Carbon Stripping

CARBON STRIPPING THE PRACTICAL ALTERNATIVES

Presented at the International Gold ExpoSeptember 7, 1989Reno. NevadaJohn L. Fast, P.E.Consulting Process EngineerDenver Mineral Engineers, Inc.

Many methods are used commercially for recovering gold and silver from loaded activatedcarbon. The major processes include:

(1) Atmospheric Zadra Stripping

(2) Pressurized Zadra Stripping

(3) Anglo American Research Laboratory (AARL) Method

(4) Alcohol Stripping

(5) Micron Elution Procedure

Each process is briefly described. Variations to and combinations of the basic methodsare also highlighted. The advantages and disadvantages of each procedure are discussed.

I. INTRODUCTION

Activated carbon has long been known to exhibit a strong affinity for the extraction ofgold from cyanide solutions. It was not, however, until methods for chemically desorbinggold from loaded carbon were developed, that the process came into widespread use for goldrecovery from ore. These procedures allowed the carbon to be recycled for further goldrecovery.

Today, many options are available to the recovery plant designer and operator for thestripping of gold from carbon. Each method has advantages and disadvantages, which shouldbe evaluated when deciding which process to use.

II. ATMOSPHERIC ZADRA STRIPPING

Atmospheric pressure Zadra stripping was the first commercially successful processdeveloped for stripping gold from carbon. The process was developed by J. B. Zadra, andothers, at the U.S. Bureau of Mines (USBM) in the early 1950s. The results of theirresearch were first applied at Golden Cycle Gold Corporations Carlton Mill at Victor,Colorado in 1951. The results of Zadras work were published by the USBM as RI #4843 (1).This publication is still in print and is actually the foundation for the other strippingprocesses. The process is still widely used today because of its simplicity.

The Zadra process consists of circulating a 1% sodium hydroxide and 0.1% sodium cyanidewater based solution upflow through a stationary bed of loaded carbon at a flow rate ofabout 2 bed volumes per hour at about 200 deg-F. Gold that was previously adsorbed on thecarbon as a sodium or calcium/gold cyanide ion pair (2) is desorbed from the carbon by areversal of the adsorption kinetics. Gold is recovered from the pregnant strip solution by electrowinning onto steel wool.

The gold depleted solution is then reheated and recycled to the carbon bed for reuse(see Figure 1).

The process generally takes about 48 to 72 hours. Typically the gold content of carbonis reduced from 150 oz Au/ton of carbon to less than 3 oz. Au/ton of carbon.

The Zadra process is characterized by simplicity of system design and operation. Mildsteel equipment is normally used. Manual control is the standard. Fluctuations in flow and temperatures can reduce stripping efficiency but the only adverse effect is an extension of the required stripping cycle time. After the desorption vessel has been filled with loaded carbon and solution flow is started, the only operator attention required is periodic system checks typical of any process plant operation.

The main disadvantage of the original Zadra process is its low rate of desorption. Itis much slower than the alternatives. This necessitates larger carbon inventories andlarger equipment than other faster processes.

Stripping temperature is the most significant operating parameter so solutions are keptas close to boiling temperature as is practical. Since many mines are at high elevations,with resultant low boiling points, the reduction in stripping rate can be significant when compared with operations at near sea level altitudes.

Buildup of miscellaneous ions in solution after continued recycling also reducesstripping efficiency. To alleviate this problem, most operations routinely bleed afraction of their strip liquor inventory and replenish with fresh solution.

The efficiency of the electrowinning cells is also significant to stripping efficiency.High levels of gold in recycled eluant result in a reduction in stripping rate asillustrated typically by Figure 2(3).

III. PRESSURE ZADRA STRIPPING

Continued research at the USBM revealed that the Zadra process stripping rate could beincreased greatly by stripping at higher temperatures (4). A comparison of the increase in stripping rate with temperature is shown in Figure 3. To operate at higher temperatures, the process must operate at pressures higher than the vapor pressure of the solution. High pressure operation is accomplished by means of a high pressure solution pump and a stripping columnpressure control regulator.

In practice, a solution containing about 1% sodium hydroxide and 0.1% sodium cyanide atabout 280 deg-F and 65 PSIG, is circulated through a pressure vessel filled with loadedcarbon at a flow rate of 2.0 bed volumes per hour. The time required for pressurestripping is generally from 10 to 14 hours.

Barren strip solution is typically pumped through a heat recovery heat exchanger and asolution heater. The solution then flows up through the bed of carbon and overflows nearthe top of the stripping vessel. The solution is cooled by exchanging heat with barrensolution and flows through a back pressure control valve, to the pregnant solution holding tank. Pregnant solution is pumped from the pregnant solution tank through electrowinning cells where gold is recovered by electrolysis. Barren solution is then returned to the barren solution tankfor recycle (see Figure 4).

High temperature limits are generally constrained by pressure and temperaturelimitations of system components, such as vessel design pressures and gasket temperaturelimits. USBM research indicated that increases in stripping efficiency could be achievedup to 356 deg-F. Above 356 deg-F cyanide was decomposed and metallic gold precipitated inthe carbon. Plant practice generally indicates that about 300 deg-F is the upper limit for maximum stripping efficiency.

Pressure stripping columns are normally sized with a height to diameter ratio of about4 to 1. Internal solution distributors and collectors are used to provide even flow ofsolution throughout the carbon bed. The majority of pressure strip vessels are constructed from stainless steel, but many carbon steel vessels are providing satisfactory service.

Solution flow rate has little effect on stripping efficiency in the range of 1 to 4 bedvolumes per hour. Low solution flow rates produce slightly higher efficiencies in mostcases, but the increase is not significant. Stripping efficiency decreases as flow ratesare increased above 3 to 4 bed volumes per hour. The design solution flow rate isgenerally based on a compromise between reduced elution time and increased equipment costs at higher flow rates.

Most columns are operated with upflow of solution, but some plants have selected toelute by downflow. The advantage to downflow is reduced potential for binding of flowdistribution screens by tramp material in the carbon. Upflow operation means that thecarbon bed is always flooded, and insures that the carbon is continually contacted bystrip solution.

The extent of instrumentation is generally determined by operator preference. Automaticsolution temperature control and column back pressure control are the minimum automationrequired.

Solution bleeding is required to prevent the buildup of contaminants, which reducestripping efficiency. The amount of solution bleed required varies from about 1/3 of theeluant volume per cycle, to as low as the residual eluant on the carbon during carbontransfer. Control of the amount of solution purged from the system is done either on aroutine scheduled basis or by monitoring stripping efficiency and bleeding as efficiencydrops.

OPERATING SCHEDULE:

The following is a typical operating schedule for a Pressure Zadra stripping cycle:

SOLUTIONTIME

Load ColumnTransfer Water90 minutes

Elution0.1% NaCN, 1% NaOH480 minutes

Carbon CoolingFresh Water60 minutes

Unload ColumnTransfer Water30 minutes

TOTAL11 hours

IV. AARL STRIPPING

The Anglo-American Research Laboratories (AARL) stripping procedure (5) was first usedon a large scale in 1980 at the President Brand Gold Mine in South Africa. Since thattime, its application has become standard practice in South Africa and Australia.

The process involves a series of procedures generally starting with an acid washfollowed by a water wash to remove residual acid. The carbon is then soaked for about 30minutes in a solution containing about 3% sodium cyanide and 1% sodium hydroxide. Highquality fresh water at about 230 deg-F is then pumped through the pressurized strippingvessel to produce the pregnant eluant. Gold is recovered from the pregnant eluant byelectrowinning and the barren eluant is discarded (See Figure 3).

It is interesting to note that the Zadra stripping procedures researched by the USBM,originally envisioned presoaking carbon with a caustic cyanide solution followed byelution with deionized water. This idea was discarded in practice, in favor of the simplerone step caustic cyanide elution.

The main advantageof the AARL process is the ability to strip a batch of carbon to low gold residuals in an8 hour shift. This offers the potential of either designing for multiple stripping cycleseach day and reducing equipment sizes in new plant design, or increasing daily capacity inexisting mills by stripping on additional shifts each day.

Disadvantages of the AARL process include the requirement for high quality water, highwater consumption, the potential for mixing acid with cyanide, and the necessity forautomated controls.

ACID WASHING

With the AARL process, elution is normally proceeded by acid washing the carbon. Acidwashing is used with all of the other stripping systems, but it is mentioned specificallywith the AARL procedure, because AARL systems generally use the stripping vessel to acidwash and acid washing is, therefore, controlled as part of the stripping sequence. Acidwashing has been shown to typically increase the efficiency of AARL stripping.Hydrochloric acid is always used in AARL systems with concentrations generally around 3%by volume.

Acid washing is currently being done both in a separate vessel from the strippingcolumn and in the elution vessel. Factors favoring acid washing in the elution vesselinclude: (1) eliminating a carbon transfer which reduces gold losses from fine carbonbreakage of loaded carbon, (2) conservation of heat if hot acid washing is employed and(3) reduction in stripping cycle time by eliminating a carbon transfer. Factors favoringinstallation of a separate acid wash vessel include: (1) less potential for mixing acidand. cyanide in the event of operator error or equipment malfunction, and (2) lessrigorous requirements for materials of construction in the stripping circuit since acidproof equipment is not required.

Following acid washing, the carbon is rinsed with fresh water to prevent acid andchlorides from entering the strip circuit.

PRESOAK

The presoak step is started by preheating the carbon with hot water. This is followedby soaking the carbon bed with hot (90 deg-C) 3 WT% NACN/1 WT% NAOH solution for about 30minutes. Reduced elution efficiency is experienced if soak solutions concentration areless than 3% NACN but elution rates remain relatively constant with soak solutionconcentrations above 3% NACN. Changes in the duration of soaking time, for most carbons,has little effect on stripping efficiency.

ELUTION

Elution is generally performed using about 6 bed volumes of good quality water at arate of about 2 bed volumes/hour.

The quality of water used has a substantial effect on stripping efficiency with theAARL procedure. The implementation of a hot acid wash step has been shown to reduce waterquality requirements to a certain degree.

The stripping efficiency is virtually independent of eluant water flow rate in therange of 1 to 5 bed volumes per hour. Selection of design and operating flows is made onthe basis of equipment costs and time constraints

Eluant water temperature has a very significant effect on stripping efficiency.Operation at 236 deg-F requires operating pressures of 10 to 15 PSIG to prevent flashingsteam in the system. Operating temperature limits of 236 deg-F are widely specifiedbecause of temperature limitations of the butyl rubber lining material utilized to linethe strip vessel. Higher temperatures also accelerate the decomposition of cyanide.

The last bed volume of eluant water is generally introduced at ambient temperature tocool the carbon for transfer out of the column.

PROCESS CONTROL

Due to the timed cyclical nature of the procedure, a programmable logic controller(PLC), automatic pump starting and automatic valves are generally used to time andsequence the system. Carbon is loaded and transferred manually, but the sequencing ofvalves and pumps during the strip cycle is controlled by the PLC.

ELECTROWINNING

Electrowinning of gold from the pregnant solution is done on a batch basis. Thesolution pH is increased to 12 by the addition of sodium hydroxide and electrowinning isstarted. Solution is circulated through electrowinning and back to the pregnant solutiontank until acceptable barren levels are achieved. The solution is then discarded.

OPERATING SCHEDULE:

The following is a typical operating schedule for an AARL stripping cycle:

SOLUTIONTIME

Load columnCarbon90 minutes

Acid Wash3% HCl20 minutes

Water RinsePotable Water90 minutes

Pre HeatPotable Water30 minutes

Pre Soak3%NaCN,1%NaOH30 minutes

ElutionPotable Water180 minutes

CoolingPotable Water30 minutes

Carbon TransferTransfer Water30 minutes

TOTAL7 hours 50 minutes

V. ALCOHOL STRIPPING

Further research at the USBM showed that the atmospheric pressure Zadra stripping cyclecan be made to operate much faster by the addition of alcohol to the strip solution (6).

Figure 6 shows the dramatic laboratory results obtained by adding 20% ethyl alcohol toa Zadra solution. Several different alcohols were investigated. Methanol, ethanol, andIsopropanol were all found to increase the gold desorption rate. Ethanol and methanol werefound to perform almost equally, but were substantially better that Isopropanol.

In plant operation alcohol stripping normally requires about 12 to 16 hours to stripcarbon to less than 3 oz. Au per ton of carbon. This is achieved at flow rates in therange of 2 bed volumes per hour operating in series flow with electrowinning cells.

The main drawback to the alcohol stripping process is the potential for fires. Fireshave been reported at several alcohol stripping operations. The electrowinning section isespecially vulnerable to fires because of the potential for sparks.

Ethanol is generally used rather than methanol. This is due to ethanols greatly lowerhealth risks from exposure to vapors. There are, however, isolated examples of operationsusing methanol.

Ethylene or propylene glycol are frequently used, rather than alcohol, to increase thespeed of atmospheric pressure Zadra stripping (7). Typical strip times with glycol are 24to 36 hours.

Glycols are generally used, rather than alcohols, because they are virtuallyuninflammable. The disadvantages of glycols are their inferior strip rate increase andhigher costs.

A typical glycol stripping solution contains 20 to 25 wt% ethylene or propylene glycol,and 2 wt% sodium hydroxide. Sodium cyanide is sometimes added to the solution but it isfrequently unnecessary. The solution is heated to about 190 deg-F and pumped through thecarbon stripping vessel at a flow rate of about 2 Bed Volumes per hour. Gold and silvervalues are recovered from the pregnant solution by electrowinning and the barren solutionis reheated and recycled through the stripping vessel. Glycol consumption is typically inthe range of 20 to 40 gallons, per ton of carbon stripped.

VI. MICRON STRIPPING

The most recently developed stripping procedure being used commercially was developedat Micron Research, in Australia (8). The Micron method involves pretreatment of loadedcarbon, with a caustic cyanide solution followed by elution with an alcohol mixture.

The Micron elution procedure takes advantage of the enhanced stripping rate achievedwith alcohol, but confines the alcohol to the closed stripping unit. Fire dangers arereduced quite substantially, as the pregnant eluant that is subsequently processed forgold recovery does not contain alcohol.

The elution unit is configured like a packed bed distillation tower with a heater onthe base of the column, an overhead condenser, a reflux pump and the loaded carbonfunctioning as the tower packing.

Loaded carbon is first presoaked with sodium cyanide/sodium hydroxide solution. Thepresoak solution is drained from the carbon bed and an alcohol solution is added to thevessel.

The unit is then switched to the batch distillation mode. Within a few hours, thealcohol is concentrated in the overhead condenser tank. The tower bottoms solution is thenfree of alcohol and loaded with gold solution which has been stripped by the refluxingaction in the column.

The Micron process consists of the following operations (See Figure 7):

(1) Presoak

The carbon is first soaked in a solution of 1 to 2 % sodium hydroxide and 5 to 10 %sodium cyanide at ambient temperature. Carbons with particularly high concentrations ofgold, silver, or copper may require solutions containing up to 20% sodium cyanide. Thesolution is then drained from the carbon until free of excess moisture.

(2) Desorption

About 0.5 Bed Volume of alcohol is added after the carbon bed has drained. Methylalcohol is used in the majority of applications, but ethanol is occasionally applied.Acetonitrile may be substituted for the alcohol, but its higher cost generally discouragesits use.

Heat is then applied to the base of the desorption vessel. Organic vapors rise throughthe carbon bed and are condensed in the overhead condenser. The condensate is pumped backto the top of the carbon bed and is sprayed on the carbon. The downflowingcondensatewashes the gold values from the carbon particles into the boiler section below.

(3) Alcohol Recovery

When desorption is completed, as indicated by gold solution concentration reaching aconstant level in the column boiler, alcohol recovery commences. The condensate recyclespray is stopped and the alcohol is allowed to boil out of the pregnant solution. Theboiling is terminated when the temperature in the boiler rises to the boiling point of thewater solution. The pregnant liquor is then drained from the desorption vessel. The carbonis then steam stripped to recover residual alcohol.

The Micron process produces a very concentrated eluant free of alcohol, with gold andsilver values two or three times higher than those in the loaded carbon. This is in directcontrast with the Zadra and AARL procedures, which produce eluant concentrations one ortwo orders of magnitude lower. The high solution grades make recovery methods such aschemical precipitation and aluminum foil electro-deposition very attractive.

The micron eluted carbon also appears to have a somewhat higher level of activity thancarbon eluted by other methods. This may reduce the need to reactivate carbon asfrequently at some operations.

The entire stripping cycle takes about 8 hours. Over 20 licenses for this process havebeen issued, but none are in the United States.

VII. VARIATIONS

There are numerous variations to, and combinations of, the basic processes asillustrated by the following:

(1) Glycol or alcohol are sometimes added to pressure strip or AARL strip operations toincrease stripping rates.

(2) A caustic/cyanide presoak may be used in a Zadra system.

(3) A hot water wash is sometimes used at the end of a pressurized Zadra strip to gainsome of the advantage of the water elution used in AARL stripping.

(4) In situations where large amounts of copper load onto carbon with the gold andsilver, a two stage strip may bebeneficial. Copper may often be selectively eluted with a cold caustic/cyanide solution.This is then followed by one of the standard stripping methods.

(5) Carbon is normally stripped batch wise, but moving bed continuous elution systemshave occasionally been used with both Zadra and AARLprocedures.

(6) Electrowinning may be done under pressure to avoid repressuring solution on eachpass through the carbon.

(7) Zinc precipitation may be substituted for electrowinning.

VIII. CONCLUSIONS & RECOMMENDATIONS

Each new project should be evaluated individually to determine the best procedure forthe particular ore and site specific circumstances.

In general atmospheric pressure Zadra stripping is favored for smaller projects wherethe increased size of equipment can be justified by a simplification of the system. It mayalso be preferred for areas where extreme ease of maintenance and operability are apriority due to a lack of skilled manpower.

The pressurized Zadra system has recently been the preferred process in the UnitedStates for most medium to large sized projects. This is due to its significant costadvantage over atmospheric pressure Zadra systems.

The AARL process is the preferred process in Australia and South Africa, except wherewater balance or water quality problems exist. Several recent United States projects havealso elected to use AARL systems. The AARL process should definitely be considered forlarge projects with sophisticated operators.

Alcohol stripping has generally fallen out of favor due to the flammability concerns.Glycol stripping frequently is used to increase capacity in existing Zadra operations, buteconomics usually favor conversion to pressure stripping if continuing operation isplanned.