Queenstake Resources 

Jerritt Canyon Mine 

February 23, 2010 

Ore Roaster Circuit General Description: 

There are two identical Roaster Circuits (East and West) with independent off gas handling 

systems located at the Jerritt Canyon Mine. The Roasters operate as two-stage, countercurrent, 

fluid bed type units which fluidize and oxidize refractory ore in a pure oxygen environment. The 

Roaster system is designed to oxidize sulfide bearing materials and organic carbon found in the 

refractory ore. 

To begin the roasting process, ore is fed uniformly into each Roaster through a fluidized feed 

distributor that maintains a barometric seal. The ore that is fed into the Roaster has already been 

mixed with pulverized coal to increase the fuel value. A coal star feed system and Roaster bucket 

elevator feed airslide are responsible for supplying a consistent ore/coal blend to the Roaster. 

In the first stage, which lasts approximately 24 minutes, the ore forms an aerated bed where it 

becomes almost completely oxidized. The bed will be at approximately 110 inches water gauge 

(WG) while the temperature is typically around 1,030o

F. Once the first-stage retention time is 

reached the ore is discharged to the second stage at the bottom of the bed through the lower 

interstage transfer box. This method of transfer is utilized to prevent oversized feed material and 

accumulation of refractory brick spallings. 

Control of the process retention times of the ore in the first and second stage is important as the 

times are directly proportional to the fluid bed level. These levels are measured indirectly by the 

pressure drop in inches of water column from the bottom to the top across the fluidized bed. The 

pressure taps measure the drop in inches of water column and are located at the bottom of the 

stage beds and the freeboard. 

In the second stage, the roasting process lasts approximately 12 minutes with operation at a 

temperature of approximately 700o

F and 65 inches WG. Once processing is completed in the 

second stage of the Roaster, the final product exits from the bed underflow. This discharge, like 

the first stage, is controlled by one of two radial action discharge cone valves. The amount of 

oxidized material or calcine exiting the second stage secondary cyclones cannot be controlled. 

From the bottom of the second stage, the final product is piped through a second stage windbox, 

through one of two Roaster discharge quench heads, and discharged into the quench launder. 
Particulate Control: 

Control of particulates in the Roaster off gas is very important and starts with the two stage 

cyclones located on the first and second stages of the Roaster. The exhaust gas from the first 

stage roasting process contains entrained dust particles containing enriched gold that is partially 

roasted because of the relatively short retention time. The material must be returned to the 

Roaster for more complete combustion to avoid gold loss and overloading downstream gas 

cleaning equipment. The first stage collected dust is returned via a stainless steel primary and 

secondary cyclone system before entering the gas handling circuit. The first stage primary 

cyclone returns collected dust back into the first stage of the Roaster reactor. The first stage 

secondary cyclone returns collected dust to the upper interstage transfer box, where the first 

stage underflow exits, for entrance into the second stage Roaster reactor. 

The refractory lined primary and dual, unlined stainless steel second stage secondary cyclone 

removes approximately 95 percent of the entrained dust from the exhaust gas before discharge to 

the interstage box. The exhaust gas from the second stage of the roasting process will contain a 

large amount of dust that tends to be sticky. This dust must be removed from the second stage 

exhaust gas to prevent the interstage windbox from plugging and scale build-up on the gas 

dispersion nozzles called the tuyeres. The second stage primary cyclone returns collected dust 

back into the second stage of the Roaster. 

The primary and secondary cyclones are designed to have control efficiencies between 90 and 95 

percent. The efficiency is highly dependent upon three variables. First, the system pressure drop, 

which is dependent upon the volume of gas flowing through the cyclones. Next, the size of the 

dust particles exiting the Roaster, which is dependent upon the adequacy of the grinding circuit. 

The final variable is the dust loading in the gas stream, which is dependent upon the gas velocity 

in the Roaster and the particle size of the feed. 

Pure oxygen is fed into the bottom of the second stage of the Roaster through the second stage 

windbox. Non-sifting tuyeres, which are gas dispersion nozzles, in the second stage are used to 

distribute gas into the bed. The windbox then distributes the oxygen across a distributor plate to 

provide uniform flow of gas across the second stage bed. The 95 percent oxygen is used to 

fluidize the second stage and complete combustion of the residual organic carbon from the first 

stage burn. The oxygen travels upwards through the second stage bed, into the freeboard, and 

exits through the second stage primary and secondary cyclones where dust is partially captured 

before entrance into the interstage windbox. 

The second stage exhaust gases, which contain approximately 70 percent oxygen, are used to 

fluidize the first stage bed and initiate the combustion of the coal and refractory components in 

the feed ore. Sifting tuyeres within the refractory dome distribute gas into the first stage bed. The 

exhaust from the first stage will contain approximately 20 percent oxygen and discharges to the 

gas handling circuit. The Gas Quench Tower and Entoleter, which follow the cyclones, provide 

additional particulate removal before treatment in the remainder of the gas handling circuit. 

In addition to oxygen, the exhaust gas contains SO2, CO, NOx, and Hg vapor as a result of 

combustion processes and ore composition. The NOx and CO levels are kept in line through 

control of the first stage bed and freeboard temperatures. The SO2, and Hg vapors are discharged 
into the gas handling circuit for removal before discharge to the atmosphere through the stack. 

Particulate Control Work Practices 

Work practices to maintain proper operation of the East and West Roaster's First and Secondary 

Cyclones include the following parameters: 

Maintain temperature in the discharge lines of each cyclone to avoid plugging in the cyclone 

discharge or bridging in the cone valves above 800o

F. 

Maintain the content of Hg in the ore between zero to 30 parts per million (ppm), and determine 

sulfur content in the ore through ore characterization. 

Particulate Control Maintenance 

The Roasters are inspected three times a shift on a daily basis. Inspections include ore sampler 

operating, feed bin levels, visible leaks in the Roasters and cyclones, operation of the coal feed 

system, baghouse operations, Roaster feed valve operation, etc. Weekly and monthly inspections 

and maintenance are conducted by the mechanical and electrical departments as part of 

preventative maintenance measures. 

The SOPs for Roasters, including appropriate routine maintenance of the Roasters and training 

requirements of personnel. 

Particulate Control Set Points 

 

The main set points for the Roaster's primary and secondary cyclones that are integral to the to 

the proper functioning of the gas handling system include the following: 

The temperature settings on the cyclones ore discharge must remain above 800o

F. 

The Roaster first stage freeboard is set at -2.0 inches of WC; 

The Roaster ore throughput is set at 90 tph. This set point is a system cap that controls the 

instantaneous feed rate; and 

The Hg content of the ore must be maintained between zero to 30 ppm, and the sulfur content 

will be determined through ore characterization. 

Particulate Control Alarms and Interlocks 

 

There are several alarms associated with the Roaster and cyclone operation to warn of problems. 

The alarms that pertain specifically to proper functioning of the gas handling are set as follows: 

The Roaster first stage freeboard high alarm is set at 17.5 inches of WC; 

The Roaster first stage freeboard low alarm is set at -27.5 inches of WC; 
The Roaster first stage freeboard deviation alarm is set at 10.0 inches of WC; 

The cyclone ore discharge temperature is set to alarm if the temperature falls below 800o

F. The 

alarm is set to notify the operator of a temperature deviation. The temperature drop is usually an 

indication of plugging in the circuit line, which will need to be cleared. 

An interlock on the Roaster first stage freeboard is set at 17.5 inches of WC. 

Particulate Control Recordkeeping 

The following data is recorded: 

The cyclone ore discharge temperature is logged every hour on the Roaster Area Daily 

Production Log; 

The Hg and sulfur content of the ore is logged once per shift in the Mill Production Report. The 

sample that is used to measure these values is from a composite sample that is collected every 15 

minutes; 

Total ore processed for each hour of operation and the hours of operation of each Roaster circuit 

on an hourly basis; and 

The origin of the ore, coal, or other material, and the amount of each type of material processed 

in the Roaster circuit on a monthly basis. 
Quench Tower 

Hot dust-laden off gas from the Roaster first stage secondary cyclone is pulled to the inlet of the 

gas quench tower, where it is sprayed with fresh or seepage water. The hot gas is cooled from 

approximately 1,200 to 165oF. The cooling step reduces the total gas volume by about 40 

percent. The volume of gas decreases in direct proportion to the absolute temperature and 

indirect proportion to the pressure. This is demonstrated by the ideal gas law, where pressure 

multiplied by the volume of the gas is equal to the number of moles times the universal gas 

constant times the absolute temperature (PV=nRT). Most of the heavier PM, larger than two 

microns in diameter is captured in the water and leaves the quench tower as a dilute slurry. The 

cooled gas stream then enters the Entoleter wet dust scrubber. The resulting slurry from both the 

Entoleter and the gas quench tower discharge to the seal tank. The seal tank serves as a 

barometric seal for the gas quench tower and the Entoleter, both of which operate below 

atmospheric pressure. 

 

The gas quench tower and the Entoleter are located within the Roaster structure above the top of 

the Roaster. Seal tanks are located one floor below the gas quench tower and the Entoleter within 

the Roaster building. Hot (1,100 to 1,200o

F) gas from the Roaster first stage secondary cyclone is 

routed through a 30 inch stainless steel duct to the inlet at the top of the gas quench tower . An 

Inconel 625 gas inlet nozzle projects two inches through the lid to maintain a sharp temperature 

transition into the tower, which minimizes corrosion and solids buildup on the inlet nozzle. As 

the hot gas passes through the inlet nozzle, four water spray nozzles quench the gas stream. 

These nozzle assemblies are located at 90" angles just below the inlet. Each six-inch nozzle 

projects a 12 gpm flat spray perpendicular to the direction of the gas stream. Consequently, the 

gas stream is completely surrounded by a wall of water, which penetrates the gas stream to its 

core. 

There are four additional spray nozzles in the upper section of the Gas Quench Tower that spray 

water onto the vessel walls to minimize corrosion and solids deposition. The hot gas evaporates 

approximately eight gpm of water and are cooled to approximately 165oF in the process. As the 

gas passes through the water sprays, the Gas Quench Tower vessel diameter increases to 48 

inches. Both the cooling and increased diameter serve to slow down the gas from the inlet 55 to 

60 feet per second (fps) to approximately ten to 11 fps. 

Another water nozzle located near the bottom of the Gas Quench Tower projects a water spray 

upwards against the gas flow. Approximately 60 gpm of water is sprayed upwards from this 

nozzle-This fifth nozzle accomplishes the majority of the gas cooling. The spray travels upward 

to the inlet and then falls to the bottom such that heat transfer occurs in both directions. 

Approximately four gpm of water is condensed in the lower section of the Gas Quench Tower so 

that the net amount of water evaporated in this unit is greater than four gpm. The gas leaving the 

Gas Quench Tower is saturated with water at 130 to 140o

F and contains some water mist. Slurry 

discharging from the Gas Quench Tower passes through a six-inch drainpipe to the Seal Tank 

located directly below the Entoleter. The Gas Quench Tower operates at a draft of about -22 

inches of water column differential pressure and requires a barometric (vacuum) seal, provided 

by the Seal Tank. 

Water flow to the upper quench nozzles in the Gas Quench Tower is manually controlled by 
means of a local flow meter and valve located at each inlet. Water flow to the lower spray nozzle 

is controlled by a flow controller, which receives a flow rate input from an orifice plate in the 

three inch water line feeding the nozzle. A temperature controller (TI-48001) located in the gas 

stream leaving the Gas Quench Tower may override the water flow set point. The water flow rate 

to the lower nozzle may be read from the DCS (FI-48001), but the upper nozzle flow rates may 

only be read at the local flow meters. If the water flow rate to the lower nozzles fails, a flow 

switch on the DCS will activate the interlock to the ID Fan which cascades to shut down the 

Roaster. 

High Pressure Water Systems 

Two high pressure water systems service the Roaster Gas Quench Tower and the Entoleter 

circuits. One system involves fresh water pumps located in the fresh water pump-house. This 

system is utilized approximately two percent of the time. The other system involves the seepage 

water booster pump located on the bottom floor of the Roaster building, and is utilized 

approximately 98 percent of the time. The seepage water booster pump provides water flow to 

the Gas Quench Tower and the Entoleter. 

Fresh Water Pump System 

A separate high pressure water pump and pipe feed each Roaster gas handling circuit so that 

failure of one pump will only shut down one Roaster. The three high-pressure booster pumps 

(two primary, one spare) are located on the northwest corner of the fresh water pump house. The 

east pump is dedicated to the East Roaster, the west pump is dedicated to the West Roaster, and 

the center pump is a common spare. Although there is a crossover between the two lines at the 

pumps, this crossover is normally shut and should only be opened at the operation foreman's 

instruction. 

The total water flow for each line can be read from the flow meters directly above the pumps and 

should be 200 gpm. Individual flows may also be monitored on the DCS through FIC-48001 for 

the Gas Quench Tower and FIC-48010 for the Entoleter. Significant deviations (50 gpm) from 

this 200 gpm total indicate a problem in the fresh water supply that must be investigated and 

corrected. 

Seepage Water Booster Pump 

The seepage water booster pump is located on the bottom floor of the Roaster building and is 

used to boost the seepage water pressure to a level high enough to provide 200 gpm flow to each 

of the Gas Quench Towers and 80 gpm to each Entoleter. Individual water flows may be 

monitored on the DCS through FIC-48001 for the Gas Quench Tower and FIC-48010 for the 

Entoleter. Significant deviations (50 gpm) from this 200 gpm total indicate a problem in the 

seepage water supply that must be investigated and corrected. 

Quench Tower Work Practices 

Work practices to maintain proper operation of the quench tower include the following 

parameter: 
Maintain total inlet water flow at 200 gpm. 

Quench Tower Maintenance 

Once per month, or more often, each Roaster is taken offline to inspect the Gas Quench Tower, 

Entoleter, and Gas Scrubbers. The inspection of the Gas Quench Tower includes checking the 

crossover between the Gas Quench Tower and the Entoleter and clean if needed. The sprays are 

checked for proper operation and flow. The piping and drain pipes are inspected and cleaned as 

needed. Any deficiencies are noted and an appropriate work order is written. All problems noted 

are corrected before the unit is placed back into operation. 

A visual inspection is conducted while service is being done for loose or missing nuts/bolts, 

cracks, breaks, defects, leaks, and condition and security of oil hydraulic-lines and wiring. The 

items will be cleaned, inspected, tested, adjusted, tightened, or replaced as required. For the 

pumps, a weekly check of the condition of the shaft seal, a check for excessive heat, couplings 

are checked, and the nozzles are inspected to make sure they are clean and working. 

Quench Tower Set Points 

The set points and normal operating conditions for the Gas Quench Tower, that are integral to 

the proper functioning of the gas handling system are as follows: 

Total inlet water flow at 200 gpm. 

Quench Tower Alarms and Interlocks 

Alarms associated with the Gas Quench Tower operation are as follows: 

The inlet water flow will have a low alarm at 50 gpm and a low-low alarm set at ten gpm. Upon 

warning, the valve and pump operation is checked. 

The water flow deviation alarm is set at 25 gpm. 

The inlet water flow interlock data low flow rate of ten gpm and will shut down the ID Fan. 

Quench Tower Monitoring 

Monitoring for the Gas Quench Tower includes the following: 

Inlet water flow will be monitored to keep at 200 gpm. 

Once per day the totalizer readings will be taken off the fresh and seepage water flow meter 

feeding the Quench Towers and Entoleters. 

Quench Tower Recordkeeping 

The following parameters will be monitored and recorded: 
The inlet water flow rate will be logged every four hours and data entered on the Roaster Daily 

Production Log. 

The totalizer readings will be taken off the fresh and seepage water flow meters feeding the 

Quench Towers and Entoleters and will be recorded on the Water Management Log Sheet. 
Dust Scrubber - Entoleter 

The Dust Scrubber, or Entoleter, is a high efficiency liquid/gas confractor that utilizes a patented 

Centripetal Vortex Contractor to clean contaminated gases of particulate matter before gases are 

discharged to the scrubbing system. 

The Entoleter is located next to the Gas Quench Tower and is connected to it with a short section 

of rectangular duct. Saturated gases from the Gas Quench Tower, laden with mist, dust, SO, and 

Hg vapor enters the Scrubber tangentially near the bottom. Maximum feed to the Entoleter is 

5,200 standard cubic feet per minute (scfm) at 185"F. The inner wall of the lower section is 

protected from abrasion with a ceramic shield. Gas is forced through the stationary vane cage in 

the lower section, where it comes in contact with water from the high-pressure seepage/fresh 

water line. The water spray is broken into tiny droplets by the action of the swirling gas as it 

passes through the vanes of the cage. The water droplets capture dust particles as the gas passes 

up through the Scrubber. The resulting slurry is drained from the bottom of the Entoleter to the 

Seal Tank. The gas and mist pass up into the disengaging section of the Entoleter where the 

water droplets are collected on the vessel walls. The clean gas passes out of the Entoleter through 

the top flange and then travels down through the fiberglass reinforced plastic (FRP) duct to the 

SO, Scrubber. 

Efficient scrubbing by the Entoleter increases with increasing pressure drop. The pressure drop 

across the scrubber is controlled by the vane cage height and, to a lesser extent, by the water flow 

rate. The vane cage height is automatically set by the DCS based on the differential pressure set 

point used in the DCS; however, the vane cage height can be adjusted manually from the control 

room. The primary water flow to the Entoleter is through the upper spray nozzle. This water 

flow, at 40 to 60 gpm, is directed to spray onto the vane cage and is controlled automatically by a 

valve in the control room. The secondary spray nozzles are used to keep the scrubbing unit clean 

and flushed. This water flow, at 1 5 to 20 gpm, is controlled by manual control valves located at 

the scrubbing unit. 

The pressure differential across the Entoleter should be maintained at 25 inches of water column 

(WC). Pressure differential indicator PDI-4801 measures the total pressure drop across both the 

Gas Quench and Entoleter. Allowing eight inches of WC differential pressure for the Gas 

Quench Tower, the total drop for both units should be kept at 33 inches of WC. 

The Entoleter has a total of three drains to the seal tank. Two drains are provided in the lower 

section, while the third is fitted into the upper section. All three of the drains must be kept open 

at all times when water is flowing to the Entoleter. Blockage of any of these drains will result in 

slurry backup into the Entoleter. The center drain is the main drain for the bottom section and 

will normally carry most of the slurry flow. The outboard bottom drain is a standby overflow for 

the lower chamber and will not normally carry any flow. If slurry is flowing from this outboard 

drain it indicates that the center drain is plugged and needs to be cleaned. The drain from the 

upper chamber carries slurry collected on the wall of the upper chamber. Slurry flow rate through 

this upper drain is very low compared to that through the main drain. 
In comparison, the Entoleter out performs the conventional scrubber and is more efficient for the 

designed facility. A Venturi Scrubber was first proposed to be mounted ahead of the low energy 

unit and to be operated at 34 inches of WC differential pressure. A Entoleter was placed in 

service on a trial basis. Subsequent tests proved that the Entoleter operations at I 8 to 1 9 inches 

of WC differential pressure could replace both the proposed Venturi Scrubber and the original 

low energy unit while achieving efficiencies as high as 99.9 percent, meeting the new stricter 

codes. The better efficiency of the Entoleter provides considerable savings in energy and water. 

High Pressure Water System 

Two high pressure water systems service the Roaster Entoleter and Gas Quench Tower. One 

system involves fresh water pumps located in the fresh water pump-house. This system is 

utilized approximately two percent of the time. The other system involves the seepage water 

booster pump located on the bottom floor of the Roaster building, and is utilized upwards to 98 

percent of the time for providing water flow to the Entoleter and Gas Quench Tower. 

Fresh Water Pump System 

A separate high pressure water pump and pipe feed each Roaster gas handling circuit so that 

failure of one pump will only shut down one Roaster. The three high pressure booster pumps 

(two primary, one spare) are located on the northwest corner of the fresh water pump-house. The 

east pump is dedicated to the East Roaster, the west pump is dedicated to the West Roaster, and 

the center pump is a common spare. Although there is a crossover between the two lines at the 

pumps, this crossover is normally shut and should only be opened at the operation foreman's 

instruction. 

The total water flow for each line can be read from the flow meters directly above the pumps and 

should be 200 gpm. Individual flows may also be monitored on the DCS through flow controller 

(FIC-48001) for the Gas Quench Tower and flow controller (FIC-48010) for the Entoleter. 

Significant deviations from this 200 gpm total indicate a problem in the fresh water supply that 

must be investigated and corrected. 

Seepaqe Water Booster Pump 

The Seepage Water Booster Pump is located on the bottom floor of the Roaster building and is 

used to boost the seepage water pressure to a level high enough to provide 200 gpm flow to each 

of the Gas Quench Towers and 80 gpm to each Entoleter. Individual water flows may be 

monitored on the DCS through FIC-48010 for the Entoleter. 

Entoleter Work Practices 

Work practices implemented for the Entoleter include the following to maintain proper operating 

ranges and to ensure proper functioning: 

Maintain the pressure differential drop at 25 inches of WC to maintain proper and efficient 

particulate cleaning. This is accomplished by the DCS automatically adjusting the vane cage 
height. 

Entoleter Maintenance 

Once per month, or more often as needed, each Roaster is taken offline to inspect the Entoleter, 

Quench Tower and Gas Scrubbers. The inspection of the Entoleter includes checking the 

crossover between the Quench Tower and the Entoleter and cleaned as needed. The vane cage is 

inspected and cleaned as needed. The sprays are to be checked for proper operation and flow. 

The fiberglass reinforced piping on the discharge of the unit is inspected and cleaned as needed. 

The drain pipes are inspected and cleaned as needed. Any deficiencies are noted and appropriate 

work orders are written. All problems noted are corrected before the unit is placed back into 

operations 

Entoleter Set Points 

The following are the set points for the Entoleter that are integral to the proper functioning of the 

gas handling system: 

The pressure differential drop is maintained at 25 inches of WC. 

Entoleter Alarms and Interlocks 

The system controls are set up to alarm before gas emission control parameters are outside of the 

required set points to ensure emission standards are maintained as follows: 

The alarms for pressure drop across the Entoleter low alarm is set at 15 inches of WC and the 

high alarm is set at 60 inches of WC; 

The deviation alarms for pressure drop across the Entoleter are set at five inches of WC; and 

The differential pressure will interlock at the low of 15 inches of WC, shutting down the ore feed 

and oxygen flow. 

Entoleter Monitoring 

The following are monitoring practices for the Entoleter: 

Differential pressure will be monitored continuously and logged hourly. 

Control data for the system will be monitored and displayed on the DCS monitors. These data 

will be tracked and logged. Further readings will be taken in the field and the data logged on the 

appropriate forms. 

Entoleter Recordkeeping 

The following parameters will be monitored and recorded: 
Differential pressure will be logged every four hours and the data entered into the Title V log. 

Water flow will be logged every four hours and the data entered on the Roaster Area Daily 

Production Log. 

 

Daily data from the DCS are also logged into the current data historian unit. 
Sulfur Dioxide Scrubber 

The SO2 packed bed scrubber measures five feet in diameter, 32 feet tall, and is constructed of 

0.5 inch thick FRP. Within the Scrubber is a l4-foot bed packed with two inch polypropylene 

Intalox saddles and a 600 gallon sump. 

The SO2 Scrubber is designed to use a dilute (ten percent) soda ash solution to scrub SO2 from 

the quenched waste gas leaving the Entoleter; however, the scrubber does remove some 

elemental Hg, which is collected from the Scrubber reservoir. A lower soda ash content is 

necessary to avoid scaling and precipitation. The sulfur dioxide in the waste gas is absorbed into 

the recirculating soda ash solution. The solution is pumped from the integral sump at a nominal 

rate of 300 gpm to the spray nozzles located above the packed bed. 

Upon entering the SO2 Scrubber, waste gases travel through a duct opening located below the 

packing section which is sloped to allow for liquid drainage into the Scrubber sump. The waste 

gas passes up through the 14-foot packed bed where it is countercurrent with the dilute soda ash 

solution. The soda ash solution is pumped from the nozzle near the bottom of the sump and is 

discharged into the recirculation pumps through a manifold. The amount of sulfur is to be 0.I 

percent by weight of the gas stream with a range from zero to 1.7 percent with approximately 

0.25 percent of the total sulfur being present as SO2. The amount of sulfur that is converted to 

SO2 is determined by utilizing the Mill Blending report, that provides the ore feed number for the 

sulfur/ sulfide content of 0.75 percent, and the monthly reagent report, that has data for soda ash 

consumption, at approximately 4.94 pounds per ton. The calculation is as follows: 

. 4.941bs/ton of soda ash x 64 (molecular weight of SO2) / 106 pounds of SO2: 2.98 lbs of 

SO2; and 

. 2.98 lbs of SO2 X 32 (atomic weight of sulfur) / 64 (molecular weight of SO2) : l.49 lbs of 

sulfur/sulfide. 

Based upon the above calculation, approximately 1.5 lbs of sulfur per ton is converted to SO2. 

Taking into account that 0.75 percent of the ore feed is sulfur/ sulfide, the following is derived: 

. 0.75 (of I ton) x 2000 lbs per ton / 100 : 15 lbs/ ton of the sulfur/ sulfide in the ore; and 

. Since only 1.5 lbs of sulfur per ton of SO2 is produced, 1.5 x 100 / 15 lbs/ton: 10 percent. 

Therefore 10 percent of the sulfur is converted to SO2. No other sulfur species have been 

identified. 

At the discharge manifold the four inch line is converted to a two inch pipe that provides a bleed 

stream discharge to maintain the sump level, this is controlled by the DCS sump level controller 

with a control valve. The main line of the pump returns to the Scrubber spray nozzle through the 

magnetic flow meter. 

The pH of the soda ash solution needs to be below 8.0 to avoid scale formation in the packed 
beds and Scrubber piping. Soda ash that is fed to the SO2 Scrubber is converted to sodium 

bicarbonate. The soda ash solution provides a great deal of buffering thereby aiding pH control. 

As the pH drops, more soda ash solution is needed. Soda ash make-up may be sent to the sump 

or into the recirculation line. The pH controller, controls the soda ash feed rate through a control 

valve. A bypass line allows for manual soda ash flow control while maintenance is performed on 

the control valve. The soda ash make-up flow rate is measured through the magnetic flow meter 

DCS readout as well as the sump level and pH level. The magnetic flow meter provides a side 

stream for the pH probe to allow control of the soda ash make-up rates (1.2 gpm of ten percent 

soda ash at 100oF). 

Differential pressure is measured to detect buildup in the packed bed and is measured by taps 

located in the inlet and outlet ducts. There is significant potential for Na2CO3, scale to form on 

the packing in the SO2 Scrubber, as well as Hg and the Tails Gas Scrubbers. This scale can lead 

to excessive differential pressure in any of the Scrubbers, which may result in an interlock shut 

down of the Gas Handling circuit. The scrubbed gases pass through an eight inch Chevron-type 

mist eliminator screen located at the top of the column before moving to the ID Fan. The mist 

eliminator is designed for 99 percent removal of all droplets greater than 28 microns. Located at 

the top of the Scrubber is the water spray that keeps the mist eliminator from becoming clogged 

and helps trap mist. The water flow rate, targeted at 10 gpm, is controlled by a ball valve and 

local flow meter and is checked each shift. 

There are no threshold values for the SO2 Scrubber to meet. The Hg Scrubber can deal with SO2

being present, in fact in other installations the Hg scrubbing occurs before the SO2 removal. 

Design parameters have been determined for optimal operation of the SO2 Scrubber. The 

recirculating soda ash solution is cooled by a tubular heat exchanger using cold water from the 

seepage or fresh water. The purpose of cooling the solution is to keep the temperature of the 

recirculating solution entering the Scrubber at approximately 85oF. 

The optimization of pH was considered as a result of an evaluation and report completed by 

Focus Environmental, Inc. in December 2007. As discussed previously, effective SO2 removal is 

achieved by addition of soda ash to control pH. This is critical for efficient oxidation of Hg 

because SO2 will act as a reducing agent inhibiting effective oxidation in the Hg Scrubber. The 

chemical reactions occurring in the Scrubber is detailed as follows: 

SO2 + Na2CO3 →Na2SO3 + CO2 and; 

SO2 + Na2CO3 + 1/2 O2→ Na2SO4 + CO2 

These reactions occur best at neutral to slightly alkaline conditions. As a result, an initial target 

pH of 8.0 has been chosen as the set-point pending further optimization of the SO2 Scrubber. The 

pH will be monitored and controlled to ensure efficient removal of SO2. 

SO2 Scrubber Maintenance 

Maintenance for the SO2 Scrubber includes monthly scheduled inspections, cleaning, testing, 
adjusting, tightening, and replacing of parts as needed. Defects recorded in the visual inspections 

will be addressed during scheduled maintenance. Weekly inspections and calibrations are 

conducted on the pH probes. 

SO2 Scrubber Set Points 

The following are the set points for the SO, Scrubber, that are integral to the proper functioning 

of the gas handling system: 

Operating pH set at 8.0 based upon Focus Environmental, Inc. Reports from2007 and 2008; and 

Scrubber solution temperature set at 85oF as a result of the addition of the heat exchanger. 

SO2 Scrubber Alarms and Interlocks 

The alarms for the SO2 Scrubber to notify of necessary parameters out of range are as follows: 

A high pH alarm will trigger at pH of 8.5 and a low alarm will trigger at a pH of 7.5; 

pH deviation alarm is set at 0.2 pH; 

high alarm will warn of the heat exchange temperature out of range at 95oF and an additional 

high-high alarm will trigger at 104o

F; 

A heat exchanger deviation alarm is set at 3o

F; and 

A high alarm to warn of a high pressure drop is set at 20 inches of WC. This was increased from 

the original high alarm of eight inches of WC to match the increase of the operation set point. A 

new low alarm is set at five inches of WC. 

The pressure drop deviation is set at five inches WC. 

The SO2 has interlocks for the following: 

The scrubber will interlock at a low pH of 7.0, shutting down the plant ore feed and oxygen flow; 

and 

The heat exchanger will interlock at a high temperature of 104o

F, shutting down the ore feed and 

oxygen flow. 

SO2 Scrubber Monitoring 

The following are monitoring practices for the SO2 Scrubber: 

SO2 Scrubber differential pressure will be monitored continuously with the DCS and logged 
hourly; 

SO2, Scrubber solution pH control room readout will be monitored continuously with the DCS 

and logged hourly; SO2 Scrubber solution pH field measurement will be monitored and logged 

three times per shift, coincident with the hourly logging of pH in the control room; 

Soda ash injection rate for the entire gas handling system is monitored continuously at the entry 

to the Roaster gas handling circuit; 

 

Soda ash concentration will be monitored once per shift by titration method Precipitates 

(turbidity) in the scrubber solution will be monitored one per shift; and 

SO2 Scrubber solution temperature will be monitored continuously with the DCS and logged 

three times per shift. 

SO2 Scrubber Recordkeeping 

The following parameters will be monitored and reported: 

SO2 Scrubber differential pressure will be logged hourly and the data entered on the Title V Log; 

SO2 Scrubber solution pH control room readout will be logged hourly and the data entered on the 

Title V Log; 

SO2 Scrubber solution pH field measurement will be logged three times per shift and the data 

entered on the Gas Handling Sample Log. This sample will be taken from the recirculation pump 

discharge; 

Soda ash injection rate will be recorded on the Gas Handling Sample Log; 

Soda ash concentration will be determined through a titration conducted each shift and logged on 

the Gas Handling Sample Log; 

Turbidity measurements will be logged on the Gas Handling Sample Log; and 

SO2 Scrubber solution temperature will be logged three times per shift and the data entered on 

the Gas Handling Sample Log. 
Induced Draft Fan 

The ID Fan is a Robinson single-stage surge-less blower that boosts the Roaster gases through 

the off gas cleaning system by increasing the pressure. The ID Fan receives and boosts the gas 

from the SO, Scrubber and discharges approximately 5,000 scfm of gas downstream to the Hg 

Scrubber (Appendix T, page l6). The ID Fan consists of a 300 hp blower that boosts the waste 

gas pressure byapproximately3.T pounds per square inch (psi), increasing the temperature by 

11"F. The pressure boost of 3.7 psi at 10,700 cfm was determined by the Differential Static 

Pressure versus Volume Chart in the Figure 3.2 below. This is an appropriate pressure increase 

for the efficiency necessary for the Gas Handling System. 

There are three water nozzles within the ID Fan unit. Two nozzles are fitted in the ID Fan 

housing and one nozzle is fitted into the ID Fan suction. The SO, Scrubber may discharge gas 

containing sulfuric acid mist. The water spray nozzles function to minimize possible corrosion 

and prevent excessive pressure increases by keeping the gas saturated and cooled. The water 

sprays also reduce solids buildup on the fan blades, which could result in significant shaft 

vibrations. Plant air is fed to the nozzles, atomizing the water spray for more efficient cooling. 

Excess water and gas are blown down a drain in the bottom of the ID Fan housing and 

transferred to a gas/water separator; where the gas is recycled to the blower suction and the water 

is transferred to the Roaster Quench Sump. The ID Fan suction damper maintains a negative 

pressure environment, which is regulated by the Roaster pressure controller. The Gas Handling 

train and Roaster will shut down in response to a failure of the ID Fan. 

The increased gas pressure, initiated by the ID Fan, has two effects on the removal of Hg. 

Increased pressure would correspond with an increased flow rate as well as an elevated quantity 

of Hg vapor at the liquid/vapor interface. However, the increased pressure would also correspond 

with reduced residence time of the gas in the Hg Scrubber. In controlling the pressure, the system 

maintains a desired level of efficiency. The ID Fan will shut down in response to: temperatures 

in excess of 200'F; flow in the Gas Quench Tower than ten gpm; head greater than 60 inches at 

the blower discharge; and vibrations in the blower bearing in excess of four mils. Normal 

operating conditions consist of an average gas flow rate of approximately 5,000 scfm at 9.6 

actual pounds per square inch (psia) and temperature range at the ID Fan suction damper 

between 130 and 140"F. The temperature of the gas discharge ranges between 140 and 150"F 

and approximately 12.5 psia. Fresh water flow to a spray nozzle is one gpm. Turbine bearing 

temperatures must be below 200'F. 

Induced Draft Fan Work Practices 

Work practices implemented at the ID Fan include maintaining the proper function of the fan to 

ensure proper flow rate, vibrations, temperature, unusual noises, draft, washing of the fan, 

resetting the multilin, mister system, interlocks and gas flows. 

Inspections will be completed on a weekly basis, as well as every six months. Weekly inspection 

includes checking oil levels, as well as the temperature and vibration of the shaft bearings. Also 

included in the weekly inspections is the gear reducer's oil level. The semi-annual inspection 

includes a steam clean of the fan and the lubrication of the fan couplers. Work practice 

procedures include visual inspections while service is being done for loose or missing nuts and 
bolts, cracks, breaks, defects, leaks, and condition and security of oil hydraulic-air lines and 

wiring. Defects will be recorded and addressed. 

Induced Draft Fan Maintenance 

Maintenance for the ID Fan includes scheduled inspections, cleaning, testing, adjusting, 

tightening, and replacing of parts as needed. Defects recorded in the visual inspections will be 

addressed during scheduled maintenance. Maintenance is closely related to the monitoring 

procedures. 

Induced Draft Fan Set Points 

There are not direct set points for the ID Fan. The ID Fan operation is controlled by the first 

stage freeboard. However, there are operating ranges that are monitored for the load on the fan, 

turbine bearing temperature, and turbine vibration levels, which are listed in Appendix U in the 

ID Fan section. 

Induced Draft Fan Alarms and Interlocks 

The alarms and interlocks for the ID Fan are identified in Appendix U. The alarms and 

interlocks, which are not recorded, include a high and high-high blower discharge pressure, a 

high and high-high blower bearing vibration, and a high and high-high amperage. 

Induced Draft Fan Monitoring 

The following parameters will be monitored on the DCS, but not recorded as they do not directly 

pertain to the functioning of the gas handling system: 

The ID Fan amperage. 

Turbine vibration of the inboard and outboard bearings 

The temperature of the turbine bearings. 
Hg Scrubber 

Elemental Hg in the waste gas from the ID Fan is further scrubbed out in the Hg Scrubber and 

the cleaned gas is sent on to the Tails Gas Scrubber. Waste gas from the Roaster contains Hg 

vapor derived from mercuric compounds in the ore that oxidize within the Roaster. Most of the 

Hg vapors are condensed in the SO, Scrubber and the Hg Scrubber captures the remainder. This 

waste gas is contacted countercurrent with a dilute, ten percent soda ash solution in the 20-

footpacked bed. The gas contact time in the bed is approximately 5.7 seconds. The pH of the 

Scrubber solution is controlled by addition of soda ash. A bleed stream from the Hg Scrubber is 

combined with the bleed from the SO, Scrubber and sent to the effluent tank and on to the tails. 

The Hg Scrubber and Tails Gas Scrubber are 38 feet tall packed tower, five feet in diameter, and 

have 5/8 inch thick FRP walls. Each tower has a 600 gallon sump, an eight inch Chevron-type 

four-pass mist eliminator with a water spray, and two spray heads for solution recycle (one 

operating, one spare). The towers are rated at six pounds per square inch gauge (psig) pressure 

and 100 inches of WC vacuum. The Hg Scrubber has a 2}-foot bed packed with two inch 

titanium Intalox saddles while the Tails Gas Scrubber has a ten foot bed packed with two inch hiflow polypropylene packing. The Tails Gas Scrubber has upper and lower nozzles for sump 

recycle to allow for future conversion to a 2O-foot packed bed should the need arise. In every 

other aspect, the construction of the Hg Scrubber and the Tails Gas Scrubber is identical. 

The Hg Scrubber solution recycle stream is taken from the four inch nozzle on the sump and sent 

to a header that feeds three recycle pumps. The inside pump is the primary recycle pump for the 

Hg Scrubber. The outside pump is the primary recycle pump for the Tails Gas Scrubber. The 

middle pump is a spare, which can serve either the Hg Scrubber or the Tails Gas Scrubber. Each 

pump operates at not less than 150 gpm. The four inch line from the pump discharge recycles 

solution to one of the spray heads at the top of the tower. The sump level controller operates the 

bleed stream control valve on the one inch line from the pump discharge to maintain the sump 

level. The pH probes are located in the four inch recirculation line. 

Soda ash is fed to the Hg Scrubber sump from the ten percent soda ash makeup tank. The soda 

ash makeup rate is controlled by the pH controller set at 8.0 pH. Raw water is controlled by a 

ball valve and is monitored locally. Raw water is also piped to the Tails Gas Scrubber sump with 

its flow rate monitored locally. Soda ash is fed to the Tails Gas Scrubber sump under the control 

of the pH controller. A high pH alarm is included in this controller to warn of high pH 

conditions, which can result in plugging of the packed bed due to sodium bicarbonate scale 

formation on the packing. The pressure drop across the Hg Scrubber column is measured on the 

DCS. 

Hg Scrubber Work Practices 

Work practices implemented at the Hg Scrubber include maintaining proper operating ranges of 

the following parameters: 

Maintain the differential pressure from the Hg Scrubber gas inlet to Tails Gas Scrubber discharge 

is set at eight inches of WC; 
Maintain the pH setting of 6.9; and 

Maintain the ORP set at 825 mV. 

Hg Scrubber Maintenance 

Maintenance for the Hg Scrubber includes monthly scheduled inspections, cleaning, testing, 

adjusting, tightening, and replacing of parts as needed. Defects recorded in the visual inspections 

will be addressed during scheduled maintenance. Weekly inspections and calibrations are 

conducted on the pH probes. Examples of the maintenance logs with detailed inspection can be 

found in Appendix X. 

Hg Scrubber Set Points 

The following are the set points for the Hg Scrubber that are integral to the proper functioning of 

the gas handling system and are detailed in Appendix U: 

The pH is set at 6.9. 

The ORP set at 825 mV. 

Hg Scrubber Alarms and Interlocks 

Several alarms exist for the Hg Scrubber to warn of deviations in set points as follows: 

The low pH alarm is established at pH of 6.5 and the high alarm at pH of 7.5; 

The pH deviation alarm is set at 0.2pH; 

The ORP will high alarm at 850 mV and low alarm at750 mV; 

The ORP deviation alarm is set at 25 mV; 

The low pressure measured in the Hg Scrubber will alarm at five inches WC and will high alarm 

at 20 inches WC. The alarms were set to meet the proper operating range of the Scrubber; and 

The pressure drop deviation is set at five inches WC. 

The interlocks for the He Scrubber are as follows: The Hg Scrubber deviation of pH at 6.5 will 

interlock the system and shut down the ore feed and oxygen flow. 

The Hg Scrubber ORP deviation at 750 mV will interlock and shut down the ore feed and 

oxygen flow. 

Hg Scrubber Monitoring 

The following are monitoring practices for the Hg Scrubber: 

Differential pressure will be logged daily; 
Solution pH control room readout will be monitored continuously with the DCS and logged 

hourly; 

Solution pH field measurements will be logged three times per shift, coincident with the hourly 

logging of pH in the control room. This sample will be taken from the recirculation pump 

discharge; 

Hg solution ORP control room readout will be monitored continuously with the DCS and 

logged hourly; and 

Hg solution NaOCI concentration will be monitored and logged three times per shift. 

Hg Scrubber Recordkeeping

The following parameters will be monitored and recorded: 

Hg Scrubber differential pressure will be logged daily and the data entered on the Title V Log. 

Hg Scrubber solution pH control room readout will be logged hourly and the data entered on the 

Title V Log. 

Hg Scrubber solution pH field measurements will be logged three times per shift and the data 

entered on the Gas Handling Sample Log. This sample will be taken from the recirculation pump 

discharge. 

Hg solution ORP control room readout will be logged hourly and the data entered on the Roaster 

Area Daily Production Log. 

Hg solution NaOCI concentration will be logged three times per shift and the data entered on the 

Gas Handling Sample Log. 

Daily data from the DCS is also logged into the current data historian unit. 
Tails Gas Scrubber 

The Tails Gas Scrubber is a l0-foot packed bed. As stated in the Hg Scrubber section, elementary 

Hg in the waste gas from the ID Fan is further scrubbed out in the Hg Scrubber and the cleaned 

gas is sent on to the Tails Gas Scrubber. Waste gas from the Roaster contains Hg vapor derived 

from mercuric compounds in the ore that oxidize within the Roaster. Most of the Hg vapors are 

condensed in the SO, Scrubber and the Hg Scrubber captures the remainder. This waste gas is 

contacted countercurrent with a dilute soda ash solution in the 20-foot packed bed. The pH of the 

Scrubber solution is controlled by addition of soda ash. A bleed stream from the Tails Gas 

Scrubber is combined with the bleed from the SO, and sent to the effluent tank and on to the 

tails. 

The Hg Scrubber and Tails Gas Scrubber are 38-foot tall packed towers, five feet in diameter, 

and have 5/8 inch thick FRP walls. Each tower has a 600 gallon sump, an eight inch Chevrontype four-pass mist eliminator with a water spray, and two spray heads for solution recycle (one 

operating, one spare). The towers are rated at six psig pressure and 100 inches of WC vacuum. 

The Hg Scrubber has a 20-foot bed packed with two inch titanium Intalox saddles while the Tails 

Gas Scrubber has a 10-foot bed packed with two inch high-flow polypropylene packing. The 

Tails Gas Scrubber has upper and lower nozzles for sump recycle to allow for future conversion 

to a 20-foot packed bed should the need arise. In every other aspect, the construction of the Hg 

Scrubber and the Tails Gas Scrubber is identical. 

The Tails Gas Scrubber solution recycle stream is taken from the four inch nozzle on the sump 

and sent to a header that feeds 3 recycle pumps. The inside pump is the primary recycle pump for 

the Hg Scrubber. The outside pump is the primary recycle pump for the Tails Gas Scrubber. The 

middle pump is a spare, which can serve either the Hg Scrubber or the Tails Gas Scrubber. Each 

pump operates at not less than 150 glm. The four inch line from the pump discharge recycles 

solution to one of the spray heads at the top of the tower. The sump level controller operates the 

bleed stream control valve on the one inch line from the pump discharge to maintain the sump 

level. The pH probes are located in the four inch recirculation line. 

Soda ash is fed to the Tails Gas Scrubber sump from the ten percent soda ash makeup tank. The 

soda ash makeup rate is controlled by the pH controller set at 8.0 pH. Raw water is also piped to 

the Tails Gas Scrubber sump with its flow rate monitored locally. Soda ash is fed to the Tails 

Gas Scrubber sump under the control of the pH controller. A high pH alarm is included in this 

controller to warn of high pH conditions, which can result in plugging of the packed bed due to 

sodium bicarbonate scale formation on the packing. The pressure drop across the Hg Scrubber 

column is measured on the DCS. 

Tail Gas Scrubber Work Practices 

Work practices implemented at the Tails Gas Scrubber include maintaining proper operating 

ranges of the following parameters: 

Maintain the pH of the solution at7.5; 
No visible precipitate in scrubber solution; and 

Maintain the pressure drop across the Hg Scrubber inlet to the Tails Gas Scrubber discharge at 

eight inches WC. 

Tail Gas Scrubber Maintenance 

Maintenance for the Tails Gas Scrubber includes monthly scheduled inspections, cleaning, 

testing, adjusting, tightening, and replacing of parts as needed. Defects recorded in the visual 

inspections will be addressed during scheduled maintenance. Weekly inspections and 

calibrations are conducted on the pH probes. Examples of the maintenance logs with detailed 

inspection can be found in. 

Tail Gas Scrubber Set Points 

Following are the set points for the Tails Gas Scrubber that are integral to the proper functioning 

of the gas handling system: 

Solution pH of 8.0. The pH of 8.0 is based upon the Focus Environmental, Inc. analysis results 

for the optimal pH of the SO, and Hg Scrubber. 

Tail Gas Scrubber Alarms and Interlocks 

The alarms for the Tails Gas Scrubber necessary to ensure proper functioning for air pollution 

control includes the following: 

 

Low pH alarm has been set at pH of 7.5 and a high alarm at pH of 8.5; 

The pH deviation alarm is set at 0.2; 

The pressure differential across the Tails Gas Scrubber will low alarm at five inches WC and 

high alarm at 20 inches of WC; and 

 

The pressure differential deviation alarm is set at five inches of WC. The Tails Gas Scrubber 

interlocks include an interlock for a pH of 7.0, shutting down the plant feed and oxygen flow. 

Tail Gas Scrubber Monitoring 

The following are monitoring practices for the Tail Gas Scrubber: 

Tail Gas Scrubber differential pressure will be monitored continuously with the DCS and logger 

hourly: 

Tails Gas Scrubber solution pH control room readout will be monitored continuously with 

the DCS and logged hourly; 

Precipitates (turbidity) in the scrubber solution will be monitored one per shift; and 
Tails Gas Scrubber solution pH field measurements will be logged three times per shift, 

coincident with the hourly logging of pH in the control room. This sample will be taken from the 

recirculation pump discharge. 

Tail Gas Scrubber Recordkeeping 

The following parameters will be monitored and recorded: 

Tails Gas Scrubber differential pressure will be logged daily and the data entered on the 

Title V Log; 

Turbidity measurements will be logged on the Gas Handling Sample Log; and 

Tails Gas Scrubber solution pH field measurements will be logged three times per shift and the 

data entered on the Gas Handling Log.