## DRAFT

MERCURY MASS BALANCE AND EMIS SIONS FACTOR ESTIMATES FOR GOLD ORE PROCESSING

FACILITIES

Prepare d for
U.S. ENV IRONMENTA L PROTECTI ON AGENCY Region 9 75 Hawthorn e St., San Fra ncisco, C alifornia , 94105

Work Assignment No. R0920 EPA Region 9 Date Prepared January 14, 2001 Document Control No. REPA2-0920-002 Contract No. 68-W-99-009 Prepared by Booz Allen & Hamilton, Inc Contractor Work Assignment Manager Stuart Strum Telephone No. (210) 244-4233 EPA Work Assignment Manager John Katz Telephone No. (415) 972-3283 1i TABLE OF CO NTENTS
Section Page
1.0 INTRODUCTI ON 1
2.0 EMISSION S FACTORS AND MASS BALANCES FOR GOLD ORE PROCESSING UNITS 3
2.1 Ore Roasters 3 2.2 Autoclaves 11 2.3 Carbon Regeneration Kilns 12 2.4 Electrowinning

Cell Rooms 14 2.5 Retorts 15 2.6 Dore Furnaces 16
3.0 SUMMARY AND CONCLUSI ONS 17
4.0 REFERENCES 201

1.0 Introduction Toxics Release Inventory reporting by

the gold mining industry has shown that these facilities may be large sources of mercury emissions to air. Air emissions of mercury f rom gold ore processing occur at a maximum rate of thousands of pounds per y ear, a much higher rate tha n that observed at coal-fired electric generating plants, which emit an ave rage of 250 pounds per ye ar merc ury . In order to id

entify the spec ific sour

ces of me rcury emissions f

rom gold processing, the range of

possible mercury loa

ding and partitioning for each proc ess type with a signific ant poten

tial has be en evalu

ated. Go ld ores a

re trea ted by a variety o f proce sses to recover the gold content of the ore . All gold recovery processes rely

on leaching relatively

low concen tration of gold fr om ores us ing cy

anide sol utions. B ecause

gold ha s a high value, it is economic to recover gold from ore s with low concentrations of finely disseminated gold (as low as 0.05 ounces/ton). Ores that contain gold that can be r eadably e xtracted by crushing

and leaching the ore with cy

anide solution are referred to as “oxide” or “ free milling” ores (Ref. 1). However, gold recovery

from some ores by le aching is not efficient, with as little as 30 percent of the gold content of the ore removed by cyanide le aching. These ores are c alled refractory ores. Poor gold recovery

from refractory

ores is caused by

the presence of gold in sulfide mineral grains (usually

pyrite, iron sulfide) that do not re act with the cya nide leach solution, or by adsorption of dissolved gold onto carbonaceous material pre sent in the ore (known as “preg robbing”). Refractory

ores can be treated by

thermal combustion processes (ore roasting) or in pressure oxidation vessels (autoclaves). These processes oxidize sulfide minerals and carbon, and the gold content of the treate d ore ca n then be e

xtracted by cya

nide lea ching. Becau se the g old containi ng cy anide sol ution (pr egnan t leach or liquor)has rela tively dilute g old conce

ntrations , gold is c oncentr

ated by adsorpti on onto activated c arbon, e ither in a dsorption columns or

in carbon added to the leaching proc ess (carbon-in-leach or car bon-in-pulp units). The adsorbed gold is then eluted from the adsorbed carbon by stripping with ammonia, nitric acid, and/or steam processes. The gold is then converted to a solid from the e luate by elec trowinning (electroplating of gold onto cathodes) or by precipitation and filtration by a dding zinc powder to the concen trated so lution. The

recove red go ld may

have sig nifican t mercur y c ontent, w

hich is separated by

retorting, which is a distillation and condensation process. The g old is then further refined in a melting furnace to sepa rate impurities into a slag. The resulting gold is re ferred to as dore, an d may contain s ignific ant silver . Dore is t hen tran sferre d to refineries to b e purifie d into pure gold or bullion. Mercury is present in many g old ores (typically ranging from 1 to 200 mg/kg ), and can be convert ed to ele mental me

rcury and emitte d to the ai

r during these tre atment pr

ocesses because of the phy

sical and chemic al prope

rties of mercury

(Refs. 2, 3). Merc

ury in the go ld ore is typically present as mercury

sulfide (cinnabar) and as an amalg am with the elemental gold. The following processes have mercury present in the feedstock and are per formed under chemical and ph ysi ca l c on di ti on s t ha t i nd ic at e t he y ar e p ot en ti al em is si on so ur ce s o f m er cu ry: Ore Roasters Ore Autoclaves Carbon Regeneration Kilns Electrowinning

Units Mercur y Re torts Dore Furnaces
2.0 Emissions Factors and Mass Balances for Gold Ore Processing Units
2.1 Ore Roasters2

Uncontro lled Roast er Emis sions Most roasters are operated with coal as the c ombustion fuel, although fuel oil or natural gas may be used as fuel, or may

be added to the ore-coal feed mixture to provide sufficie nt heat of combustion to roast the ore. Roasters are ty pically opera

ted with combustion chamber temperatures of 1,800 o F to 2,500 o F (1,000 o C to 1,400 o C), and the ore is heated to a temperature of approximately 1,300 o F (700 o C) to compl ete oxidati on of ore constitue nts that int

erfer e with gold extr action. Combustion is u sually perfor med in the

roaster by f eeding fuel and heated a ir to the combustion chamber. The ore feed, including any mixed fuel, is fed to the roasting chamber and exposed to the heat produced by fuel combustion through multiple hearth or fluidized bed processes. Roaster feed rates ra nge from less than one ton per hour for small oper ations that roast ore concentrates, to over 200 tons per hour f or large-scale opera

tions that roast lower grade ores.
Becau se the ro asters a

re oper ated to oxid

ize the ca rbon and sulfide mi

nerals that interf

ere with gold re covery

by c ya nide lea ching, roasters are de signed a nd opera ted to ef fective ly b

urn these constitue nts. These

constitue nts provid

e some of the heat of combus tion to the r

oasting process , and ore composition is considered in the design of the roasting process. The carbon and sulfide burned in the roas ter prod

uce ga seous com

bustion pr

oducts (SO 2 and CO 2 ) that are emitted from the roaster in the flue gas. Roasters ar e designed and operated to remove at least 95 percent of the ore sulfide and carbon that interfere with gold recovery

(Ref. 4). Effective roaster performance requires nea rly complete oxidation of ore sulfide minera ls and carbon. Roaster performance for a specific ore depend on the bulk mineralogical composition of the ore, the mode of occurrence of g old in the ore, feed ore grain size distribution and the phy sical properties of the calcined ore.
The mass balance assumes an ore fee dstock mercury concentration of 20 mg/kg, occur ring primarily as cinnaba

r (HgS). Some mercury

may also be pre sent as a constituent of disseminated metallic gold present in the ore. Mercury present in cinnabar is converted to metallic mercury during roasting by

the chemical reaction:

HgS + O 2 ? Hg + SO 2 Because the ore is raised to a tempe rature of at least 1,300 o F (700 o C) during the roas ting pro cess, virtually all of the merc

ury produce d by oxidation of cinnabar in the ore w ill be volatilized and entrained in the f

lue gas (Ref. 2) . Element

al mercu ry produce d during

roasting is complet

ely volatilize d as the r

oasters heat the ore abov e the boil ing point of merc ury (674 °F, 356.58 °C) (Ref. 5) . Mercur

y pr esent as an alloy ed consti tuent in fi nely dissemina

ted gra ins of meta

llic gold or silver in the ore will not be affected by the roasting process and will be retained with the calcined ore. The amount of mercury released to flue gas during roasting of the ore is therefore dependent on the proportion of mercury present in cinnabar and alloy ed with metallic gold or silver. A rchaeo logica

l studies of

gold ar tifacts a nd gold mi

nes have indicate d that unr

efined gold can con tain 30 we ight per

cent mer cury (Ref. 6) . Based

on the bulk

ore comp ositions pr

esented in the mining and metallurgy literature (gold concentrations from 1.5 mg /kg up to 20 mg/kg, and total mercury c oncentrations from 1 to 200 mg/kg), it is assumed that most of the mercury present in gold ores is present as cinnabar. T his analysis assumes that 80 perc ent of the mercury presen t in t he ore i s in ci nnaba r, and is rel eased t o flue gas dur ing roa stin g. For a typic al ore with a total mercury concentration of 20 mg/kg, conversion of 80 percent of mercury in the ore to elemental mercury vapor through oxidation of cinnabar results in the following mass bala

nce and emission r

ates: Ore Total Mercury

Content 20 mg/kg 0.04 lb/ton Mercury Emitted in Off Gas 16 mg/kg 0.032 lb/ton3

per Mass of Ore Roasted Mercury Retained in 4 mg/kg 0.008 lb/ton Calcined Ore
The mass balance and uncontrolled emission rates for the r oaster process should be adjusted for the merc ury content o f ore to d

evelop si te-spec ific emis sions estimates. Av ailable d ata indic ate that gold ore mercury

concentrations range from 1 mg /kg to 200 mg/kg (Refs. 1, 3, 7). Mercury emissions from coal-fired boilers have been studied to determine e missions rates and effectiveness of control technologie s. Coals typically contain 0.1 mg/kg mercury , and because of the large volumes of coal burned to g enerate electricity, electric generating plants are a signific ant sourc e of mer

cury emissions (

Ref. 8). Coal-fir

ed boilers (CFBs)

share some basic similarities in processes and operations with ore roasters (Ref. 9). B oth operate at similar combustion temper

atures a nd proce ss config

urations , includin

g fluidi zed bed an

d fixed hea

rth combustion chambers. Ore roasters with hearth combustion chambe rs are typic ally config

ured with multiple hearths, to ensure complete combustion of ore components. Mercury emissions have also been evaluated for other c ombustion processes, including municipal waste combustors (WMC) and hazardous waste incinerators (HWI) (Ref. 8).
Factors that differ between CFB and ore roasters include mercury content of the feed and flue gas, flue gas sulfur dioxide concentration, and f lue gas mercury

speciation. Coals used as boiler fuel h ave mer cury concen trati ons ra nging fr om 0. 03 to 0.24 m g/kg, wi th an a verage concentration of 0.1 mg/kg (Ref. 10) . Analyses of untre ated flue gas from CFB ha ve detected mercur y c oncentr

ations up t

o 25 ìg/m

3 , with an a

verag e conce

ntration o f 15 ìg/m 3 . Gold ores have mea sured me

rcury concen trations f rom less than 1 mg/k

g to 200 mg

/kg for ore depo sits in the weste rn U.S. Comp arison of

the ore f eed rat e and air flow rat es for or e roaste rs at Jerr itt Cany on, untre ated roa ster off

gas ha s an estima

ted merc ury concen tration of 1.5 to 15 mg

/m 3 , up to 1,000 times greater than the ty pical mercury

concentration in CFB flue gas ( Ref. 11). Detailed studies of treatment technologies for mer cury re moval from CFB emissions have shown that speciation of mercury has a major impact on mercury removals, with some species completely remove d and complete pass through for other forms of me rcury in c ertain control types. I mportant mercury

species in combustion gases include elemental mercur y vapor (H g 0 ), oxidized mercury vapor (Hg 2+ ) and mercury

associated with particulate matter (Hg P ). Studies of mercury speciation in CFB flue gas indicate that ther e is considerable variability in flue gas mercury speciation, ranging from g reater than 20 to 80 percent mercury as Hg 0 in flue g as. Most of the remainder of the flue ga s mercury is pre sent as Hg 2+ , with par ticulate mercur y ty pically less than 10 percent of the total flue gas merc ury (Refs. 10, 3, 12) . While the mechanisms of coal speciation in CFB flue gas are not completely understood, ava ilable information indicates that coal chloride content is positively c orrelated with the amount of Hg2+ in the flue gas, as chloride catalyzes the oxidation of mercury in the gas phase. Thermody namically, Hg

0 is thermodynamically

stable at combustion temperatures, while Hg 2+ is stable at lower temperatures (Ref. 12). Oxidation of the mercury in cooling flue gas is limited by reaction kinetics where the flue gas has a low chloride content, with g reater than 90 per cent of the vapor mercury as Hg 0 in flue gas from combustion of coals with less than 100 mg/kg chloride (Ref. 12). Speciation of mercury

in roaster off gas has not been studied; howeve r, the process for conversion of cinnabar during ore roasting is well understood. The oxidation of the sulfur present in cinnabar produces sulfur dioxide. The stoichiometry of this reaction shows that each atom of sulfur oxidized consumes two atoms of oxygen. Bec ause oxygen is a lso consumed by burning of the fuel in the r

oaster, there is n

ot suffic ient oxy

gen pr esent to o xidize the sul fur to sulfate ion. The reaction also releases ele mental mercury , which is stable at roasting temperatures. Because roa ster off gas may have mercury

concentrations up to 1,000 times greater than ty

pical CFB flue gas, oxidation reactions may not contribute as much to the4

conversion of Hg 0 to Hg 2+ in roaster off gas, especially prior to gas cooling. Similarly , adsorption onto particulate matter will not be as significant, as particulate ma tter concentration in roaster off gas should be similar to CFB flue gas, since the source of gas particulate c ontent is fly ash from fuel combustion in both gas streams. The process evaluation for non-ferrous metals smelting estimated that gas emission Mercury speciation is 85 percent Hg 0 , 10 percent Hg 2+ and 5 percent Hg P (Ref. 8). This speciation estimate for roaster off g as mercury is used to evaluate removal efficiencies and mass balances for treatment options.
Roaster Emissions Data Evaluation
Review of the Nevada DEP Bureau of Air Quality permit files re vealed that a number of gold ore roasters have pe

rmit limits f

or merc ury emissions; however , emissions test data were a vailable for only one fa

cility, the Jerritt Cany

on mine, which has two roasters with a combined operating capacity of 250 tons per hour (Ref. 11). I n addition, mercury a

nalyses wer

e performed on the outlet gas after all treatment processe s, data on inlet and outlet mercury concentrations for each process were no t collect

ed. Tota l mercur y a naly ses were

perfor med, the s

peciation of merc

ury in the treated off gas was not eva luated. The tests were performed in April and May of 2000. The test parameters and emissions data are listed in Table 1. These data show the potential for variability in roaster emissions that probably r eflect the influence of ore content on the mercury emission rates. Retesting of the West Roaster in May 2000 yielded similar results as the pre vious test, indicating that short-term variations in roaster operating conditions were not the source of the higher emissions at this unit. The feed rates and controlled emissions data at Jerritt Cany on were compared using the g eneral mass balance and emissions factor relationships previously described. Based on the regional average ore merc

ury conce ntration of 20 mg/kg, and a combined roaster feed rate of 215 tons/hr, the
Table 1. Operating Parameters a nd Mercury

Emissions Rates for Jerritt Canyon Mine Ore Roasters
Unit Feed Ra te Mercur y Em ission Rate East Roaster 107 tons/hr 0.31 lb/hr West Roaster 104 tons/hr 1.01 lb/hr
assumed loading of mercury in the roaster feed was 8.61 pounds per hour merc ury. With 80 percen t of the or e mercu ry partitioned to the r

oaster o ff gas , the tota

l untrea ted emissi ons rate

is estimated at 6.89 pounds per hour of mercury . The combined stack emission rate of 1.31lbs/hr mercury indicates an overall removal rate of 81% by all of the emissions controls in operation at Jerritt Canyon. I

n order to attribute mercury removals to the individual treatment processes, the mercury speciation and removal efficiency for each process must be evaluated
Process c ontrols us ed at Jerr itt Cany on include

a wet ve nturi scr ubber fo r gas c ooling a

nd some particulate removal, a sodium hy droxide scrubber for sulfur dioxide removal, a sodium oxychloride p acked b ed scrub ber for mercur y r emoval, a nd a wet e lectrost

atic pre cipitator

(ESP) for final particulates removal. Estimates of mercury speciation in the roaster off gas and r emoval efficiencies for the individual treatment processe s are based on process knowledge for ore roasting and information on mercury removal technology

performance derived from studies of coal-f ired utili ty b oilers.
Control Efficiencies Roaster Off-Gas Quench5

Quenching of roaster off g asses by wate r spraying may be per

formed to reduce the flue ga s temperature from approximately 1,000 o F to 400 o F to allow treatme nt for control of p articula tes, sulfur dioxide, and mercury. Roaster off gas quenching is usually performed by

spray towers, or by wet venturi scr

ubbers that also provide some particulate removal. Be cause quenching is the first treat ment s tep fo r roast er off ga s, in fluen t gas me rcury conc entrat ions will typicall y range from 1.5 t o 15 mg/m

3 with 85 percent present as Hg 0 (Ref. 8, 11). Cooling of the roaster off g as will result in some oxidation of Hg 0 , but the amount of vapor mercury oxidized is expected to be considerably le ss than that observed in CFB flue gas, due to the hig h mercury

concentrations relative to the amount of chlorine in the roaster off g as available to cataly ze the oxidation reaction. Some mercury

may also be adsor

bed onto particulates during cooling, as adsor ption equilibri um conce ntrations on the soli d phase in

crease as tempe rature decrea ses. Adsorption to solids is also limited by the high mercury concentration in roaster off ga s relative to particulates contributed to the gas stream as fly ash. Based on these considerations, roaster of f gas quenching is expected to have maximum mercury re moval efficiency

of 10 perce nt , with ty

pical re movals of 5 per c ent or le ss. The ma ss balanc e relati onship for off ga s quench

ing is: Untreated Hg Emission Rate(lb/hr) = (0.8 x Ore Hg Conc. (lb/ton)) x Ore Feed Rate (ton/hr) Quench process removal (lb/hr) = 0.1 x Untreated Hg Emission Rate(lb/hr) Effluent roaster off gas Emission Rate = 0.1 x Untreated Hg Emission Rate(lb/hr) Based on these emission factors, the Jerritt Cany on roasters processing gold ore containing 20 mg/kg mercury

at 215 tons/hr will intake 8.6 pounds of mercury per hour and emit mercury in the untreated roaster off ga s at a rate of 6.9 pounds per hour. Wet off ga s quenching at Jerritt Canyon is estimated to remove mercury at a rate of 1.7 pounds per hour. Bec ause mercury removal during roaster off ga s quench occurs through oxidation and dissolution in the quench solution and entrainment of particulate mercury in the liquid phase, dry g as cooling processes such as dry spray cooling or boiler heat re covery

will not re sult in any

mercur y r emoval.
Sulfur Dioxide Scrubbers
Roaster o ff gas

is often tr

eated to remove s ulfur dio xide (SO 2 ) from generated by combustion of sulfur in the fuel and oxidation of sulfide minerals in the ore. The gas strea m is fed to spray towers or packed bed towe rs where the gas flows cou

nter to a sodium hy

droxide (N aOH) or

milk of lime (Ca

OH) solut ion, which react s with the SO 2 in the gas stream. Studies of CFB flue g as treatme nt have shown that SO 2 scrubbers can remove up to 90 perce nt of the Hg +2 present in the gas stream as the mercury dissolves into the liquid phase, but does not provide any remova l of Hg 0 from the f

lue gas . Beca

use sign ificant c hloride c ontent ma

y be present in the roa ster off

gas, some conversion of Hg 0 to Hg +2 may occur in the cooled gas stream, and the Hg +2 component of the roas ter off gas could b e effe ctively removed by t he SO 2 scrubber solution. Both mercury and chloride content of the gas stream may be variable, depending on the ore c omposition and coal quality. Base d on typical ore a nd coal chemistry va

lues, an average merc

ury remova

l of 25 percent may

be provid

ed by SO 2 scrubbers. Under conditions where coal chlor ide content is low and ore M ercury

content i s high, e ffectiv e mercu ry removal by SO 2 scrubbers may

approach 0 per cent. Based on ty

pical conditions, the expected mass balance relationship for mercury removal by SO 2 scrubbin

g is: Inlet Hg Rate(lb/hr)

= (0.8 x Ore Hg Concentration (lb/ton))x Ore Feed Rate (ton/hr) x 0.9
SO 2 Scrubber Process Removal Rate (lb/hr) = 0.25 x Inle t Hg Emission Rate(lb/hr)
SO 2 Scrubber Process Outlet Emission Rate = 0.75 x Inlet Hg Emission Rate(lb/hr)
Based on these e

mission fa

ctors, th e SO 2 scrubber process at Jerritt Cany on processing gold ore6

containi ng 20 mg /kg mer cury at 215 tons

/hr will int

ake 6.9 p ounds of me

rcury per hour and emit mercur y in the trea ted roas ter off gas at a r

ate of 5. 2 pounds pe

r hour. SO 2 scrubbin

g at Jerr itt Canyon is estimated to remove mer cury at a rate of 1.7 pounds per hour.
Particulate Removal De vices
Availabl e contro l technol ogies in use for p articula

te contr ol at gol d ore roasters ar

e elect rostatic precipitators (ESPs) and baghouses. Bag houses typically

provide highly e ffective control for particulates, but require more maintenance a nd must be operated under very controlled condition s (gas f

low rate , pressur

e drop) t o achiev e optimum pe

rforman ce. ESPs pr

ovide some advantages in performance over wider operating conditions and may require less maintenance than baghouses. Wet and dry ESPs both function by gener ating an electric field that imparts an electro static charge t

o partic ulates in t he gas stream ( provided the part iculates have some electrical conductivity

), and then collecting the charg ed particles on a charged c ollector plate. Dry ESPs rely

on mechanical processes to remove the collec ted particulate matter from the charged collector plate, while c ollected particulates are removed fr om wet ESPs by a film of water flowing over the collector pla te. Studies of removal efficiencies for these tec hnologies applied to CFB flue gas show that removal e fficiency is strongly cor

related with the fraction of Hg P in the gas stream, with nearly complete removal of Hg P by both ty

pes of devices (Ref. 13). For unscrubbed gas streams, the pa rticulate control devices do not provide any effective removal of Hg O or Hg +2 components in the gas stream. Based on the performance tre nds for these devices in CFB flue g as treatment and the expected composition of roaster off gas, the particulate c ontrol devices are expected to provide a removal efficie ncy of up to 9 pe

rcent f rom roas ter off gas. Bas ed on ty pical mer cury concen trations i n roaster off gas and an a

verage removal eff

iciency of 5 perce nt , the mass balance relationship for Wet ESPs and other particulate control devices can be summarized by the following relationships.
Based on 90 perc ent pass t hrough from off gas que nching and 25 pe

rcent pa ss throug

h from SO 2 scrubbing, multiplying

these pass through rates gives the inlet loading factor: 0.9 x 0.75 =0.675. Inlet Hg Rate(lb/hr)

= (0.8 x Ore Hg Concentration (lb/ton))x Ore Feed Rate (ton/hr) x 0.675 Particulate Process Removal Rate (lb/hr) = 0.05 x Inlet Hg Emission Rate(lb/hr)
SO 2 Scrubber Process Outlet Emission Rate = 0.95 x Inlet Hg Emission Rate(lb/hr)

Because the effectiveness of the mercury removal by par

ticulate control devices is dependent on the fraction of HgP , particulate removal by ESP prior to gas cooling (known as hot side ESP) may not provide significant mercury removal. This configuration is used at some CFBs, but is not typically used for particulate control in roaster emissions. Based on the se emission factors, the particulate process at Jerritt Cany on processing gold ore containing 20 mg/kg mercury

at 215 tons/hr will intake 5.2 pounds of mercury per hour and emit mercury at a rate of 4.94 pounds per hour. Par ticulate removal for the Jer

ritt Cany on loading

rate is e stimated t

o remove mercur y a t a rate of 0.26 pounds per hour. Carbon Adsorption Fixed bed carbon adsorption units have been used to control mercur y emissions at municipal waste combustors a

nd hazard ous waste inciner ators in the U.S. and

Europe, and their performance is well understood (Ref. 8). Eff ective performance of the units require that the7

influent gas stream is sufficiently cooled so that mercury

can be effectively

adsorbed on the carbon, and to prevent oxidation losses of the carbon matrix (Ref. 13). Ty pically, g ases are cooled below 300 o C for effective mercury removal by ca

rbon adsorption. Factors affecting carbon bed adsorption performance inc lude residence time of the gas phase in the c arbon bed, bed temperature, and carbon par ticle size. Longer re sidence times, smaller carbon particle sizes and lower temperature rang es can result in mercury removal efficiencies of nearly 100 percent. As mercury

is adsorbed in the fixed bed, the carbon will eventually approach is adsorptive capacity and will allow some mercury to pass through the filter. The adsorptive capac ity of the carbon can be increased by impregnating the carbon with eleme ntal sulfur. Mercury

adsorbed by the carbon then reacts with the sulfur to produce Hg S, which has a much lower vapor pressure than elemental mercury

and will not be desorbed from the filter under normal opera ting conditions. Carbon filters can be regener ated by che

mical stripping or retorting of the adsorbed mercur y, a nd the mer

cury can be r ecover ed from the rege

neration proces s.
Me rc ur y ad so rp ti on fr om ga s p ha se s h as be en re po rt ed at 50 to gr ea te r t ha n 9 5 p er ce nt re co ve ry. Lower recove

ry eff iciencies by c arbon adsorption are attributed to the presence of Hg +2 in the vapor ph ase, ele

vated te mperatu re of the inlet ga s, or insuf

ficient r esidenc e time. Pro

perly designed and operated car bon adsorption filters can achieve mercury removals of 90 perce nt , especially f

or gas streams where Hg +2 has already

been removed through scrubbing processes. Mercury removal by ca

rbon adsorption under appropriate operating conditions provided below.
Based on 90 perc ent pass t hrough from off gas que nching , 25 perc ent pass t

hrough from SO 2 scrub bing, and 95 per cen t pass throu gh by parti culat e remov al, mu ltip lying the se pas s thro ugh rates gives the inlet loading factor: 0.9 x 0.75 x 0.95=0.641. Inlet Hg Rate(lb/hr)

= (0.8 x Ore Hg Concentration (lb/ton))x Ore Feed Rate (ton/hr) x 0.641 Carbon Adsorption Process Removal Rate (lb/hr) = 0.9 x Inlet Hg Emission Rate(lb/hr) Carbon Adsorption Process Outlet Emission Rate = 0.1 x Inlet Hg Emission Rate(lb/hr)
Based on these e

mission fa

ctors, th e carbon adsorpt

ion proce ss at Jerri

tt Cany on feed r ates, processing gold ore containing 20 mg/kg mercury

at 215 tons/hr, will intake 4.94 pounds of mercury per hour and emit mercury at a rate of 0.446 pounds per hour. Particulate remova l for the Jerritt Canyon loading ra te is estimated to remove mercury at a rate of 4.446 pounds per hour. Large volume ore roasters have gas flow r ates that may excee d 10,000 ft 3 /m. Therefore, carbon filters used for this gas stream will have to be desig ned to allow sufficient flow and residence time for mercury

removal, and may

require management of sig nificant volumes of carbon materia l. Other Mercury Removal Tec hnologies Several other technologies are a vailable for mercury

removal from roaster off ga s, including mercury scrubber technologies and selenium adsorption filters. Mer cury scr ubbing is achieved by sodium oxychloride packed bed scr ubbers, and sulfuric acid scrubbers, including the Bolkem and Outokumpu processes. Mercury can also be removed from gasse s by the Boliden_Nor zink process, which reacts mercurous chloride with mercury to for

m mercuric chloride, which is then removed from the stripper solution. A sodium oxychloride packed bed sc rubber is used for mercury removal at Jerritt Canyon. Roaster off gas testing data indicate s that with 25 percent removal of mercury by sulfur dioxide scrubbing, the estimated performance of the sodium oxychloride packed be d scrubber is 56 percen t, avera ged ac ross the two units. While this is a sig

nifican t amount of

mercur y r emoval, the large ore feed a

nd gas flow rates for these units result in significa nt mercury e

missions (1.318

lb/hr, total) even with the mercury scrubber in operation. These data indicate that mer cury removal process es will ha

ve to per form at hi gh removal eff

iciencie s to achie

ve acce ptable mercur y e missions for larg e volume r oasters that are used to pr

ocess me rcury rich go ld ores. Other available control technologies re ly on scrubbing of mercury

from roaster off gas by reaction of the Hg O in the gas phase with a scrubber solution that oxidizes the Hg to Hg +2 , which is dissolved into the scrubber solution. These technologies all require remova l of mercury

solids or regeneration of scrubbe r solutions that includes generation of solid or liquid mercury bearing residual streams. The most commonly use d mercury scrubbing process is the Boliden-Norzink process , which r

eacts me rcurous chloride solution wi

th vapor mercury

to produc e mercu

ric chloride, by the f

ollowing reaction: HgCl 2 + Hg ? 2Hg Cl The scrubber solution is regenerate d by reac tion with chlorine gas, with period removal of mercur y by precipi tation and settling

or filtra tion. The H gCl 2 solution is fed to a packed bed tower an d react s the roa ster off

gas, fl owing c ounter to

the solution flow in the tower. This treatme nt proce ss is ty

pically used whe re SO 2 will be recovered to genera te sulfuric acid, and mercury content in the acid will restrict use of the acid as a commerc ial product. Reported mercur y r emoval e fficien

cies for this proc ess are

great er than 9 0 per ce

nt. The Bolkem and Outokumpu processes both remove merc ury from the g as stream by r eaction with a sulfuric acid scrubber solution. I n the Bolkem process, roaster off g as is scrubbed with a concentrated sulfuric acid solution in a series of two dry ing towers. The reaction produces Hg 2 S0 4 , which is removed from the scrubber solution by reaction with sodium thiosulfate, which produce s a prec ipitate of

HgS that is remove d in a thickener or filter. I n the Outo kumpu proc ess, roaster off gas is scrubbed in conc entrated sulfuric acid at 150 o C to 180 o C, produc ing Hg SO 4 , which precipitates and is filtered from the scrubber solution. Mercury conta ining solids from these processes can be recove red by re torting. Selenium filters are used to control mercury emissions at two non-ferrous metals smelters currently ope

rating in Sweden (Refs. 14, 15). 90 per cent r emoval efficiencies have been reported for this process. The gas stre am is passed through a fixed bed filter of amorphous selenium, which re acts to form HgSe , which r emains in t

he filter . The filt

er mater ial reac hes its adsorptive capacity

as the selenium is reacted with vapor Hg, and c an be regenerate d by retorting or chemical processing. The g as stream must also be sufficiently cooled, (less than 400 o C) to prevent thermal decomposition of HgSe in the filter.
Mercur y c an also be

removed by s crubbing

with a sel enium solu

tion in 20 to

40 perce nt sulfuri c acid solution, and this technology has reported removal efficiencie s of 90 to 95 per cent (Ref. 14).
Mercur y sc rubber t echnolog ies requ ire that t he units a re oper ated unde r proper condition s, including maintenance of scrubber solution chemistry , maintenance of scrubber towers and manage ment of me

rcury containi ng trea tment res iduals an

d hazardo us treatment chem

icals (sulfuric acid, chlorine gas, selenium, Hg Se). Generally , these technologies have been used where r oasting of sulfide

ores pro duces SO 2 bearing off gase s that can be treated to produce sulfuric acid as an economic product. While gold ores may contain sulfide minerals, the ores typically do not contain as much sulfide as ore concentrates of othe r metals, which have been treated to remove non-sulfide metals. These technologies can be used to treat mercuric roa ster off ga ses produ

ced by roasting gold ore s, but may

have to be

adapted to gas st reams tha t typ ically contain l ower SO 2 concen trations.
2.2 Autoclaves9

Sulfide minerals that interfere with gold recover y through c

yanide lea

ching process can also be treated in autoclaves. Autoclave treatment of or es is a wet process, water is added to crushed or e to form a slurry or

pulp that is acidified and then oxidized under elevated pressure and tempe ratur e cond itio ns. Au tocla ving i s not used f or tre atmen t of or es con taini ng high concen trations o f carbonate mine rals, as the car bonates r equire h igh leve ls of sulfu

ric acid addition to lower the pH to the desired range for the pulp. Autoclaves are also not used for treatme nt of ore s that hav e poor g old recovery due to the

carbon c ontent of

the ore. Sulfuric

acid is added to the pulp to lower the pH of the feed material to 1 or lower , as this promotes the oxidation of sulfide minerals. The acidified pulp is pumped into the pressurized reaction chamber, where the material is heate d to 180 o C to 220 o C, and is pressurized to 345 pounds per square inch in an oxyg en atmosphere (Ref. 16). The pulp is agitated in the r eaction chamber, and the high tempera ture and oxyg en press

ure cau se rapid

oxidation of

sulfide mi

nerals. The stron

gly oxidizing conditions promote complete oxidation of sulfur in the sulfide minerals to sulfate. For cinnaba r, which contains most of the

mercur y in the ore, the rea

ction is: HgS + 2O 2 ? HgSO 4 Because of the controlled conditions that promote oxidation, greate r than 90 per cent of the mercur y pr esent in the pulp is c onverte d to oxidized me rcury . The sulf

ide conte nt of the ore is compl etely ox idiz ed to a llow gold re covery fro m the t reated ore of 9 0 perce nt or m ore th rough cyanide le aching (Refs. 1, 16). At the discharg e from the pressurized reaction chamber in the autocla ve, the pu lp is trans ferre d to a fla sh chambe r, wher e pressu re is rel ieved an

d the pulp liquid is converted to steam. Steam may be r ecovered and returned to the pre ssurized reaction compartment to reduce heat loss from the process. Of f gas from the flash chamber is trea ted in a venturi to remove acid mist and cool the gas stream, and a dditional mist and particulate removal may be per

formed in a cy

clone or similar unit. Autoclave emission testing has been performed a t Twin Creeks, where two autoclave units are operated in series to oxidize sulfide ore minerals. The testing was perfor med at a feed rate of approximately 204 tons per hour, and the or e has a mercury

content 198 mg/kg (Ref. 17). The mercury feed rate to the autoclave proce ss for the test is estimated at 80.78 pounds of mercury per hour. The measured emission rate downstrea m of the venturi and cy

clone during the test was 0.0153 pounds of mercury per hour. This indicates that the controlled mercury emission rate was 0.02 per cent of the process. The venturi and c yclone control sy

stem is a wet cooling and cleaning process, the effe ctive removal rate of mercury is 90 percent for the Hg2+ component of the gas stream, and zero per cent r emoval for the Hg o in the aut oclave o ff gas . The com

pletene ss of the oxidation of sulfide minerals for ore treatment is ty pically 95 perc ent, and a similar degree of oxidation of mercury will occur in the autoclave. The removal efficienc ies and speciation of the untreated off gas provide the f ollowing relationship between the untreated and tre ated gas streams. The removal eff icience s for ea

ch speci es of mer

cury in the con

trol devic e show tha

t: Total Treated Mercury

Discharge Rate = 1 x Untreated Hg o + 0.10 x Untreated Hg 2+ While the oxidation performance of the autoclave process results in 95 perc ent oxidation of me rc ur y: Total Treated Mercury

Discharge Rate = 1 x Untreated Hg o + (0.10 x 19 x Untreated Hg 0 ) Total Treated Mercury

Discharge Rate = 3 x Untreated Hg o Combining these relationships, the combined generation, remova l and treated emission rates are as follow s: Total Untreated Mercury Emission Rate: 0.122 pounds of mercury pe r hour10 Total Mercury

Removal Rate:0.107 pounds of mercury per hour Total Mercury

Emission Rate = 0.1 x Hg 2+
The ore feed concentra

tion and e mission ra

tes measu red at th e Twin Cre eks autoc lave tes t indicate that the me

rcury emission r

ates and removal s are as

follows:
Uncontr olled emis sion rate

= 0.13 pe

r cent of autocla ve merc ury feed ra te (ore feed rate times mercury fraction in ore) Removal Efficiency

for Venturi/Cyclone

system: 85 per cent (90 percent removal of 95 per cent of mercury as Hg 2+ )
Controlle d Emission Ra

te = 0.02 p er cent

of autoc lave mer

cury feed ra te The process information and test data show that large scale autoclaves can be used to trea t mercur y r ich ores with merc

ury emission r

ates wel l below one per ce nt of the me

rcury feed ra te in the aut oclave. Low me

rcury emission r

ates in a utoclave s are at

tributed t o the proc ess conditions, including high oxyg en pressure that promotes wet reaction oxidation of ore sulfide minerals, and operating temperatures tha t thermodynamically

favor formation of Hg2+ . The oxidized mercury is less volatile and is more effe ctive controlled in wet gas stream treatment processes. 2.3 Carbo n Regener ation Kiln s Carbon used to collect dissolved gold from cy anide solutions is stripped by one of sever al wet chemical processes to genera te a concentrated gold solution. The stripped car bon is then regenerated to restore

its ability to adsorp gold befor e being returned to gold rec overy proc esses. The most c ommon reg enerat ion techn ique is the

rmal tre atment in r

otary kilns. I n order t o prevent oxidation of the carbon in the kiln, steam is introduced to maintain a reducing environment. The elevated temperatures and r educing environment in the kiln chamber promotes volatilization of adsorbed mercury into the kiln off gas as Hg o . The feed rates for carbon regeneration kilns varies from less than 0.25 to 1.5 tons per hour . Higher volumes of carbon adsorbe nt must be r

egene rated a t faciliti es that us e carbon in direct contac t with the le ach sy stem, such as c arbon in l each an d carbon in pulp sy

stems. The

range of merc ury emission r

ates is dependent on the mercury content of the carbon feed, since the kiln opera tion will ensure greater than 90 per cent volatilization of the mercury present in the kiln feed (Ref. 1). Design studies for gold r

ecover y pr ocesses have shown that the

mass load

ing of me rcury to carbon a dsorption media is de

pendent on the ore

mercur y c oncentr

ation in the

leachin g proc ess, the cy anide concentra

tion of the

leach so lution, an

d the use o f additive s in the lea ching process t

o suppress mercury

dissolution into the leach solution. Roasted ores and ores with low mercury content prior to any tre atment will result in lower mercury concentrations in the leach solution, resulting in lower mercury loading to carbon. Le aching studies have shown that the proportion of mercury

leached from the ore is also related to the cy anide concentration of the leach solution (Ref. 1) . Cy anide sol

utions with

150 mg/l s odium cy

anide we re found to dissolve

very little mercury , while sodium cyanide solution at a conc entration of 2,000 mg/l was found to leach 80 percent of the mercury

from the ore. Regional studies of Nevada or es have found that at ty

pical ore concentrations of 20 mg/kg mer cury, 20 per

cent of the ore mercury

is leached into the cyanide solution, resulting in gold to mercury

concentration ratios of nearly 1:1 in the leach solution (Ref. 1). Addition of sodium sulfide (Na 2 S) or sodium bisulfide (NaHS) has been shown to suppress mercury dissolution into the cyanide leach solution, and this method has been successfully used to minimize mercury control requirements in gold processing oper ations at Jerritt Canyon and at mines in Australia (Ref 1) . Once the carbon has been loaded w ith gold and11

mercur y f rom the a dsorption process , the go ld is recovered by circula ting a c oncentr

ated ca ustic cyanide solution throug

h the carbon to desorb the gold into the strip solution (Ref. 1). Targ et gold concentrations are reporte d to be on the order of 150 ounces per ton gold on loade d carbon, and less than 5 ounces per ton gold on carbon af ter stripping has been completed (Ref. 1). The amount of mercury

retained on the carbon after g old recovery is dependent on the type of mercur y cya nide comp lex prese

nt in the cya

nide solut ion and ca

rbon strip process used to remove t he adsor bed gol d (Ref. 1 ). Gold str ipping f rom car bon under

atmosphe

ric ge nerally removes less mercury

from the carbon that stripping operations perfor med at elevated pressures and temperatures. Based on results of stripping studies, mercury c oncentrations on stripped carbon range from 2,000 mg /kg to 200 mg/kg at facilities where me rcury- bearing ores are processed. Lower

mercury concentrations in stripped carbon are produce d where additives are used to suppress mercury dissolution into leach cyanide solutions (throug h addition of mercury suppressants such as Na 2 S), or whe

re the str ipped ca rbon has b een tre ated with a nitric a cid solution prior to kiln regeneration. Emissions testing of kiln off gas at the Smoky Valley mine detected mercury

emissions at 0.01 lb/hr, reflecting low mercury loading in the stripped carbon at this facility (Ref. 18). Using a midpoint of potential mercury c oncentrations in stripped carbon, a value of 1,100 mg/kg mercury is used for this mass balance evaluation.
At this con

centra tion the e mission fa

ctor for regeneration

kilns is der

ived as f ollows: Mercury Emission Rate (lbs/hr) = Kiln Feed Rate (tons/hr) x 2,000 (lbs/ton) x 0.0011 For a kiln with a feed rate of 0.5 tons/hr, the resulting uncontrolled mercury

emission rate is 1.1 lb/hr mercury. T

his shows that regeneration kilns may be significant sources of mercury emissions. Because the rege

neration kilns operate in a steam environment, the mercury will desorb from the carbon in the kiln chamber as Hg o . Particulate control devices such as baghouse s and venturi scrubbers do not provide any effective control of vapor phase Hg o (Refs. 8, 12). Some Hg o present in the off gas fr om the kilns

may be adsor bed onto p articula

te carb on entra

ined in the

gas, and this component of the mercury present in the off gas will be removed by the control devices. Only a minor fra

ction of the carbon will be entrained into the off ga s, and the Hg o : particu late ratio will not be near the ratio of 10,000 needed for e ffective adsorption of mercury in the off gas (Ref 3). At most 10 per cent of the mercury will be adsorbed by e

ntrained carbon in the off gas and removed by

particu late con trol devic es. At 10 per cent removal efficiency for particulate control devices for the kiln off g as, the controlle d emission r

ate for the expected merc

ury loading to the kiln i s: Controlled Emission Rate (lbs/hr) = Uncontrolled Emission Rate (lbs/hr) x 0.9 This indicates kiln off gases treated for pa rticulate removal under expected mercury loading rates is 0.99 pounds per hour. Carbon adsorption filters can be used to effectively treat kiln emissions for removal of mercury

. Once the off ga s has bee

n suffic iently

cooled, t reatmen t of the of f gas c an be pe

rformed to result in an effective removal efficienc y of 90 per c ent or greater. The emission fac tor for kiln off gas treated by t he carb on adsorption proc ess is:

Controlled Emission Rate (lbs/hr) = Uncontrolled Emission Rate (lbs/hr) x 0.1 For a carbon with 1,100 mg/kg a dsorbed mercury

and a feed rate of 0.5 tons per hour, the pr ec it ed em is si on ra te fo r t he ca rb on ad so rp ti on tr ea te d s tr ea m i s 0 .1 lb /h r m er cu ry.12

2.4 E lectro win ning Cell R ooms The gold bearing solution produced by the carbon stripping process is piped to gold rec overy process es. The two major pr ocesses used for gold re covery

are ele ctrowinn

ing and the Merr ill Crowe process. In e lectrowinning, the concentrated g old solution produced by car bon stripping is pumped to an electrolytic c ell where gold is plated onto the cathode as the solution flows through the electrowinning cell. I n the Merrill Crowe process, the solution is placed in a pressure reaction vessel, and zinc powder is added to the solution, forming zinc cyanide solution and precipitating gold, silver and merc ury from solution. The pre cipitated metals are then recove red in a f ilter pre ss. Metals em

issions ha ve from a variety of elec troplatin

g proc esses ha

ve been evaluat ed, with misting of

electro plating solution by

off ga ssing of

the gas produce d at the a

nodes. The mist entrains dissolved metal to produce emissions (Ref. 19). Mercury is volatile and will be emitted from the electrowinning process eve n when mist from the process is controlled. Mercury is more electro positive than gold , and sele ctively plates on to the ca thodes in t

he electrowinnin

g cell before gold and silver plate out onto the cathode. B ecause elemental mercury is a liquid, it may weep from the cathode and collect in a pool in the bottom of the elec trowinning cell vessel. Gas forming on the ca thode (pr imarily ammonia f

ormed by reducti on of cy

anide an ion) act s as a stripping gas promoting mercury volatilization. The aqueous solution does not provide a barrier to mercury vola

tilization (MSHA guidance). Based on the vapor pressur e of mercury

and volatilization rates from metallic mercury , the emission rate for mercury from electrowinning cells is: Emission Rate (lb/hr) = Cathode Area (ft 2 ) x Mercury F raction in plated metal x 0.02 (lbs/ft 2 /hr) (Refs. 1, 20). For a typic al electrowinning unit configuration, the ca thode area is 115 ft 2 . For an electro plate composition that is 40 percent mercury , 50 percent gold and 10 percent silver, the mercury fraction would be 0.4. This would result in an emission rate of 0.92 pounds per hour. The mercur y e mission ra

te will be directly proporti on to the me

rcury content o f the ele ctroly

te solution, and mercury pr

ecipitation prior to electrowinning through Na 2 S may reduce

the mercury concentration by

more than 95 percent (Ref. 1). Mist control through scr ubbing will not provide any merc ury remova

l, as the mercury

has been reduced to Hgo through the electrowinning process. Carbon adsorption of the vented gas stre am from the electrowinning cells can provide removal of 90 percent or more of the vapor phase mercury , resulting in a controlled emission factor of: Emission Rate (lb/hr) = Cathode Area (ft 2 ) x Cathode Mercury Fraction x 0.02 (lbs/ft 2 /hr) x 0.1 With the example configuration described above, the mercur y emission rate af

ter carbon adsorption treatment is expected to be 0.092 pounds per hour. After the electrowinning proce ss has been completed, the cathodes are re moved from the cells. The cathodes are ty

pically steel wool, and the pla ted steel wool is removed from the drained electrowinning cells and is placed in char ge pots for retorting. The cha rge pots should be covered when the cathode materia l is not being placed into them, and ventilation should be provided in the work area to prevent worker exposure. Ventilation exhaust will contain Hg o due to volatilzation of mercury from the c athodes. The emission rate will depend on the surface a rea of the expo sed merc

ury and the a mount of time the mate

rial is expo sed in the

electro winning cell room. Ventilation exhaust should be treated in a carbon adsorption filter prior to discharg e. 2.5 Retorts13

Retorts are used to separate mercury from the gold recovered by electrowinning or zinc precipitation. Depending on the composition of the gold solution, the recove red metal may contain from trace amounts to more than 50 wieght per cent mercury . Based on reported production rates, most facilities in Nevada that produce commerc ial mercury indicate that typical mercury production rates are 15 per cent of the gold rates (Ref. 22). In the r etorting process, the recovered gold, mercur

y and silver mixture is placed into the char ge pot and loaded into the retort. Retorting is usually done for 12 to 24 hours at 600 o C to 700 o C, to remove around 99 per cent of the merc ury . The cha

rge is heated un

der a pa rtial vac uum, and the mercu

ry vaporize s and is recovered in a water-c ooled condenser. The liquid mercury that is bled from the condenser tube is piped in

to a colle ction ves sel. The r emaining gold an d silver a t the end o f the retorting

process typ ically contains about 1,0 00 to 8,000

mg/kg mercury

(Ref. 1) . The con

denser a llows some mercury to discharge in the off gas, a nd loss of 0.4 to 0.7 per cent of the mercury from the condenser has been reported (. 23) . Based on this relationship, the emission factor for mercury from the gas discha

rge af ter cond enser r ecover y is : Mercury Discharge (lbs/hr) = Charg e Weight (lbs) x Mercury Fraction x 0.0055 For a retort charge

of 1,000 pounds with a mercury composition of 15 per cent, the total emission of merc ury from the c ondense

r during a retor t cy cle would be 0.825 p ounds of me

rcury . Some facilities operate a carbon adsorption filter be tween the condenser and the vacuum pump. The cooled gas downstream of the condense r is expected to allow 95 per cent removal efficienc y for a properly maintaine d carbon adsorption filter, resulting in a treated me rcury discha

rge rate of:

Mercury Discharge (lbs/hr) = Charg e Weight (lbs) x Mercury Fraction x 0.0055 x 0.05 For the example mass and composition retort charge c onsidered above, this would result in a batch emission amount of 0.04 pounds of mercury

. 2.6 Dore Furnaces
After the ore has been removed f rom the retort, it is melted in a refinery furnace to produce a commerc ial mixture o f gold a nd silver known as d

ore. The gold is he ated to a

pproximate ly 1,500 o C with a flu

x material

that pre ferent

ially absorbs i mpurities , such as b ase and metallic metals. Most of the remaining mercury is volatilized in the dore furnace. The dore melt is poured into bars, and any f

lux slag that hardens on the bars is removed with a mechanical c hipper. The cleaned bars are then further processed to produce gold bullion (99.5 perce nt pure gold). Studies of retort efficiencies have shown that the r etorted gold contains 1,000to 8,000 mg/kg mercur y. The largest refinery

furnaces have a throughput of 5 tons pe r hour, with most units processing 0.25 to 0.4

tons per hour. The

mercur y e mission ra

te for dore furnaces is d

erived a s follows: Mercury Discharge (lbs/hr) = Charg e Rate (pounds/hr) x 0.0045 Mercury content of Feed
For a furnace fee d rate of 600 pounds per hour, the mercury emission rate is 2.7 pounds per hour. Because the furnace

melt contains gold, silver and mercury in their metallic forms, the mercury emitted will be present as metallic mercury . Because current controls on the fur naces are particulate controls such as baghouses and we t scrubbers, little control of the mercury emissions is expected. Most furnaces heat the dore by electrical induction. Because there is no combustion process associated with this heating method, relatively low volumes of fume gas are collected by the exhaust system for the furna ces. These gas streams could be c ooled and treated by carbon ads orp ti on f

il ter s, w it h an ex pec ted mer cur y rem ova l ef fic ien cy of

90 p er c ent or g rea ter . It should also be noted that the melting and refining of g old and silver mixtures that have not been retorte d may

result in h

igher e mission ra

tes than presente d in this analy ses. Site- specific14

information on the compostion of furnace feed materia ls would be needed to estimate emission rates for these conditions. Furnace emissions with carbon f ilter controls have emission rates es ti ma te d b y: Hg Emission Rate (lbs/hr) = Charge Rate (pounds/hr) x 0.0045 Hg Content of Feed x 0.1

3.0 Summ ary and Conclusions Emissions factors and mass balance evaluations of gold ore processes and control technologies have been estimated based on source testing data, process engineering information on mercury concentrations and behavior in the processes and c ontrol technologies, and estimates derived from mercury

emissions and controls from other industrial processes with similar emission type s. The mercury

feed rate for the processes ma y differ sig

nificantly among

facilities and at individual sites over the life of a mine because of the var iable mercury

content of gold ores and the differe nt methods us ed for or

e proce ssing an

d gold re covery . Gold and mercur y ha ve some si

milaritie s in their geochemical behavior, and both a re associated with mineral districts where gold de position has occurred through hy

drothermal and volcanic processes, including de posits in Nevada, the leading gold producing state, Califor nia, and other mineral districts. Lower mercury concen trations a re possib le in gol d ores as sociate d with cop per porphyr

y mineralizat

ion and g old deposits that have been subjected to geologic metamorphism (elevated temperature and pre ssure conditions) after gold deposition. Deposits of these ty pes are found in Washington, Arizona, South Dako ta, Monta

na, I daho and

South Caro lina. Dev elopment of repr esentat ive, site- specific emissions factors and mass balance evaluations will require asse ssment of ore mercury concentration, ore mineral composition, ore treatment methods, and g old extraction and recovery processes. The emissions factors and control efficiencies for the processes with significant potential for mercury emissions are listed in Table 2. Where site-specific process config urations or mercury concentrations in ore are present, ac tual emissions may vary

significantly f

rom the estimated emissions factors. For example, facilities where gold re covered from electrowinning or zinc precipitation processes is not retorted prior to furnace smelting may have

higher emission rates from the refinery

furnace than facilities where re torting is used to separate mercury from the furnace feed material. B ecause gold ore processing and recovery

uses physical and c

hemical processes that also concentrate mercur y in its elemental form, most mercur y emitted from the individual processes is expected to occur as elemental mercury vapor. Mercury

removal by we

t chemical scrubbing processes has be en shown to be of limited effectiveness for elemental mercury data in extensive speciation and removal efficiency studies performed on coal-fired boiler e missions. Me asureme

nt of merc

ury concen trations i n industria

l gas str eams also present s some difficulties, and individual site evaluations should include mass balance evaluations that measure mercury

concentrations in the solid phases (process input and output streams) and treatment residuals (adsorption media and scrubber solutions). Statistical methods should also be employed in the deve lopment of sampling and analy sis schemes to account for temporal and spatial va riability in the mer

cury concen tration in gold ore feedsto cks at ind

ividual fa cilities.
Based on the information reviewed for this eva luation, the following aspects of mercury emissions from gold mines should be considered in development of any emissions control and pollution preventi on strate gies:

C The over

all poten tial for mercury

emissions i s relate d to merc ury content o f ores, and the me

rcury concen trations i n ore may vary on a short-term or

long ter m basis as the ore is excavated and processed.

C Removal efficiencies of mercury control technologies is highly dependent on mercury speciation. Many gold ore processes convert mercury to its elemental15 form, whi ch exhibits p oor rec overy in many control te chnolog ies.C Available information indicates that autoclave treatment of g old ores produces much lower mercury emissions than roasting ; howeve

r, the me rcury retaine d in the sol id phase a fter au toclavin g may result in h

igher e missions in downstre am proce sses, such as car bon reg enerat

ion kilns, e lectrow

inning c ells and f

urnace s.

C The site-specific configuration of proc esses may have a large impact on emissions from an individual process. For example, gold recovered by electro winning or zinc pr

ecipitat ion that is n

ot retort ed prior to smelting

may result in higher mercury

emissions at the smelter furnace.

C Control technologies that reduce air emissions may transfer mercury

to other media that require treatment or management, including scrubber solutions, proce ssed tailings and spent adsorption or filtration media. Control techniqu es that re move mer

cury as the sul fide com

pound ar e most stab le, with lower vap or press ure than metallic mercury and lower aqueous solubility than mer cury salts. Mec

ury sulfide should be stable a s long as it is not managed in an oxidizing environment in a medium with sufficient sulfide content to produce acid gener ating conditions.

Table 2. Process Emission Factors and Control Efficiencies
Process U

nit Emission Con trol Emissions Factor (lb/hr) Comments Ore Roasters Untreated (0.8xOre Hg Conc. (lb/ton)) x Ore Feed Rate (ton/hr) 80 percent Hg volatilized Quench (0.8 xOre Hg Conc. (lb/ton)) x Ore Feed Rate (ton/hr) x 0.9 No remov al with dr y pr ocess SO 2 Scrubber (0.8xOre Hg Conc. (lb/ton)) x Ore Feed Rate (ton/hr) x 0.675 Not used where ore sulfide Particulate Control (0.8xOre Hg Conc. (lb/ton)) x Ore Feed Rate (ton/hr) x 0.641 Baghouse, wet or dry ESP, Carbon Filter (0.8xOre Hg Conc. (lb/ton)) x Ore Feed Rate (ton/hr)x0.064 Effective for cooled gas Autoclaves Untreated Ore Hg Conc. (lb/ton) x Ore Feed Rate (ton/hr) x 0.0013 Hg oxidized form Venturi/Cyclone Ore Hg Conc. (lb/ton) x Ore Feed Rate (ton/hr) x 0.0002 Wet Scrubbing effective for Carbon Regeneration Kilns Venturi/Cyclone Kiln Feed Rate (tons/hr) x 2,000 (lbs/ton) x 0.0011 Strip process may off load Hg prior to regeneration Carbon Filter Kiln Feed Rate (tons/hr) x 2,000 (lbs/ton) x 0.0011x0.1Electrowinning Units Untreated Cathode Area(ft 2 ) x Hg metal fraction x 0.02(lbs/ft 2 /hr) Hg recovery from solution prior to e lectrow

inning will reduce emissionsCarbon Filter Cathode Area(ft 2 ) x Hg Fractionx0.02(lbs/ft 2 /hr)x0.1 Ventilation discharge will be at suitable temperature for carbon filterRetorts Condenser Charge Weight (lbs) x Mercury Fractionx0.0055 Loss rate from condense r assumes covered collection container for liquid Hg Carbon Filter Charge Weight (lbs) x Mercury Fraction x 0.0055 x 0.05 Effective for gas flow ra te andDore Fu

rnaces Particulate Control Charge Rate (lbs/hr) x 0.0045 Hg content of Fe edCarbon Filter Charge Rate (lbs/hr) x 0.0045 Hg content of Fe ed x 0.1 Gas may re quire precoolingCarbon Filter Charge Rate (lbs/hr) x 0.0045 Hg content of Fe ed x 0.1 Gas stream may require26

4.0 References 1. U.S. EPA, 1994, Technical Resource Document Extraction and Bene ficiation of Ores and Miner als, Volume

2: Gol d. Of

fice of Sol id Was te, Sp ecia

l Waste Br

anch . 401 M .Stre et NW, Washingt on, D.C. 2. Menne, D. M. 1998, Mercury in Gold Processing – Volume I : Metallurg y. http://memb ers.iine

t.net.au /~menne/ hg1.htm 3. Letter from George Conger, Newmont Gold Company , to Leo Drozdoff, Nevada Department of Environmental Pr

otection , re: Requ

est to Tes t Santa Fe

Ores and Concent

rates wi th Newmont Processing Technologies. Date d April 11, 1997.
4. Hubbar

d, G. and J. P. D’Acie

rno, 1999 . CFB Roa sting for Gold Or e Proces sing, in Proceedings of the 15 th International Conferenc e on Fluidized Bed Combustion. http://www.etis.net/balpyo/15icfbc/99-0158.PDF27

5. Be nt or , Y in on . 2 00 2, Ch em ic al El em en t. co m - Me rc ur y. http://www .chemic aleleme nts.com/e lements/h

g.html 6. R.A.Ixer. 1996. Ore petrog raphy and a

rchaeological provenanc e. Feature Mineralogica l Society Bulletin 113, 17-19. 7. U.S. Department of Interior, B ureau of Land Ma nagement. 2001. Marigold Mine Expansion Project F inal Envir

onmental Impa ct Stateme

nt. 8. U.S. Environmental Protection Agency , 1997 Mercury Study Report to Congress, Chapter 8. An Evaluation of Mercury Control Technologies and Costs. EPA-452-R-97-010.
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