4.40 67 3.93

60 . . . . . . . . . . . . . . . . . . . . . . 301 5.16 117 4.12 67 4.73

301 117 67

81 . . . . . . . . . . . . . . . . . . . . . . 1.4 251 6.10 117 3.64 67 5.06 1

82 . . . . . . . . . . . . . . . . . . . . . . 251 2.24 117 3.63 67 5.04

251 117 67

109 . . . . . . . . . . . . . . . . . . . . . 251 4.06 117 3.52 67 5.06

251 117 67

136 . . . . . . . . . . . . . . . . . . . . . 251 5.28 117 3.31 67 4.97

251 117 67

158 . . . . . . . . . . . . . . . . . . . . . 1.4 201 6.49 117 3.02 67 4.82 1

159 . . . . . . . . . . . . . . . . . . . . . 201 2.31 117 3.09 67 4.85

201 117 67

223 . . . . . . . . . . . . . . . . . . . . . 201 4.72 117 2.28 67 4.83

201 One cube of SN disintegrated. 67

201 67

238 . . . . . . . . . . . . . . . . . . . . . 201 5.19 2.17 67 4.84

Total mL of acid added: . . . . . 7.0 7.0 12.6

Total mequi. of acid added: . . . 250 250 450

ition of acid. pH prior to add 1

pH after addition of acid. 2

NOTE.—Blank cells indicate no data.
13

Table 10.—Element concentrations in cemented backfill samples after washing with H SO for 227 days, milligrams per liter 2 4

Analyte CN sample SO sample

SN sample

Al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.00.4480.1

Ba . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.010.003ND

Ca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 640716623

Cd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.010.321.8

Cu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.100.070.68

Fe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.66.897.6

K ............................................................................................................. 3.91.4 3.0

Mg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.2125110 

Mn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.865.364.8

Na . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.08.79.4

Pb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.901.63.9 

Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.634.7157

Zn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.9733.7336

pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.94.72.2

SO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1,8072,0503,622

ND No data.

Early in the experiments, Ca concentrations reached a

plateau, apparently limited by gypsum (Ca SO " 2H O) 2 4 2

solubility. It was obvious from the sample analyses that iron

oxyhydroxides precipitated during dissolution of the cemented

backfill and limited the solubility of Fe.

In the CN sample wash, all concentrations of metal ions

(except Mn) stayed below 10 mg/L. This result can be

attributed to the stability of the sample and its acidneutralization capacity. The pH at the end of the experiment

was about 5.5.

Concentrations of several metals were greater in the SO

sample than in the CN sample. pH was about the same, so

solubility was probably not limiting; it is suspected that more

of the SO sample had been attacked and more metal ions were

freed.

FIELD WATER QUALITY

Concentrations of As, barium (Ba), and Pb exceeded the

primary MCL's specified under the Safe Drinking Water Act in

selected samples (table 11). Also, concentrations of Fe, Mn,

and SO exceeded secondary MCL's. However, during most 4

visits, all concentration levels were below or near the MCL's.

EC was highest in the sample from the sump and lowest in

sample DH1 from level 1. Higher EC correlated most closely

with higher concentrations of Ca, K, Mg, and SO . No 4

meaningful differences in pH, Eh, or most heavy metals were

detected; an exception was that Pb and Zn concentrations were

higher in specific samples from the sump on level 2.

Concentrations of Ca, Mg, K, and SO were higher in the 4

sump and stope samples than in the drillhole samples. The

higher concentrations of Ca, Mg, and SO , in conjunction with 4

near-neutral pH values, suggest that acid production by

oxidation of sulfide minerals is being buffered by dissolution

of carbonate minerals, or more significantly, the cement added

to the backfill. High concentrations of Na balanced by high

concentrations of HCO were detected in some samples; this 3

occurrence appears to be unrelated to chemical reactions within

the backfill.

The saturation indices (SI) calculated using WATEQ4F

(Ball and others, 1987) indicate equilibrium or slightly

undersaturated conditions for most minerals (&1 # SI # 1)

(table 12). Ferrihydrite is one of few minerals that was

supersaturated (SI $ 1), indicating a potential to precipitate.

Other minerals, such as gypsum, that commonly precipitate

from water discharging from sulfidic wastes were

undersaturated (SI # &1) and therefore were not expected to

precipitate. Mixing water high in HCO3 , a condition !

observed at several sites, with water in equilibrium with

calcite may result in precipitation of calcite.
14

Table 11.—Average and range (in parentheses) of element concentrations in water samples from cemented backfill in test mine

Analyte DH1 (22) DH2 (14) DH3 (7) DH4 (8) Sump (10) Stope (9) 

Element, mg/L:

 Al . . . . . . . 0.20 (0.01-0.66) <0.01 (<0.01-0.64) 0.45 (0.08-0.66) 0.03 (0.01-0.10) 0.29 (0.01-1.19) 0.10 (0.01-0.65)

 As . . . . . . . <0.10 (<0.10-0.40) <0.10 (<0.10-0.22) <0.10 <0.10 <0.10 (<0.10-0.20) <0.10 (<0.10-0.60)

 Ba . . . . . . . 1.12 (0.59-1.79) 0.88 (0.74-1.02) 0.22 (0.17-0.27) 0.39 (0.26-0.44) 0.06 (0.02-0.09) 0.08 (0.06-0.12)

 Ca . . . . . . . 25.81 (12.20-35.60) 27.38 (24.00-29.01) 27.72 (23.00-36.50) 28.38 (21.80-32.63) 230.50 (10.62-434.30) 66.42 (29.16-99.70)

 Cu . . . . . . . 0.04 (0.01-0.09) <0.01 (<0.01-0.06) 0.04 (0.02-0.06) <0.01 0.04 (<0.01-0.20) <0.01 (<0.01-0.04)

 Fe . . . . . . . 0.57 (0.05-2.46) 0.39 (0.06-0.47) 0.61 (0.27-0.94) 0.22 (0.10-0.31) 1.56 (<0.01-8.70) 0.23 (0.02-0.56)

 K ........ 20.27 (12.90-58.10) 21.96 (17.40-24.70) 11.55 (7.70-15.10) 11.38 (9.50-17.16) 44.79 (9.40-124.80) 51.35 (41.10-75.29)

 Mg . . . . . . . 2.46 (1.39-3.07) 3.45 (2.97-3.83) 2.51 (1.86-3.21) 2.00 (1.83-2.33) 38.48 (1.85-66.76) 7.17 (3.05-11.45)

 Mn . . . . . . . 0.17 (0.05-0.44) 0.08 (0.07-0.11) 0.06 (0.03-0.08) 0.05 (0.03-0.08) 0.86 (0.02-1.69) 0.41 (0.18-0.89)

 Na . . . . . . . 57.97 (34.69-84.41) 184.44 (157.17- 179.58 (136.39- 221.31 (54.12- 72.18 (13.70-170.64) 211.21 (183.67-

245.06) 244.92) 302.73) 262.93)

 P ........ 0.18 (0.03-1.04) 0.08 (0.03-0.27) 0.10 (0.03-0.27) 0.10 (0.03-0.41) 0.22 (<0.05-1.47) 0.33 (0.03-1.20)

 Pb . . . . . . . <0.05 (<0.05-0.13) 0.06 (<0.05-0.10) 0.06 (<0.05-0.08) <0.05 (<0.05-0.08) 1.01 (0.06-6.84) 0.17 (<0.05-0.46)

 Si . . . . . . . 9.45 (3.85-12.80) 7.96 (3.11-8.91) 13.33 (10.20-17.29) 10.70 (5.99-12.73) 9.60 (5.22-12.36) 13.68 (8.45-19.45)

 Zn . . . . . . . 0.05 (<0.01-0.34) 0.02 (<0.01-0.05) 0.04 (0.01-0.07) <0.01 (<0.01-0.03) 0.42 (0.25-0.79) 0.04 (<0.01-0.16)

HCO , mg/L . 3 261.53 (235.00- 617.58 (549.00- 484.33 (450.00- 672.43 (638.00- 134.75 (79.00-366.00) 304.10 (234.00-

290.00) 866.00) 518.00) 762.00) 518.00)

SO , mg/L . . 4 3.18 (0.15-8.4) 9.84 (2.10-16.20) 39.78 (33.85-46.13) 17.45 (7.12-20.67) 663.10 (140.27- 384.94 (271.81-

1595.45) 503.58)

EC, S ...... 0.46 (0.25-0.52 1.08 (1.00-1.13) .98 (0.85-1.09) 1.41 (1.32-1.45) 1.83 (0.54-3.02) 1.60 (1.26-2.01)

Eh, mV ..... 180.77 (88.00- 160.36 (-68.00- 170.67 (80.00- 144.86 (0.00-194.00) 208.90 (177.00- 150.67 (3.00-207.00)

265.00) 238.00) 282.00) 256.00)

pH . . . . . . . . . 7.21 (6.29-7.98) 7.36 (7.02-7.58) 7.15 (6.69-7.42) 7.13 (7.06-7.26) 7.46 (6.50-9.35) 7.63 (6.98-8.17)

Temp., EC . . 29.59 (26.60-31.90) 34.58 (33.20-37.50) 33.33 (31.40-36.00) 40.63 (39.00-42.30) 27.38 (25.30-31.40) 35.68 (31.40-40.60)
15

Table 12.—Saturation indices of water samples

Mineral DH1 DH2 DH3 DH4 Sump Stope 

Albite ...................... !0.518 NC 0.040 0.523 &1.284 !0.158

Calcite ..................... !0.325 0.200 0.860 !0.119 0.086 0.282

Dolomite .................... !1.801 !0.559 0.597 !1.362 &0.978 !0.396

Gypsum .................... !3.414 !3.027 !1.274 !2.401 &2.779 !0.587

Illite ........................ 3.910 NC 2.808 4.638 0.677 4.667

Kaolinite .................... 5.621 NC 3.353 6.291 2.979 12.905

Ca-montmorillonite ........... 4.584 NC 2.451 5.527 1.503 4.868

Siterite ..................... !0.305 NC NC NC NC NC

Silica gel .................... !0.505 !0.631 !0.416 !0.394 !0.563 !0.473

NC Not calculated.

DISCUSSION AND CONCLUSIONS

Previous research into the environmental impacts of

mine waste sandfill demonstrated that sandfill mineralogy,

as well as exposure to O and water prior to mine flooding, 2

will control the release of metals to ground water. Secondary

minerals formed when sandfill is exposed to O during mine 2

operations will be dissolved, and oxidation of sulfides by Fe3+

may occur after the sandfill is flooded. Addition of cement is

expected to decrease the surface area of sulfides exposed to

oxidation, to decrease the diffusion of air into the backfill prior

to flooding by increasing residual water saturation, and to

augment the buffering capacity provided by the natural

mineralogy. These factors should result in the release of less

metals than would occur from sandfill alone.

The addition of cement affected the chemistry of backfill

under the influence of an external source of acidic water.

Sufficient H SO was added to the solution contacting the 2 4

cemented backfill to drop the pH to ~2.0. In three different

tailings samples, the pH of the solution contacting the backfill

increased. The acid-neutralization capacity, as well as the acidneutralization rate, of all three samples was different.

Particle-size distribution of the tailings used to make a

cemented backfill appeared to be very important in controlling

the rate of chemical attack. Material with a wider distribution of

particle sizes created concrete with a slower chemical reaction

rate with acid than material with uniform sizes of particles. It is

possible that the slower reaction rate results from slower

diffusion of acid into the sample, which in turn may be caused

by smaller pore sizes.

The SN and SO samples had very similar distributions of

particle sizes and smaller ranges of sizes than the CN sample.

These samples exhibited very rapid neutralization of the acid

wash flowing over the surface of the cemented tailings, which

suggests a rapid penetration rate. This, in turn, could be

interpreted as related to the pore size of the concrete.

The presence of sulfide minerals in the tailings is important

in determining the acid-neutralization capacity of the tailings.

The main difference between the SO and SN samples was the

estimated pyrite content; the quantity of Ca and Mg carbonates,

as estimated from the strong acid dissolution, was approximately the same. Therefore, the SN sample had a greater potential to

generate acidic water than did the SO sample. After an approximately equivalent period of acid neutralization by the two

samples, the wash contacting the SN sample remained acidic.

It appears that the additional acid exposed sufficient reduced

sulfide minerals to allow oxidation and acid production. The

acid-producing capacity of tailings will be an important consideration prior to their use as cemented backfill.

Backfilled stopes in rock with low hydraulic conductivities will

probably constitute preferential flow paths after mine flooding;

however, the rate of flow through the backfill will be much less

prior to flooding. Furthermore, addition of cement decreases the

hydraulic conductivity of backfill, which will result in less water

flow through the backfill than if uncemented sandfill is used.

Preferential flow may develop where shrinkage occurs near stope

boundaries or where the backfill has deteriorated because of sulfide

oxidation. However, the effect of increased flow will be offset by

decreased surface area contact and by armoring of preferential flow

paths by insoluble precipitates.

Considering all factors, the overall impact of cemented

backfill should be less than the effect of uncemented sandfill.

Factors relevant to the contamination potential of cemented

backfill that warrant further study include (1) the relationship

between residual saturation of backfill and the addition of

cement, (2) the reduction in sulfide oxidation and buffering

resulting from armoring of preferential flow paths, (3) changes

in the strength of cemented backfill in the long term caused by

sulfide oxidation, and (4) the quality of residual pore water

after backfilling.
16

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