Table of Contents
Acid-washed carbon is loaded into the top of the column from a feed vessel and allowed to rill into the feed hopper. The carbon flow from the feed vessel is automatically choked and will only feed as carbon is withdrawn from the base of the column. There is sufficient surge capacity in the feed hopper to allow recharging of the column without interruption to the process.
Comparison of Batch and Continuous Elution Systems
For the Continuous Elution process to be adopted, it must demonstrate clear advantages over the existing batch systems. The comparison is against a batch AARL system having a separate acid wash column.
Mechanical
The continuous column design is simpler than the batch because the former is carried out at atmospheric pressure. For carbon discharge, only one control valve is required compared to three valves for batch elution.
Equipment such as pumps and heaters are only one sixth the size of those in a batch system of equivalent capacity.
The control circuit is simpler, no logic controller is required for sequencing solution flows.
In terms of plant availability, this would depend upon the amount of flexibility designed into the continuous system.
Operational Comparison
The most operable elution plant design incorporates a single batch column which allows the acid wash and elution processes to be carried out in the same column. This system is ideal for the small elution plants treating up to 2 tons of dry carbon per batch. The combined process is easily automated and a full cycle takes less than six hours in total. The only restriction is on the butyl rubber lining used in the column. The maximum recommended temperature for this lining is 110 dec. C which is the optimum operating temperature for the AARL elution. Suitable alternative column materials are available, but the coat is prohibitive. Relining the smaller columns does not usually present a serious problem. Column sizes above two ton capacity, however, are more difficult to remove and repair. This is one reason the trend on larger plants is to separate the acid wash and elution process with two separate columns.
Another argument for the use of two columns is to prevent inadvertent mixing of acid and cyanide. The point is not strictly valid as acid can be transferred between columns with the carbon. There are also effective ways of isolating the acid and cyanide lines to a single column using the ‘atop and block’ valve to detect leakage.
Another point in favor of the two column system is that acid washing can be carried out at lower temperatures than elution, thus allowing a large range of materials to be used for the acid wash column.
The ACE plant requires a separate acid wash facility irrespective of its capacity. It therefore only compares favorably from an operability standpoint with the two column batch plant.
Metallurgical Results
The carbon used for the initial evaluation of the metallurgical performance of the system was not typical of that generally used in gold plants. It was an extruded carbon of low activity, further compounded by a high silver content of 4,500 ppm with a gold loading of 1,400 ppm.
The normal elution system for treating this carbon is an AARL Split Elution, operating at 100 deg. C. A total of 20 bed volumes (10 + 10) are required to eluate the gold down to 200-300 ppm, an elution efficiency of 82%.
The results from the pilot run were not encouraging due to the extruded carbon use.
The indication from the test runs, however, is that the elution efficiency is at least equivalent to the batch system. However, definitive comparisons were not possible because of the type of carbon tested. Figure 2 shows batch elution curves for the extruded carbon tested compared with a ‘typical’ coconut shell carbon. These curves were derived from laboratory tests to highlight the poor elution characteristics of the extruded carbon and to predict the likely recovery with a coconut shell carbon (Table 1).
These results indicate good recoveries could be obtained from a coconut shell carbon, even though some flow and temperature problems existed with this run.
The most significant assay from the second pilot run is the eluate gold value which peaked at 184 g/t Au (Table 2). This is equivalent to a solution: carbon mass flow ratio of 7:1, compared to 20:1 (10 parts fresh eluant and 10 parts recycle eluant) required for the split elution used in the main plant operation. The laboratory batch elution test peaked at a much lower eluate value of 105 g/t Au (Figure 2), thus demonstrating a marked benefit to be derived from the continuous system.
