CALGON CARBON CORPORATION

The Mining Record
The Voice of the Mining Industry

By William D. Faulkner, Manager of International ,Marketing Calgon Carbon Corporation

Until the present decade, the Merrill-Crowe cyanide leach, zinc precipitation method was the dominant processing technology for gold-bearing ore not amendable to flotation or other physical recovery processes. However, the recent trend is to recover precious metals through cyanide leaching combined with adsorption on granular activated carbon.

Of the top ten U.S. gold producers, eight now use activated carbon in pulp and/or in columns. Within the past year, the following new gold recovery plants in North America have selected the carbon approach to gold recovery: Golden Sunlight (Montana); Getty Mining (Utah); Noranda (California); Amoco/Dome (Ontario, Canada).

Before looking at the two process approaches - application of granular carbon directly to pulp, and application in columns to pregnant leach solutions from heaps or vats - it would be worthwhile to scan the history and development of adsorption media for precious metal recovery.

History

Interest in adsorption for precious metal recovery goes back almost a full century. Wood charcoal was first used commercially at the Yuanmi Mine in Western Australia in 1894. The technique involved passing pregnant solution through barrels of charcoal and then burning the charcoal to recover the adsorbed gold.

During the 1930s and '40s, the University of Arizona's T. G. Chapman investigated several adsorption and screening/flotation methods with finely ground granular carbon. Eagle Picher worked on "stagnant pool leach" in the 1940s. The major drawback to these early systems was the absence of a known way to strip the gold and recover the carbon for reuse.

By the early 1950s, J. B. Zadra of the U. S. Bureau of Mines introduced low-cost procedures for desorbing gold and silver from carbon, whereby the carbon could be recovered and recycled to the adsorption circuit, and the precious values could be recovered by electro-winning (onto steel wool) from desorption solutions.

The "Zadra Process", along with other stripping techniques involving variations of temperature, caustic (sodium hydroxide) and cyanide, opened the way for wider acceptance of carbon adsorption for precious metal recovery. The Golden Cycle Carlton Mill in Cripple Creek, Colorado, was the first modern Zadra/carbon-in-pulp (CIP) operation. In 1970, Cortez Gold used commercial scale heap leach and carbon adsorption at its operation in Nevada; Cortez later (early 1981) also adopted the carbon-in-leach (CIL) process. However, the August 1973 startup of Homestake's CIP plant in Lead, South Dakota was the first carbon in pulp operation to really gain worldwide attention.

Carbon Evolution for Gold Recovery

Although activated carbon can be produced from a variety of base materials, including nutshells, fruit pits, wood, bituminous coal and coconut shells, coconut shell type granular carbons are used almost exclusively for gold recovery. Typically, 6 x 12 or 6 x 16 mesh coconut carbons are used in pulp, and 6 x 16 or 12 x 30 mesh are used in columns.

Coconut base and other shell base carbons were essentially the only commercial granular carbons available in North America in the 1930s and early '40s. At that time the only commercial use for granular carbon was in military gas masks.

Metallurgical investigators of that era found that the gas mask shell based carbons worked better than "homemade" variety carbons for gold recovery. So they proceeded to develop and improve gold recovery processes.

Around 1970, the U.S. Bureau of Mines investigated bituminous coal based carbons for precious metal recovery and found that, from an adsorption standpoint, they compared favorably with the best coconut grades. However, the general perception was that coconut carbons stand up better in a pulp circuit than do other base material carbons. Since coconut carbon was available and worked well, there was little incentive to further qualify the capability of other base materials. To complete the historical picture, at least one significant CIP circuit used bituminous coal base carbon. Climax Molybdenum used 8 x 20 mesh bituminous to recover oxidized moly from flotation plant tails in a 16-million-dollar plant at Climax, Colorado.

As coconut carbons evolved from the military gas mask program, it comes as no surprise that manufacturing specifications for coconut carbon can be traced to the U.S. Government Chemical Warfare Service. After World War II, coconut carbon found significant use in other applications, such as catalyst support, cigarette filters, solvent recovery, and odor control. But none of these uses related to metal recovery, or even to liquid phase technology used in metal recovery.

These factors contributed to a mismatch between orientation of coconut carbon suppliers and the needs of gold process metallurgists. Unknowingly, the carbon manufacturers followed the traditional targets of carbon surface area and pore volume. The metallurgists hoped those goals would result in product consistent for gold recovery. This did not happen. Research in the late 1970s by Calgon Carbon Corporation showed essentially no correlation between the traditional parameters for surface area and pore volume with adsorption capacity for gold cyanide. The same has been reported by Dr. Ray Davidson of Anglo Research Laboratories. Calgon subsequently found out how to control gold adsorption capacity, and now markets new types of coconut carbons - GRC-11, GRC-11F, and GRC-22 - tailored to gold adsorption specifications. Carbon manufacturers now pay special attention to attrition and carbon undersize characteristics - aiming to keep these at absolute minimums.

CIP and CIL

If a gold recovery process entails sequential leach then adsorb steps, it is called carbon-in-pulp; if the process involves a simultaneous leach and adsorb step, it is called carbon-in-leach. In either case, coconut carbon is added directly to the pulp and advanced through a minimum of four agitation tanks countercurrent to pulp flow. The original practice involved air-lifting pulp and carbon to external vibrating screens, allowing the pulp to proceed to the next tank and the carbon to return to the same tank, or be advanced one tank as needed. Today, favored practices involve the use of air swept, stationary launder or peripheral screens in/on each adsorption tank. This allows the carbon to remain and circulate in each tank as the pulp flows through. Periodically, and in some cases continuously, a portion of the pulp and carbon is air-lifted or pumped upstream, countercurrent to the pulp flow.

Next, carbon loaded with 40-250 ounces of gold per ton of carbon is removed from the circuit, washed and then stripped of metal values down to a residual level of 1-3 ounces gold per ton of carbon. Gold values stripped from the carbon are electro-won, or cemented from the solution with zinc. Following carbon regeneration, the carbon is returned to the circuit.

The CIL circuit originally evolved for use on gold ores that contained preg robbing substances (such as natural adsorptive carbon or clays). By adding granular carbon as leaching occurs, gold recovery is substantially improved. There is growing consideration of CIL or hybrids of CIP and CIL processes for other than just ores that demand a CIL circuit. By combining the leach and adsorption circuits into one, around 25 percent capital may be saved on this portion of the plant. In simple terms, it must be understood that ore characteristics play a major role in proper selection of the best recovery process.

Comparisons with Merrill-Crowe

This focus on CIP and CIL does not mean that the Merill-Crowe zinc dust precipitation method is totally out of the picture. There are situations where both carbon and Merrill-Crowe technologies are comparable in total costs. And, as noted in AIME panel discussion by G.Potter, "....Every gold and silver plant, even if you are using Merrill-Crowe, should be using a carbon column somewhere in the system to scavage precious metals from bleed streams and tailings return water."

Capital and operating costs have been compared with Merrill-Crowe by a number of investigators. Dr.Chris Flemming of the National Institute of Metallurgy, South Africa, summarized these findings for North America and South Africa in the chart below.

Dravo Corporation evaluated overall costs as a function of solution grade and determined that CIP was favored up to 0.46 oz./ton.

The single most significant advantage to CIP or CIL flow sheets (on finely ground ores compared with Merrill-Crowe) is the absence of a liquid/solid separation step. Without washing or filtering the pulp, dissolved gold in the pulp tailings on the order 0.001-0.0005 oz./ton of solution, and sometimes less, is realized. These residual levels are achieved regardless of soluble head grade.

Another advantage realized in several plants is better leaching efficiency. The carbon seems to act as a reaction promoter (not a catalyst in the true sense) causing more gold to dissolve. Up to 0.003 oz./ton of ore additional leaching across the adsorption circuit has been reported.

To round out the comparison of CCD-Merrill-Crowe against carbon based systems, one should consider:

1. Carbon systems don't require solution deaeration.

2. Zinc cementation is sensitive to alkalinity and cyanide. Zinc, in the absence of excess cyanide, forms and insoluble hydroxide layer, stopping or impeding its reaction with gold.

3. Trace impurities can significantly affect the efficiency of zinc precipitation; i.e., soluble sulfides, arsenic, antimony, nickel.

There are, however, a few drawbacks to the carbon process. The carbon processes usually require pre-screening of all pulp to keep wood or other extraneous material out of the circuit. Cyanide is not regenerated in the carbon process. A fair amount of gold is held up by the carbon in the circuit, thereby having an impact on cash flow.

Cost Factor

Carbon in Columns

Granular carbon also is employed in columns in gold cyanide circuits on clear, or relatively clear solutions. In a CIP plant where cyanide and lime are added in the ball mill significant gold will be in the thickener overflow. Therefore, the flow sheet will frequently provide for carbon columns on thickener overflow, and CIP for the thickened pulp.

In heap or vat leach systems, the pregnant solution resulting from percolation of cyanide solution through the ore or waste becomes the feed to the carbon columns. In a column application, granular carbon can produce barren solutions on the order of 0.0001 oz. gold/ton of solution.

The head solution grade fundamentally influences maximum possible loading on the carbon. On tailings dam return water (where gold is barely detectable), the loading may reach 30 oz./ton of carbon. With zinc press filtrate, higher values of 40 to 60 oz./ton can be reached. With a feed of 1 ppm gold and a ton grade carbon, a theoretical loading of 900 oz. gold/ton can be achieved. However, this operational level is rare because: (1) other ions in plant solution compete to some degree with gold for adsorption, particularly for lower activity carbons; (2) deposited calcium salts or incomplete carbon regeneration interfere with recovery; (3) risk, security and cash flow factors favor operating at lower levels. Typical operating levels for a CIP circuit are 100-250 oz/ton. For a CIL circuit, the loadings are lower, usually in the range of 60-100 oz./ton.

Silver

While this discussion has been oriented towards gold, silver should not be ignored. Silver cyanide also adsorbs on activated carbon, though not as well as gold. But gold and silver will adsorb together. When silver is present along with gold, the process will possibly operate on a silver-loss basis, since significant soluble silver in the tails will likely rise first. It depends on the balance of silver to gold.

If allowed, gold will displace silver from activated carbon. Zadra showed that when working with pregnant Getchell Mine pulp, carbon could be loaded to 640 oz. gold/ton. Silver reached a maximum loading or 63 oz./ton as the gold loading approached 350-450 oz./ton. As the gold loading continued to rise, silver backed off to a level of 2.5 oz./ton at the end of the experiments. This is not uncommon for adsorption systems dealing with multiple component adsorbates. At least one mine in North Africa uses a CIP plant to process silver ore.

Conclusion

Significant advances have been made in the still evolving carbon-related technologies for gold and silver recovery. What we have experienced in the last decade is great advancement in knowledge of the adsorption process and carbon specifications as they relate to gold recovery. We know more about the balancing of pulp residence time, carbon concentration, carbon loading, and soluble tails. Also, the question of what to do to the carbon from the time it is removed from the adsorption circuit until it reenters the circuit is now receiving high priority with carbon manufacturers and metallurgists.

Finally, mechanical improvements are resulting in lower power costs, more flexible and efficient agitation, gentler, lower cost pulp/carbon separation techniques, and lower capital costs for regeneration.

Reprinted with permission from the February 8, 1984 MINING RECORD.