Help Instruction File:

Lime Products Module Overview

Provided by the Office of Surface Mining Reclamation and Enforcement (OSMRE), the Pennsylvania Department 

of Protection (PADEP), the U.S. Geological Survey's (USGS) and the West Virginia Department of Environmental 

Protection (WVDEP).
Lime Products Module Overview

Table of Contents

1.0 OBJECTIVE 5

2.0 OVERVIEW 5

2.1 Physical and Chemical Properties 5

2.1.1 Lime (CaO) 5

2.1.1.1 Slaking Equipment and Process 7

2.1.1.2 Limestone and Lime Characteristics 8

2.1.1.3 Slaking Temperature 8

2.1.1.4 Slaking Water Quality 9

2.1.2 Hydrated Lime (Ca(OH)2) 9

2.1.2.1 Hydrated Lime Equipment and Process 10

2.2 Benefits/Drawbacks, Equipment and Typical Treatment Configurations 11

2.2.1 Benefits of Lime (CaO) vs. Hydrated Lime (CaOH2) 11

2.2.2 Drawbacks 11

2.2.3 Application, Equipment, and Typical Treatment Configurations 11

2.3 Financial Analysis between Lime and Hydrated Lime 12

3.0 LIME PRODUCTS MODULE OVERVIEW 13

3.1 Layout and Workflow 13

3.2 Module Inputs 13

3.2.1 Water Quality and Flow Input 13

3.2.2 Lime Information 14

3.2.3 Chemical Consumption 14

3.2.3.1 Stoichiometric 14

3.2.3.2 Titration 15

3.2.3.3 User-Specified Quantity 15

3.2.4 Equipment 15

3.2.4.1 Storage and Dispensing System Lime/Hydrated Lime Treatment System Sizing 16

3.2.4.1 Storage and Dispensing System Operational Time for Electrical Components 16

3.2.4.2 Foundation 17

3.2.5 Other Capital Items 17

3.2.6 Other Annual Items 17

3.3 Module Outputs 18

3.3.1 Sizing Summary 18

3.3.2 Capital Cost 18

3.3.3 Annual Cost 18

3.3.4 Net Present Value 18

3.3.4.1 Financial Variables 18
3.3.4.2 Cost Categories 19

3.3.4.3 Rationale for Recapitalization Recommendations 20

3.4 Assumptions of Design Sizing and Costs 21

4.0 REFERENCES 22

5.0 FIGURES 23

Figure 1: Rotary Kiln to produce Lime at a plant near Harrisburg, Pa 23

Figure 2: Alternative view of horizontal rotary kiln used to make Lime. 24

Figure 3: Slaker at a mine drainage treatment plant near Johnstown, Pa 25

Figure 4: Slaking chamber where Lime and water are mixed with paddles at a constant temperature. 26

Figure 5: Graph showing saturated solution of Ca(OH)2 is achieved at a pH of 12.4 at 55 o

F. 27

Figure 6: Graph showing the inverse relationship between solution temperature and solubility of 

Ca(OH)2. 28

Figure 7: A 23-ton semi-truck pneumatically filling a 50- ton silo with hydrated lime. Note the fill line 

along the left side of the photo. Also, the fill line can be seen through the fence that connects the truck to 

the silo. Not the air exhaust line extending from the top of the right-hand side of the photo down to the 

reaction tank where the line is submerged to during filling to control lime dust. This 1960s-era silo lacks 

a filter bag air purification system onto of the silo. 29

Figure 8: 50-ton turn-key hydrated lime silo with bottom skirt that houses bin vibrators, electrical control 

panel, slurry tank, slurry mixers, slurry pumps, and electrical control panel (shown on silo). Note the fill 

line that connects to semi-trucks to pneumatically push 23-tons of hydrated lime up and into the top of the 

silo. 30

Figure 9: Photo of the fill line entering the top of the silo. Semi-Trucks carry approximately 23 tons of 

Hydrated Lime attach to the bottom of the fill line and pneumatically push the lime to the top of the silo. 

Note the blue housing that contains bag filters used to the evacuated exhaust air as lime is pushed into the 

silo during filling. 31

Figure 10: Alternative view of dust collection system on top of the silo. Motor and filter bags in blue 

housing remove hydrated lime exhaust dust during the filling of the silo. 32

Figure 11: Access to filter bag house to maintenance and replace filters on top of silo 33

Figure 12: Photo of dual silo cones beneath a 120-ton silo. The dual cones and hydrate feeder system 

provides redundancy incase the feeder system malfunctions. Note the orange 1-HP bin vibrator motors 

that vibrate the base of the silo to prevent lime from compacting and “bridging” across the base. The 

motors are time to vibrate the base of the silo for 30-secs every 5 minutes. The timers are adjustable. 34
Figure 13: Hydrated Lime silo and dispensing system “dry feeding” hydrate directing into mine drainage 

flowing underneath the system. This eliminates the need to make a slurry and pump a hydrate slurry to the 

dose point. Attention is required to ensure moisture from mine drainage doesn’t redecarbonate hydrate 

and cause feeding issues. 35

Figure 14: Simple hydrate system positioned overtop of a mine discharge to dry feed lime directly into the 

drainage flowing under the white building. A rock channel is used to create turbulence to aid in mixing 

and dissolving the lime into the drainage. Note the green ferrous iron hydroxide indicating the treatment 

pH is > 8.3. 36

Figure 15: Volumetric screw feed, hydrated lime slurry tank, and mixer positioned under the silo. 

Hydrated Lime and water are mixed in the slurry tank and pumped to the dose point. 37

Figure 16: Pumps used to pump Hydrated Lime slurry from the slurry tank, located under silo, through 

the red hose and to the dose point. 38

Figure 17: Electric control panel that controls dust collection bag filters that sit on top of silo and low and 

high silo fill alarms. This control panel is attached to the outside of the silo and operated when the silo is 

filled. 39

Figure 18: Electrical control panel located inside silo and controls slurry make up and pumping. 40

Figure 19: Close up of the touch screen attached to the face of the electrical control panel shown in Figure 

18. 41

Figure 20: Mixer motor on top of Hydrated Lime slurry tank used to mix the hydrate with water. Note 

exhaust fan near top of photo to improve air quality within the skirt of the lime silo. 42

Figure 21: Exhaust fan shown near bottom of silo to remove hydrate dust and improve air quality within 

the silo skirt. 43
1.0 Objective

The objectives of this overview are to: (1) Provide an understanding of the application of Lime products

in mine drainage treatment and (2) Provide an overview of the Lime Module to guide users in developing

an estimate of the cost to construct, operate, and maintain Lime treatment systems and also aid in reverse 

cost modeling of existing systems. The information is presented in two sections, Overview and Lime

Module Overview. 

2.0 Overview

A basic understanding of the chemical and physical properties, the application, and equipment 

requirements of a Lime treatment system area required to develop a treatment strategy using the 

AMDTreat software. These topics are discussed below before discussing the Lime module interface and 

functionality to provide the necessary context. The Overview and Application section is organized into 

three parts: (1) Physical and Chemical Properties and (2) Benefits/Drawbacks, Equipment and Typical 

Treatment Configurations, and (3) Application and Financial Analysis. 

2.1 Physical and Chemical Properties

The two main Lime products used in mine drainage treatment are Lime and Hydrated Lime. In 2019, 18 

million tons of Lime products were produced by 28 companies at 84 plants in 36 States. However, only 

18 companies produced Lime for commercial resale and the remaining was produced for internal use 

(USGS, 2020). Table 1 provides several characteristics for Lime products. 

Parameter High Calcium Lime High Calcium Hydrated Lime

2020 Unit Cost (Pa) $145 $160

lbs of CaCO3 /lbs reagent 1.79 1.35

$/lb $0.08 $0.073

$/Ton Alkalinity $81.20 $118.40

Typical Purity 90 to 94% 94 to 97%

Specific Gravity* 3.2 2.3

Bulk Density (lbs/gal)* 7.35 (Pebble) 3.34

Angle of Repose* 55 o 70o

Table 1: Costs and characteristics of Lime products (* National Lime Association, 2020) 

2.1.1 Lime (CaO)

Lime is produced by calcining high-Ca content limestone, containing less than 5% Mg, in horizontal or 

vertical kilns at temperatures between 1,832 and 2,372 o

F (above the theoretical calcination temperature 

of 1,568 o

F) for rapid decomposition and evolution of CO2 (Means et al., 2014) (Figures 1 & 2). The

chemical formula below indicates a maximum of 56 tons of CaO and 44 tons of CO2(g) is produced from 

calcining 100 tons of CaCO3.
The resultant product, CaO, is often referred to as by the chemical formula name, Calcium Oxide, but also 

referred to by a variety of common names including, Lime, Quicklime, Burnt Lime, HiCal Lime, Caustic 

Lime, and Calcium Lime. Herein, CaO will be referred to as Lime. It is important to note that agriculture 

lime or ag lime is a variety of crushed limestone (CaCO3) and is not, and should not be confused with, 

Lime (CaO). 

Lime is a whiteish mineral containing a crystalline structure. The purity of Lime (% CaO by wt.) is a 

function of the limestone feedstock quality and the effectiveness of calcining, however, most Lime used 

in mine drainage treatment is between 92 and 95% pure. Lime is produced in various form factors 

depending on the application and is mostly sized as Pebble (1/4” to 2 1/2”), Ground (< 1⁄4”), or Pulverized 

(100% through a No. 20 sieve). Most lime plants using CaO use the Pebble gradation (Pebble Lime). 

Some Lime treatment plants directly dose Pebble Lime into mine drainage, but most plants first slake the 

Lime to produce Slaked Lime (Hydrated Lime) before application. 

The motivation to directly dose Lime into mine drainage was mostly due to a lack of electricity at a site. 

Generally, sites with no electricity were forced to use Caustic Soda (NaOH) due to its ability to be 

manually dosed and passively mixed, but, the high chemical cost made treatment financially unreasonable

for large acidity loads. Operators and equipment manufacturers developed systems capable of dispensing 

Lime using water, solar, or battery power. Being able to dispense Lime without the need for commercial 

electric allowed Operators to take advantage of low cost of Pebble Quicklime ($/ton alkalinity) and avoid 

dependence on Caustic Soda at remote sites. 

While these systems are generally dependable and able to achieve effluent standards, the dosing of

pebble-sized lime directly into mine drainage without mechanical mixing resulted in poor dissolution and 

increased sludge mass due to unreacted Lime. This resulted in two issues. First, poor dissolution limits the 

amount of Lime available to react, causing the operator to increase Lime dosing at several times the 

theoretical amount to compensate and overcome the inefficiency. There are several causes that explain 

the poor dissolution, including the small surface area of the Pebble size, particle agglomeration, poor 

dispersion due to Lime settling to bottom of reaction ditch, and the incomplete hydration (slaking). Lime 

must hydrate (discussed later) before participating in treatment reactions and directly mixing Pebble Lime 

to large volumes of mine drainage will hydrate the outer layer of the Pebble, which will act as a barrier 

for water to penetrate and hydrate the interior. All these issues contribute to poor dissolution, increased 

Lime consumption, and increased chemical cost. 

The second issue is the unconverted and unreacted Pebble Lime ends up as sludge in the settling pond. 

The resultant sludge is very difficult to pump, and extra steps are required for desludging that increase

cost. A settling pond must be decanted to expose the sludge and then water must be pumped and mixed 

with the Pebble Lime sludge to produce a favorable % solids slurry capable of being pumped. The 

increased cost and effort of desludging caused some operators to abandon the direct application of Lime 

to mine drainage. Additionally, unreacted Pebble Lime will accumulate in conveyance ditches resulting in 

significantly increased maintenance cost. 

Both issues, increased chemical and sludge costs, have caused some operators to install an electrically 

driven Lime plant to properly “slake” the lime before dosing to improve reactivity. The process of slaking 

involves converting Lime to Hydrated Lime. Lime is mixed with water and “hydrated” or “slaked” to 
produce Hydrated Lime before mixing with mine drainage for treatment.

Equation (2) shows the process of hydrating CaO to form Ca(OH)2 is strongly exothermic and care is 

required when mixing Lime with water. Furthermore, stoichiometric relationships in equation (2) show 1 

ton of pure CaO requires 640 lbs of water to produce 1.32 tons of Ca(OH)2. Hydrated Lime is a diluted 

form of Lime and contains 25% less CaO than Lime on a weight basis. 

While the terms hydrating and slaking can be used interchangeably, the term hydrating is generally 

reserved to describe the process where minimal water is used to create a low moisture powered form of 

Hydrated Lime. Slaking is used to describe the process when excess water is used to produce a wet slurry 

of Hydrated Lime (Slaked Lime). Dry hydration is conducted by manufactures to produce commercially 

saleable Hydrated Lime (Ca(OH)2 ), which is discussed in the Hydrated Lime Section 2.13. Wet hydration 

or slaking is conducted onsite by treatment plant operators using equipment called a slaker. Since wet 

hydration involves the purchase of Lime (CaO) it will be discussed later in this section, which is devoted 

to Lime. The reason why the discussion is separated is because choosing between designing a Lime plant, 

which will slake Lime onsite to produce Hydrated Lime, or designing a plant to use manufactured 

Hydrated Lime is a very important decision that warrants separate detailed discussions. Designers and 

Operators need to understand the benefits and drawbacks of each approach to make an informed decision. 

The reason why Lime is slaked before treatment is to avoid the issues previously discussed when Lime is 

directly dosed to a wastewater stream. Lime is hydrated to improve reactivity and dissolution efficiency

mainly due to the increased specific surface area of Hydrated Lime. Murry (1956) found specific surface 

area of similar sized particles ranged from 0.67 to 1.30 m2

/g for Lime and from 13 to 26 m2/g for 

Hydrated Lime. Boynton (1980) found that surface area is the most significant physical property in the 

reactivity of Hydrates, even more than particle size. Similar-sized Hydrated Lime particles, created from 

differing feedstock and/or under differing conditions, will result in particles of differing shapes and 

porosities that can significantly affect surface area. An increased specific surface area allows more of the 

particle to be available for reaction with the wastewater, thus increasing efficiency. Therefore, specific 

surface area, not particle size, is the most important attribute to promote reactivity. 

2.1.1.1 Slaking Equipment and Process

Lime and Hydrated Lime treatment systems share many of the same componentry. Both products are 

stored in silos and use volumetric feeders dispense the product from the silo. At this point, Lime systems 

differ from Hydrated Lime systems in that the Lime is dispensed into a slaker, whereas, Hydrated Lime is 

dispensed into a slurry tank (see Section 2.121). 

Slakers are equipment designed to mix Lime and water to hydrate the Lime to form Hydrated Lime to 

generate the increased surface area and reactivity (Figure 3). While the theoretical Lime to water ratio 

required for hydration is 3.1:1.0, slaking ratios are typically 1.0: 3.0-6.0. There are several types of 

slakers, including paste, detention/slurry, and ball mill and detention/slurry. Detention/slurry slakers are 

the most popular in mine drainage treatment. The basic operation of a slaker is for Pebble Lime to be

dispensed in volumetric quantities and mixed with water in a reaction chamber to complete the hydration 
process (Figure 4). The reaction time for the hydration process varies between less than one minute and 

10 minutes. A programmable logic controller (PLC) (or some other mechanism) monitors temperature in 

the reaction chamber to maintain a reaction temperature of 170 to 190 o

F by controlling water addition. 

The exothermic reaction from the slaking process produces the heat to achieve the desired reaction 

temperature. After the Lime is slaked, the newly formed Hydrated Lime flows into a grit separator where 

impurities, such as silicates, settle from suspension. The purified Hydrated Lime slurry flows to a slurry 

storage tank for dosing. 

Slaking optimization involves completely hydrating the all Lime to create Hydrated particles that will 

fully and rapidly dissolve in water. This allows for small reaction tanks, mixers and lower annual costs. 

There is debate and conflicting information on how to achieve optimization for a given feedstock of 

Lime. Treatment plant operators should consult with their Lime producer to discuss the best slaking 

practices for their Lime. Lime manufactures have often tested their Lime and understand how to optimize 

for desirable characteristics, such as, particle size and surface area. In addition, some manufactures have 

technical assistance available to analyze slaking efficiency and optimize the process. 

The potential cost savings makes onsite slaking attractive for large Lime consumers (discussed in Section 

2.3), however, slaking does add complexity to the treatment process. Without a knowledgeable operator, 

the potential cost savings could be offset by inefficient slaking practices that lead to poor conversion and 

low surface area that increases the Lime requirement. The information provided below is aimed at making 

readers aware a few of the variables in the slaking process. Modern slaking equipment has largely 

automated the process; however, operators should still have a basic understanding of the factors that 

promote efficiency. 

2.1.1.2 Limestone and Lime Characteristics

Many remote mine drainage treatment locations only have one Lime producer available, thus treatment 

plant operators lack the ability to evaluate and select from different Lime feedstock. Lime quality depends 

on the quality of limestone and the calcination process. The concentration of iron, aluminum, silica, and 

other impurities vary among quarries and affect the quality of the feedstock for Lime production. In 

addition to Limestone quality (ASTM C25), the production and preparation of Lime (calcination temp., 

particle size, etc.) affects the slaking process, which effects slaking time, particle size, surface area, 

purity, etc. If multiple Lime manufactures are available, potential consumers should request results of 

laboratory and slaking analysis of their Lime. For purity and surface area, there are several ASTM 

standards to measure Lime characteristics, including Gradation (E 11-70), Settling Rate, Slaking Rate (C 

110) Water Treatment (C 53-63) and Industrial Waste Treatment (C 826-75), acid neutralization (ASTM 

C400-64), and there are other tests to measure slaking time, reactivity, and other qualities of interest. 

2.1.1.3 Slaking Temperature

Slaking temperature is probably the most important variable that affects surface area and particle size. In 

general, higher slaking temperatures results in smaller particles with larger specific surface areas, 

however, slaking at temperatures near boiling can cause issues. Low slaking temperatures form large 

Hydrated particles with lower surface areas that are prone to agglomeration. Therefore, typical slaking 

temperatures range from 170 to 190 o

F. 

Since controlling water input controls the slaking reaction temperature, source water that experiences 

large temperature fluctuation throughout the year can make maintaining a constant slaking temperature 

challenging. In remote settings groundwater can be used as hydration water and equation (2) indicates 1.0 
lbs of CaO would raise the temperature of 3.9 lbs of water from 55 to 180o

F, assuming no thermal loss 

from the reaction tank. In most instances, either potable “municipal” water is plumbed to the site and 

purchased for slaking or clarified water is pumped from the top of the clarifier and used for slaking. 

2.1.1.4 Slaking Water Quality

Boynton (1980) and others have reported the effect of water quality on the slaking process. Mine 

drainage contains two water quality variables that will affect the slaking process, dissolved CO2 and 

sulfate. 

Dissolved Carbon Dioxide

It is common for Appalachian underground mine water to contain hundreds of mg/L of dissolved CO2

from the reaction between acidic water in the mine pool mixing with alkaline recharge water. If treated 

mine water is used for slaking the CO2 content should be determined and evaluated for use. High 

concentrations of CO2 can cause recarbonation of the Lime to form CaCO3 during the slaking process, 

which leads to an inefficiency that will increase use and cost

.

Dissolved Sulfate

Boynton (1980) recommended not using water that contains sulfate concentrations > 500 mg/L for

slaking. He provided data to show elevated sulfate affects and slows the hydration process. Presumably, a 

gypsum coating forms a shell around the Lime particle and retards the penetration of water molecules to 

slake the particle interior. 

The effect of sulfate on slaking has always been a concern in mine drainage treatment since sites are 

remote and often treated water is the only source water for slaking. The author had a unique experience 

to compare Lime consumption slaked with low sulfate municipal water against clarified treated water 

pumped from the top of the clarifier. The treated water contained a sulfate concentration of 1,100 mg/L. 

The large treatment plant pumps at a constant rate and the mine pool water quality is stable with an iron 

concentration of 160 mg/L. The treatment plant consumes almost 20 tons of Lime per day. 

The treatment plant was designed to be able to switch between using municipal and process water for 

slaking. The plant was operated for a month using municipal water and a month using process water. 

Flow and mine pool chemistry was monitored to explain any change in Lime use and the treatment pH 

remained constant and controlled by the PLC. The PLC did not record any significant change in Lime 

usage and the plant continues to use process water. Laboratory analysis may have been able to detect 

effects on the slaking process, however, practically speaking, there was no observed difference that could 

justify the cost of purchasing municipal water. There may have been noticeable effect if sulfate 

concentrations were higher. 

2.1.2 Hydrated Lime (Ca(OH)2)

Hydrated Lime, also known as Calcium Hydroxide, has a chemical formula of Ca(OH)2. The proceeding 

paragraphs discuss commercially available Hydrated Lime produced by “Dry Hydration” and sold as a 

dry powder in 50 lb bags or 23-ton bulk deliveries. Unlike Wet Hydration, Dry Hydration involves the 

use of minimal water and the process is highly individualized by the manufacture. Small consumers 

choose Hydrated Lime over Lime due to the capital cost and increased complexity of operating a slaker. 

Hydrated Lime is relatively insoluble and, consistent with exothermic reactions, solubility increases with 

decreasing temperature (Figure 5). At a temperature of 55 0

F the solubility of Ca(OH)2 is 1.2 g/L and will 
achieve a pH of 12.4 and dissolved alkalinity of 1,621 mg/L as CaCO3 if dissolved into water containing a 

Total Inorganic Carbon (TIC) concentration of 2 mg/L (Figure 6). A saturated solution of Hydrated 

Lime is known as Lime Water. Because of the low solubility of Hydrated Lime, the use of Lime Water 

would require large make up water requirements and storage tanks. For example, a typical surface mine 

discharge of 20 gpm and acidity of 100 mg/L (as CaCO3) would require 1,778 gallons/day of Lime 

Water. Thus, Hydrated Lime is mixed with water to create a slurry, which consists of Lime Water and

suspension of unreacted Hydrated Lime particles. Modern Hydrated Lime systems can produce Slurry 

with up to 38% solids by weight. A 38% slurry provides an additional 651,700 mg/L of alkalinity (as 

CaCO3) in the form of suspended Ca(OH)2 particles to produce a highly concentrated product with a total 

alkalinity (dissolved + suspended) of 653,321 mg of CaCO3 per Liter of slurry. For perspective, the 1,778 

gallons/day of Lime Water required in the example above would be reduced to 4.4 gallons/day of 38% 

slurry. 

The dissolved alkalinity in the Lime Water is immediately available for reaction, however, the majority of 

the alkalinity in Hydrated Lime slurry is in the form of suspended Ca(OH)2 particles that require a 

dissolution step before being available for reaction. In most cases, a mechanical mixing is required to 

help dissolve and promote reaction with the mine drainage. Mixing, promotes dispersion, prevents 

agglomeration and settling, and helps to continuously abrade particles to reveal fresh reactive surfaces 

that may become coated with mine drainage precipitate.

2.1.2.1 Hydrated Lime Equipment and Process

Hydrated Lime is delivered to treatment sites by semi-truck, and pneumatically transferred from the truck 

to the storage silo (Figures 7 through 11). Storage silos commonly range from 30 to 120 tons and are 

sized from consumption estimates to hold Hydrated Lime for up to a month to prevent recarbonation and 

“bridging” and compaction issues in the silo. Vibratory mechanisms are placed near the bottom of the silo 

and periodically shake the silo to maintain free-flowing Hydrated Lime and to prevent compaction

(Figure 12). Compaction will change the mass of Lime dispensed by the volumetric screw feeders. The 

volumetric screw feeders at the base of the silo dispenses Hydrated Lime directly into mine drainage, 

known as dry application, or into a slurry tank to produce a Hydrate slurry for wet application. 

Dry Application of Hydrated Lime

Dry application involves feeding Hydrated Lime from the silo directly into mine drainage located 

underneath or adjacent to the silo (Figures 13 & 14). There has been considerable debate over the merits 

of dry vs wet application. Dry application is often selected for smaller treatment systems where the goal is 

to maintain low capital cost and simplicity or in situations where water is not available to make a slurry 

and the treated process water contains sulfate concentrations that are too high to avoid gypsum formation. 

In these cases, Hydrated Lime is directly dispensed into mine drainage by a belt or screw feeder and 

mixing is accomplished by passive (water turbulence) or mechanical means. For sites that afford onsite 

sludge disposal, the cost of installing and operating mechanical mixers may not offset the cost of 

inefficient dissolution of particles and additional sludge handling costs caused by unreacted Hydrated

Lime particles. Historically, dry application system has been plagued with poor pH control caused by

variability in volumetric dosing if the bulk density of Hydrate changes in the silo (e.g. compaction). 

Advancements in technology over the past decade have eliminated this concern and there are several 

highly efficient and effective dry application systems in the eastern U.S. Dry application systems do 

eliminate maintenance issues related to making (water requirement), mixing (mechanical mixers), and 

pumping (pumps and line clogging) of Hydrated Lime slurry.
Wet Application of Hydrated Lime

Wet application involves dispensing Hydrated Lime from a silo into a tank and mixed with water to create 

a slurry (Figure 15) . Treated mine water, either from a clarified portion of a settling pond or from the top 

of a clarifier, is the common source of slurry make up water if sulfate concentrations will not invoke 

gypsum precipitation. Some sites purchase municipal water and even fewer sites use a ground water well. 

A pump connected to a pH controller in the reaction tank controls the pumping of slurry to the dose point 

to maintain a pre-determined pH set point. Wet application has been far more popular than dry and is 

required if the dosing point is higher than the bottom of the silo or far away from the silo. Additionally, 

maintaining precise pH control is easier since pumps are generally more precise than various types of dry 

feeding systems. 

2.2 Benefits/Drawbacks, Equipment and Typical Treatment 

Configurations

2.2.1 Benefits of Lime (CaO) vs. Hydrated Lime (CaOH2)

The major reason why Lime is popular is because of its convenience and flexibility. The benefits of using

Lime as a mine drainage treatment alkali include: 

1. Lime costs between 10 and 20% less than Hydrated Lime, depending on price negotiation and 

market conditions;

2. CaO contains 25% more Lime than Ca(OH)2 since it is anhydrous ; 

3. The bulk density of Lime is approximately twice that of Hydrated Lime;

4. The practical effect of benefits #1 through #3 listed above include decreased chemical and 

transportation costs and storage requirements (smaller silo);

5. Lime is less dusty than Hydrated Lime;

2.2.2 Drawbacks

The drawbacks of using Lime vs. Hydrated Lime, include: 

1. Increased capital cost of slaker equipment; 

2. Increased operator knowledge to effectively operate slaker; 

3. Increased operation and maintenance of slaker; and

4. Inefficient slaking will lead to increased Lime demand, and increased sludge generation which 

can delay or null equipment payback. 

2.2.3 Application, Equipment, and Typical Treatment Configurations

Lime and Hydrated Lime treatment systems in Appalachia (Eastern U.S.) are typically used to treat high 

acidity load discharges despite the fact that they have significantly higher capital cost than Caustic Soda, 

Soda Ash, or Pre-Manufactured Lime slurry systems. Lime and Hydrated Lime systems contain much 

more sophisticated mechanical and electrical systems. They also require additional site attendance and 

maintenance. However, these extra costs and effort are quickly offset by the low cost per ton of alkalinity 
of Lime and Hydrated Lime that quickly make these two products the most economical choice for 

discharges containing a large acidity load. Unless dealing with extremely acidic low flow discharges, 

Hydrated Lime treatment systems start to be an option once flow rates exceed 250 gpm and become a 

very common treatment option for discharges that exceed 500 gpm. Discharges less than 250 gpm are 

typically treated with Caustic Soda, Pre-manufactured Lime Slurry, Soda Ash, or Passive Treatment 

technologies. It is important to note the flow-based criteria are provided to provide perspective and 

should not be considered guidelines. Treatment technology selection should be based on economics and 

the goal of the project. In some cases, more expensive treatment options are selected if they afford 

convenience or some other benefit important to the project. 

As previously described, Lime and Hydrated Lime equipment are very similar except for Lime systems 

contain slakers and Hydrated Lime systems contain slurry make down equipment, unless it is a dry feed 

Hydrated Lime system. Both use storage silos, dust control systems and electrical control panels, and silo 

dispensing mechanisms. It is common to purchase a complete turn-key system from a single 

manufacture. The turn-key systems contain the silo, silo dispensing equipment, slaker/slurry system, 

electrical control panel, all enclosed in a building under the silo. The system is delivered as a completed 

unit and erected using a crane by the manufacture. The operator is responsible for the foundation and 

electrical connections to the unit. 

2.3 Financial Analysis between Lime and Hydrated Lime

The decision to design a Lime plant to slake Lime onsite or a Hydrated Lime plant can be a difficult 

decision. The decision to slake or not is largely determined based on whether the potential cost savings of 

using Lime is outweighs the convenience afforded by Hydrated Lime. 

To provide perspective on the potential for cost savings, consider the treatment scenario showed in Table 

2. 

Flow Acidity Lime Purity Hydrated 

Lime Purity

Est. Annual 

Lime 

Consumption

Est. Annual 

Hydrated 

Lime 

Consumption

5000 gpm 100 mg/L as 

CaCO3

93% 96% 660 tons 845 tons

Increase in Capital Cost for 

Slaker Lime Unit Cost Hydrated Lime Unit Cost

$250,000 $140/ton $160/ton

Lime Unit Cost Hydrated Lime Unit 

Cost

Annual Chemical Cost 

savings using Lime

$92,505 $135,278 $42,773

Table 2: Treatment scenario evaluating the cost of using Lime vs. Hydrated Lime for treatment.
The discharge characteristics and costs are reasonable for an abandoned mine pump and treat project in 

Pennsylvania. Assuming the increased capital cost associated with purchasing a slaker is $250,000 and 

the annual chemical cost savings is $42,773, it would take approximately six years to payback the 

investment in Lime treatment, assuming the operation and slaking process was efficient. While there are 

more in-depth approaches for analyzing the payback, this simple analysis shows there is potential cost 

savings for moderately sized treatment projects. 

3.0 Lime Products Module Overview

3.1 Layout and Workflow

In general, inputs are on the left-hand side of the module and calculated outputs are on the right. The 

module inputs on the left-hand side are arranged into six sections: (1) Water Quality and Flow Input, (2) 

Dry Lime Information, (3) Chemical Consumption, (4) Equipment, (5) Other Capital Items, and (6) Other

Annual Items. The workflow for the module is for users to start at the top left-hand side. Enter the Typical 

Flow and Net Acidity and AMDTreat calculates the annual acidity loading. Then select Hydrated Lime or 

Lime, along with the purity and mixing efficiency and unit cost. Next select the method to estimate 

Chemical Consumption (Stoichiometric, Titration, or User-Specified Quantity). AMDTreat uses this 

information along with the acidity load to estimate the annual consumption. Users use the Equipment and 

System Installation heading to select and size treatment equipment. Additionally, users can use this 

section to specify the operational frequency and hours of operation of the treatment system so electrical 

costs are correctly modeled. Finally, users can specify additional capital and annual costs not considered 

by the module under the Other Capital Items and Other Annual Items headings. 

Module output is provided on the right-hand side of the module. Module outputs on the right-hand side 

are arranged into four sections: (1) Sizing Summary, (2) Capital Cost, (3) Annual Cost, and (4) Net 

Present Value. The Sizing Summary section provides estimates of chemical consumption and silo refill 

frequency. The estimated cost to construct and operate the Lime treatment system is provided under the 

Capital Cost and Annual Cost headings. Lastly, users can opt to conduct a Net Present Value (NPV) to 

obtain the total cost to operate and maintain a treatment system for a defined time period. 

A general overview of the module input and output sections is presented below, however, users are 

directed to the numerous tool tips located in the module that provide additional detailed information, such 

as definitions of terminology. In most cases, the tool tips are accessed by clicking on the information icon 

( ) in each of the subheadings in the module. 

3.2 Module Inputs

3.2.1 Water Quality and Flow Input: The Water Quality and Flow Input section is where users 

specify the Typical Flow and Net Acidity values. These values are used to (1) estimate the annual 

chemical consumption and (2) estimate the size of various equipment, such as, chemical storage 

tanks. 

The definitions for Typical Flow and Net Acidity can be found in the tool tip for this section. Click on 

the information icon ( ) on the right side of the Water Quality and Flow subheading. In short, 
Typical Flow is the flow rate “typically” experienced at the site. This flow rate is used to calculate the 

annual chemical consumption so one must take careful consideration to calculate this value. 

Net Acidity represents the acidity released and neutralized when the base is added to achieve the 

treatment pH. For eastern coal mine drainage, the acidity producing species will be the hydrolysis of 

Al3+, Fe2+, Fe3+, and Mn2+, the precipitation of CaCO3, the deprotonation of H2CO3 and HCO3

-

. For 

treatment pH > 10, the hydrolysis of Mg2+ and hydroxylation of cations need to be considered. Net 

Acidity is best determined by performing a cold acidity titration in the field to preserve WQ 

characteristics. This method is described in Titration Section 3.2.3.2 under the Chemical 

Consumption heading. 

3.2.2 Lime Information: Users can select to estimate the consumption and cost of either Hydrated 

Lime (CaOH2) or Dry Lime (CaO). For either chemical, users must specify the Purity of the chemical 

and Mixing Efficiency. Lime producers can provide potential purchasers with the purity for either 

product. Hydrated Lime is refined from Lime, thus purer. Default purity for Hydrated Lime is 96% 

and 93% for Dry Lime. The impurities in Hydrate and Lime are typically consist of silicate and oxide 

minerals and will not dissolve and end up as grit (during slaking) or sludge. 

Mixing Efficiency is used to simulate the percentage of chemical being dosed that is dissolving or 

participating in treatment reactions. Unlike Caustic Soda, Lime is a solid and requires mixing to 

promote and accelerate dissolution. If Lime is dosed without mechanical mixing or agitation, maybe 

only 50% of the Lime will dissolve and the remaining will be unavailable to further react since it will 

be removed during the solid/liquid separation stage. Therefore, the Mixing Efficiency may be 

estimated at 50% to properly estimate chemical consumption. Mixing efficiency can be measured by 

collecting total and dissolved water samples before chemical addition and right before settling and the 

difference between total and dissolved calcium provides insight to the Mixing Efficiency.

3.2.3 Chemical Consumption: The Chemical Consumption section offers users three methods to 

estimate the annual chemical consumption, Stoichiometric, Titration, and User-Specified Quantity. 

3.2.3.1 Stoichiometric – This method estimates the annual chemical consumption by using the 

user-specified information under the Water Quality & Flow Input and Lime Information

headings. The method uses the values for Typical Flow and Net Acidity to calculate the 

annual acidity loading in Calcium Carbonate Equivalents. 

Where: 

A.L. = Annual Acidity Load in g CaCO3/yr

Flow = Typical Flow in gal/min

Acidity = Net Acidity in mg/L as CaCO3

After the Annual Acidity Load is determined, the program uses the stoichiometric 

relationship between CaCO3 and Ca(OH)2 to re express the acidity load in terms of Ca(OH)2. 
HL = Annual Acidity Loading in g/yr expressed as Ca(OH)2

Finally, annual amount will be adjusted for the user-specified values for the Purity and 

Mixing Efficiency to determine the annual Hydrated Lime Consumption.

3.2.3.2 Titration – The titration method allows users to input the results of field or bench 

acidity titrations that empirically determine the required dose to achieve effluent standards.

The “treatment” acidity or net acidity is best determined by field conducing a cold acidity 

titration using the same treatment chemical that will be used in the final treatment system. A 

cold acidity “treatment” titration is performed by filling a 1 Liter beaker with fresh mine 

drainage and immediately titrating to various pH endpoints. The beaker contains a pH probe 

and magnetic stirrer. At each pH endpoint, a filtered sample is collected and sent for 

laboratory analysis. The information is used to determine the pH at which effluent standards 

will be achieved and the corresponding chemical dose is used to determine the treatment (or 

net) acidity. For Lime products, AMDTreat requires the treatment acidity to be entered as lbs 

of Lime products per gallon of mine drainage treated. Since the purity is inherently contained 

in the titration input value, AMDTreat does not use values for Purity or Mixing Efficiency

when the Titration method is selected. 

Where: 

Annual H.L = Annual Hydrated Lime in tons/yr

3.2.3.3 User-Specified Quantity - This method allows users to specify the annual Lime

consumption. This method is typically used when AMDTreat is being used to evaluate longterm water treatment liability using the Net Present Value calculations. 

3.2.4 Equipment: This section has two subsections: Storage and Dispensing System and Foundation. 

Users use the Storage and Dispensing System to select the size of a “turnkey” Lime/Hydrated Lime 

treatment system to determine capital cost and specify the operational time for electric components to 
accurately estimate annual electric costs. allows users to: (1) Size and cost a “turnkey” Lime or 

Hydrated Lime treatment system, and (2) Specify operational time for various electrical equipment to 

accurately estimate electrical costs. Users size and estimate the foundation size for the Lime treatment 

system using the Foundation subsection. 

3.2.4.1 Storage and Dispensing System Lime/Hydrated Lime Treatment System Sizing: Users can

size a “turnkey” Lime or Hydrated Lime treatment system in 30, 60, 90, or 120-ton silo sizes

(Figure 8) (See Section 3.2.4.2 for information on silo diameter and foundation size). The term 

“turnkey” refers to the purchase of a complete Lime/Hydrate system that includes everything 

needed to slake Lime or produce Hydrated Lime slurry. The system includes the pricing for the 

purchase and delivery of a single unit system that includes the silo, silo dispensing system, slaker 

or Hydrate slurry system, Slaked Lime/Hydrate slurry pump system (Figure 16), electrical panel

(Figures 17, 18, & 19), and controls all enclosed in the base of a skirted silo for protection. The 

cost of a foundation and installation are not included by AMDTreat offers the ability to capture 

those costs 

3.2.4.1 Storage and Dispensing System Operational Time for Electrical Components: The 

turnkey Lime and Hydrated Lime system contains several electrical components that require the 

user to specify the motor power and operational time to accurately model annual electrical costs. 

The operational time reflects the time period the component is turned on and drawing electricity. 

The following electrical components are included in the turnkey system: 

1. Dust Collector Blower: The Dust Collector Blower is used to control Lime dust when the silo 

is replenished with Lime or Hydrated Lime (Figures 10 & 11). It will only operate while the 

semi-truck pneumatically pumps Lime into the silo. The annual electrical cost to operate the 

blower is calculated by using the Silo Refill Frequency, provided under Sizing Summary, to 

estimate the number of times each year the silo will be refilled. Users specify the amount of time 

it takes to refill the silo and it is multiplied by the number of silo refills to determine the hours of 

operation each year. The Blower motor horsepower along with the operational time and electric 

rate to calculate the annual electrical cost. 

2. Bin Activator/Vibrator: The Bin Activator is a device attached to the bottom cone of the 

storage silo and periodically vibrates the side walls of the silo to prevent Hydrated Lime from 

“bridging” across the silo and to keep the silo free flowing. Users specify the motor power and 

the vibration frequency to estimate the annual electrical consumption and cost. (Figure 12)

3. Screw Feeder: The Screw Feeder is used to volumetrically dispense Lime products from the 

bottom of the silo to either a slaker or slurry make down system. The screw feeders only operate

when the control system calls for Lime products. The default operational time is assumed to be 18 

hr per day. 

4. Slurry Mixer and Pump: Hydrated Lime systems have tank that mixes water with Hydrated 

Lime to produce slurry. The slurry mixer keeps the slurry mixed and homogenized (Figure 20). 

The pump is used to pump slurry from the tank to the point where it is dosed with mine water

(Figure 16). The pump is usually controlled by a pH controller. Both devices are operating when 

the plant is treating mine water . 

5. Silo Exhaust Fan: The Silo Exhaust Fan is located within the silo skirt and keeps fresh air 

within the silo (Figures 20 & 21). The 18x18” fan can be thermostatically controlled to help 

lower the temperature within the silo skirt during the summer months if a slaker is used. 
6. Silo Space Heater: Space heaters are used heat the skirted silo space where the slaker/slurry 

system are located. The heats provide a comfortable environment for workers but also prevent 

slurry water pipes from freezing. The heaters are thematically controlled are only operated when 

temperatures drop below 40 degrees. Users should estimate the number of days the heater will be 

used to accurately estimate electrical costs. 

3.2.4.2 Foundation: The first step in estimating the capital cost of the foundation for the turnkey 

Lime/Hydrate treatment system is to specify, or accept the default, Concrete and Material 

Placement unit cost used to calculate the cost to purchase, deliver, and form (including rebar) the 

concrete foundation. The second step is to either have AMDTreat estimate the required 

foundation volume by select User-Specified and specify the foundation volume. AMDTreat 

estimates the foundation area using the information in Table 3. Foundation thickness depends on 

soil quality selected. For “Poor” quality soils (1500 lbs/ft2

), foundation thickness is determined 

by multiplying the “standard thickness” show in Table 3 by 1.5 to achieve a 5.25 ft thick 

foundation. For “Average” quality soils (3000 lbs/ft2

), foundation thickness is the 3.5 ft shown in 

Table 3. For “Excellent” quality soils (4500 lbs/ft2

), foundation thickness is determined by 

multiplying the “standard thickness” show in Table 5 by 0.75 to achieve a 2.6 ft thick foundation.

For either sizing method, the foundation volume is multiplied by the Concrete and Material 

Placement Unit Cost to estimate the capital cost of the foundation. If no foundation is required, 

users can make the Concrete and Material Placement $0.0 or enter a $0.0 yd3 for the UserSpecified foundation volume. 

Silo Size (ton) Silo Diameter (ft) Foundation Area (ft2) Foundation Thickness* (ft)

30 12 220 3.5

60 12 220 3.5

90 16 315 3.5

120 16 315 3.5

* Thickness will vary based on specified soils quality. See Section for more detail 

Table 3: Relationship between user selected Silo size and foundation area. 

3.2.5 Other Capital Items: The Other Capital Items section allows users to capture the capital cost of 

equipment and other items that are not included in this module. For example, a small number of Lime

treatment systems have electronic surveillance to notify authorities if unauthorized persons attempt to 

interfere with the equipment. Since this is uncommon it was not included the cost module, however, users 

who want to include this capability can input the cost into the Other Capital Items section to capture the 

capital cost. 

3.2.6 Other Annual Items: The Other Annual Items section allows users to capture the capital cost of 

equipment and other items that are not included in this module. For example, users could include the 
annual subscription cost to conduct electronic surveillance on the treatment system in the Other Annual 

Items section. 

3.3 Module Outputs

3.3.1 Sizing Summary: The Sizing Summary section displays important calculated module outputs, 

such as the estimated chemical consumption, storage silo refill frequency, and foundation size. The 

storage silo refill frequency provides users with an estimate of how often the storage tank will be refilled 

based on the silo capacity and estimated chemical consumption. This information is useful to help size a 

silo to hold a delivery of Lime for no more than 45 days to prevent recarbonation and bridging issues. 

3.3.2 Capital Cost: This section provides the estimated costs for the various user-specified components 

and the total estimated cost to construct the Lime system. Users can opt to estimate the installation cost by 

specifying it as a percentage of the capital cost or by entering a cost. 

3.3.3 Annual Cost: The annual cost section provides an estimate of the annual cost to operate and 

maintenance the treatment system. Users can select to have AMDTreat estimate the annual chemical cost 

or specify an annual chemical cost. Specifying an annual chemical cost is often used when AMDTreat’s 

Net Present Value calculations are being used to estimate water treatment liability for an existing 

treatment system. The annual Maintenance for the treatment system can either be specified by the user or 

estimated by assuming it is a percentage of the capital cost. The latter method assumes the more 

expensive system are more costly to maintain. Finally, users can specify the annual electric cost or 

AMDTreat will estimate the annual electric cost for all the electrical equipment described under the 

Storage and Dispensing System section. 

3.3.4 Net Present Value: The Net Present Value (NPV) section determines the cost to operate a 

treatment system component over a specified time period. The NPV calculates the present-day financial 

investment required to generate the income to pay for future operation and equipment/materials

replacement costs. Both Financial Variables and Cost Categories are required to calculate the NPV. 

3.3.4.1 Financial Variables - The Term of Analysis, Inflation Rate, and Rate of Return are 

three variables used in the NPV calculations. The default values for these terms are 

shown under the Net Present Value section of each module. Users must access the Net 

Present Value menu at the top of the main user interface to change the default values as 

they would apply to all modules used for an entire treatment system. While NPV is 

determined for each AMDTreat module activated by the user, the goal is to determine a 

total NPV for an entire mine drainage treatment system project (a collection of cost 

estimates for individual modules creates a treatment system project in AMDTreat). 

Therefore, a single value for Term of Analysis, Rate of Return, and Inflation Rate is 

applied to all modules and cannot vary between modules.

• Term of Analysis: The time period used by the NPV calculation to determine the 

financial investment required to pay for all future costs of the treatment system. 
• Inflation Rate: Represents the average price increase of goods and services over 

time. AMDTreat uses the inflation rate to calculate the future cost of the annual 

operation and maintenance (O&M) and recapitalization items. 

• Rate of Return: Describes the expected profit on an investment. 

3.3.4.2 Cost Categories - For each treatment module, AMDTreat provides a list of 

recommended equipment and materials that require recapitalization. In addition, 

AMDTreat provides recommendations (default values) for life cycle and replacement 

percentage. Users can click on the default values for Life Cycle or Replacement 

Percentage and use the +/- buttons to change the default values. In addition, users can 

select Custom Cost and enter a new cost to represent the current cost of the equipment. 

Users can add new recapitalization items or deactivate/delete existing items for 

calculating the NPV. 

An example of how the recapitalization variables are used to determine NPV is to 

consider the following hypothetical scenario. Assume a vertical turbine pump has a life

cycle of 50 years but requires the pump motor to be rebuilt every 20 years. Assume the 

present-day cost to purchase the motor is $500,000, and the cost to remove, rebuild, and 

reinstall the pump motor is $20,000. Now assume we want to determine the amount of 

investment required today (NPV) to generate the income to pay for the future cost of 

rebuilding the pump motor over a 50-year Term of Analysis, which is also equal to the 

life cycle of the pump. Assume an Inflation Rate of 5.0% and Rate of Return of 8.1%. 

The goal is to place the money in a relatively secure investment vehicle to generate 8.1% 

annually. The NPV will calculate the size of investment required to generate income for 

future costs. 

There are several ways to model the replacement cost. One way is to replace 4% of the 

present-day cost of the pump (4% of $500,000 = $20,000) with a life cycle of every 20

years. If the Term of Analysis is 50 years, then the entire pump would not require 

recapitalization since the life cycle of the pump is 50 years. However, the motor would 

require two replacements (50 years / 20 years = 2.5 rounded down to 2). 

To determine the NPV to recapitalize rebuilding of the motor, AMDTreat calculates the

future cost to rebuild the motor at each life cycle, 20 and 40 years. The program uses the 

Inflation Rate to inflate the present-day default cost to rebuild the motor in 20 and 40 

years from now. While the present-day cost to rebuild the pump motor is $20,000, the

future cost to rebuild the motor in 20 years at a 5.0% Inflation Rate is $53,065 and 

$140,799 in 40 years (Equation 3). Assuming an 8.1% Rate of Return, the 50-year NPV

for the pump is $17,422. In other words, an initial investment of $17,422 is needed at an 

annual Rate of Return of 8.1% to generate the investment income required for the two 

motor rebuilds over the 50-year life cycle of the pump. 
• Annual Operation and Maintenance Cost: By default, AMDTreat transcribes the annual 

O&M cost from the Annual Cost section to the Net Present Value section. The program 

assumes the module is being used to first estimate the annual cost for a treatment system 

component, so it automatically transcribes the annual cost to the NPV section. If this is 

not the case or the user wants to use some other annual cost, the “Use Custom” box can 

be selected to allow the user input of a different annual cost to utilize in the NPV 

calculation. 

• Recapitalization Cost: Certain treatment system components, especially mechanical and 

water conveyance equipment, require periodic replacement. The recapitalization cost of 

an item is an estimate of the amount of money required to pay for future replacement 

costs for the item. In addition to the Financial Variables described above, three additional 

values are required to calculate the NPV of recapitalization costs, the Present-Day 

Equipment Cost, the Life Cycle, and the Replacement Percentage.

• Default Cost: This represents the current cost to purchase the equipment or material.

• Life Cycle: The time frame between equipment or material replacement is termed as its 

Life Cycle. Some equipment manufacturers provide recommended life cycles for their 

equipment to provide consumers with an estimate of how long the equipment is expected 

to be operational. Some life cycles, such as those used for treatment media (limestone), 

are based on best professional judgement. Some operators prefer to periodically purchase 

and replace equipment before failure to preserve the continuity of operations, while 

others wait until failure to replace an item.

• Replacement Percentage: The Replacement Percentage is an adjustment factor to the 

Default Cost to accommodate situations where the entire piece of equipment or all of the 

material does not require recapitalization. For example, a passive treatment component 

may be designed to contain enough limestone to neutralize the acidity load for 20 years, 

however, the accumulation of metal hydroxide precipitates within the void space of the 

limestone layer may require that 25% of the limestone be replaced every 7 years to 

prevent hydraulic failure such as plugging or short-circuiting. For this scenario, the 

initial cost of the limestone making up the limestone layer is discounted by 75% and 

assigned a life cycle of 7 years to determine the amount of money required to cover the 

cost of replacing 25% of the limestone layer every 7 years over the Term of Analysis. 

3.3.4.3 Rationale for Recapitalization Recommendations:

Recapitalization recommendations are based on professional experience of the 

AMDTreat Team and may not apply to all situations. Users are encouraged to customize 

the recapitalization assumptions to their treatment scenario. AMDTeat Team members in 

Pennsylvania and West Virginia and have collective experience in design, funding, 

and/or operation/maintenance of more than 20 large Lime/Hydrated Lime treatment 

systems. The AMDTreat Team held discussions on personal experience to develop a list 
of recapitalization recommendations. Users may have different experience and opinions

than those listed. 

By default, AMDTreat includes a list of ten recapitalization items for Lime and nine for 

Hydrated Lime treatment systems. Users can delete or modify any of the default 

Recapitalization items by either deselecting the item or by setting the Replacement % to 

zero. If the item is deselected the Total Cost for the item will still be shown but the cost 

will be subtracted from the Net Present Value Cost, shown in the Net Present Value 

Heading. For example, the default value for the lifecycle of a steel storage Silo is 40

years assuming it is not sandblasted and repainted at first sight of corrosion. However, if 

a Silo will be periodically repainted, users may opt to increase the Lifecyle. Users are 

free to fully customize the replacement items, including adding new items or deleting 

default items. 

Silo: The Life Cycle of a Steel Storage Silo depends on how well the Silo is maintained 

to prevent corrosion. Silos that sandblasted and repainted at the first sight of corrosion 

have been known to last more than 60 years. On the other hand, Silos that are not 

maintained and allow corrosion to spread have required replacement after 30 years. After 

considering the condition and life of many steel Silos, the AMDTreat team feels 40 years 

is a good estimate without being able to predict the maintenance commitment. The 

estimate was developed after consultation with a Silo manufacture. 

Bin Activator, Screw Feeder, Slurry Mixer, Slurry Pump, Silo Space Heater, and 

Slurry Tank: There are many factors that affect equipment longevity; however, a major 

Lime/Hydrated equipment manufacture and numerous plant operators were requested to 

provide “typical” estimates for longevity for a variety of Lime system components. The

AMDTreat Team used the information to develop default Lifecycles for the major 

Lime/Hydrate componentry.

Hardware/Software: Many modern Lime and Hydrated Lime plants use automation and 

are computer controlled. The AMDTreat team have worked at several treatment plants 

where issues developed in the automation and hardware and software that was 15 years 

old was obsolete and could no longer be replaced or fixed. The rapid advancement in 

software and hardware can lead to significant costs to upgrade the system if issues arise. 

The AMDTreat team decided computer automation software and hardware become 

obsolete after 15 years and will require replacement if an issue arises. 

Slaker: If Lime treatment is selected, AMDTreat automatically includes the slaker as 

equipment that requires recapitalization. After consultation with a major manufacture of 

Slakers, a Lifecyle of 25 years is assumed. 

3.4 Assumptions of Design Sizing and Costs

AMDTreat is a cost estimation model that uses assumptions to provide treatment sizing and both capital 

and annual cost estimates. While there are many assumptions in the program, the assumptions that follow 

are important for the Lime module. 
1. The Stoichiometric method used to estimate the annual chemical consumption relies on a 

properly determined value for Typical Flow and Net Acidity. Many people use the Standard 

Method 2310 (Hot Acidity) procedure to determine Net Acidity. In most instances, a Hot Acidity 

titration result will not accurately describe the base requirement to achieve effluent standards and 

could over or underestimate chemical costs. A cold acidity titration is the best method to 

determine the Net Acidity encountered during treatment. 

4.0 References

United States Geological Survey, 2020, National Minerals Information Center Website 

(https://www.usgs.gov/centers/nmic/lime-statistics-and-information?qtscience_support_page_related_con=0#qt-science_support_page_related_con)

Boynton, R.S., (1980) Chemistry and technology of lime and limestone, 2nd edn. Wiley, New York. 

Means, B., Beam, R, and Mercer, J., 214. Analysis of Hydrated Lime Consumption in Circumneutral 

Underground Coal Mine Drainage Treatment. Mine Water and the Environment.

Murray, J., 1956. Summary of Fundamental Research on Lime. National Lime Association. 

National Lime Association, 2020, Lime Fact Sheet, 

https://www.lime.org/documents/publications/free_downloads/fact-properties2007rev.pdf
5.0 Figures 

Figure 1: Rotary Kiln to produce Lime at a plant near Harrisburg, Pa
Figure 2: Alternative view of horizontal rotary kiln used to make Lime.
Figure 3: Slaker at a mine drainage treatment plant near Johnstown, Pa
Figure 4: Slaking chamber where Lime and water are mixed with 

paddles at a constant temperature.
Figure 5: Graph showing saturated solution of Ca(OH)2 is achieved at a 

pH of 12.4 at 55 o

F.
Figure 6: Graph showing the inverse relationship between solution 

temperature and solubility of Ca(OH)2.
Figure 7: A 23-ton semi-truck pneumatically filling a 50- ton silo with 

hydrated lime. Note the fill line along the left side of the photo. Also, the 

fill line can be seen through the fence that connects the truck to the silo. 

Not the air exhaust line extending from the top of the right-hand side of 

the photo down to the reaction tank where the line is submerged to 

during filling to control lime dust. This 1960s-era silo lacks a filter bag 

air purification system onto of the silo. 
Figure 8: 50-ton turn-key hydrated lime silo with bottom skirt that 

houses bin vibrators, electrical control panel, slurry tank, slurry mixers, 

slurry pumps, and electrical control panel (shown on silo). Note the fill 

line that connects to semi-trucks to pneumatically push 23-tons of 

hydrated lime up and into the top of the silo. 
Figure 9: Photo of the fill line entering the top of the silo. Semi-Trucks 

carry approximately 23 tons of Hydrated Lime attach to the bottom of 

the fill line and pneumatically push the lime to the top of the silo. Note 

the blue housing that contains bag filters used to the evacuated exhaust 

air as lime is pushed into the silo during filling. 
Figure 10: Alternative view of dust collection system on top of the silo. 

Motor and filter bags in blue housing remove hydrated lime exhaust dust 

during the filling of the silo. 
Figure 11: Access to filter bag house to maintenance and replace filters 

on top of silo
Figure 12: Photo of dual silo cones beneath a 120-ton silo. The dual 

cones and hydrate feeder system provides redundancy incase the feeder 

system malfunctions. Note the orange 1-HP bin vibrator motors that 

vibrate the base of the silo to prevent lime from compacting and 

“bridging” across the base. The motors are time to vibrate the base of the 

silo for 30-secs every 5 minutes. The timers are adjustable. 
Figure 13: Hydrated Lime silo and dispensing system “dry feeding” 

hydrate directing into mine drainage flowing underneath the system. 

This eliminates the need to make a slurry and pump a hydrate slurry to 

the dose point. Attention is required to ensure moisture from mine 

drainage doesn’t redecarbonate hydrate and cause feeding issues. 
Figure 14: Simple hydrate system positioned overtop of a mine 

discharge to dry feed lime directly into the drainage flowing under the 

white building. A rock channel is used to create turbulence to aid in 

mixing and dissolving the lime into the drainage. Note the green ferrous 

iron hydroxide indicating the treatment pH is > 8.3. 
Figure 15: Volumetric screw feed, hydrated lime slurry tank, and mixer 

positioned under the silo. Hydrated Lime and water are mixed in the 

slurry tank and pumped to the dose point.
Figure 16: Pumps used to pump Hydrated Lime slurry from the slurry 

tank, located under silo, through the red hose and to the dose point.
Figure 17: Electric control panel that controls dust collection bag filters 

that sit on top of silo and low and high silo fill alarms. This control panel 

is attached to the outside of the silo and operated when the silo is filled.
Figure 18: Electrical control panel located inside silo and controls slurry 

make up and pumping.
Figure 19: Close up of the touch screen attached to the face of the 

electrical control panel shown in Figure 18.
Figure 20: Mixer motor on top of Hydrated Lime slurry tank used to mix 

the hydrate with water. Note exhaust fan near top of photo to improve 

air quality within the skirt of the lime silo.
Figure 21: Exhaust fan shown near bottom of silo to remove hydrate 

dust and improve air quality within the silo skirt.