THE GOLDEN GIANT MILL; Part 2 — Grinding Circuit Pre-treatment

The following is the conslusion of an article featured in the December, 1989, issue of The Northern Miner Magazine. It ended with a discussion of the “Hemlo treatmen process.”

In the original grinding circuit configuration, as shown in the top portion of Figure 5.1, tailings reclaim water was mixed with grinding thickener overflow, which was then pumped in a closed loop back into the grinding circuit for re-use. Reclaim water additions were governed by the volume of water carried to the leach circuit with the thickener underflow.

On repeated occasions, sampling campaigns showed that the grinding thickener overflow water quality was superior to that of tailings reclaim water reporting to the effluent treatment plant. That is, substantial removal of most contaminants was occurring naturally in the grinding circuit. It was apparent that if this phenomena would hold true after “open circuiting” the spent grinding water and directing it to the effluent treatment plant, significant cost savings could be realized in effluent treatment.

Based on the observations noted above, open circuiting of the thickener overflow directly to the effluent treatment plant was initiated on a temporary basis. Tailings reclaim water was used exclusively in the grinding circuit and only the grinding circuit thickener overflow was pumped to the effluent treatment plant. The two flowsheets in Figure 5.1 illustrate the changes made in water flows during the test.

It was expected that reagent requirements for effluent treatment would be substantially diminished as long as the reduction of trace impurities in the waters of the grinding circuit would be attained in continuous operation. Over a 10-day period, circuit results were satisfactory, and reagent requirements in the effluent treatment plant were reduced by about 40% due to the much lower contaminant levels as shown in Table 5.1.

However, small quantities of gold were dissolved in the grinding circuit because of the presence of cyanide in the reclaim water. This meant that a carbon column circuit would be required for gold recovery from the grinding thickener overflow solution.

The success of the test run led to a permanent change of effluent routing. Tailings effluents are used as make-up water in the grinding circuit, and grinding thickener overflow is pumped to the effluent treatment plant. A carbon column circuit has been installed to recover dissolved gold from the thickener overflow solution prior to effluent treatment.

Four months of continuous operation with this flow arrangement have given excellent results. It is clear that the benefits of pre-treating reclaim water in the grinding circuit meet expectations.

The Role of Natural Cyanide Degradation in the Tailings Pond

The CIP process tailings are pumped to an impoundment pond located 3 km north of the minesite. The tailings pulp is alkaline, having a pH in the range of 10.0 to 10.5 and the contained water is rich in free cyanide ions, relatively stable metallocyanide complexes and other cyanide reaction products — including thiocyanate and cyanate. The metallocyanide complexes of copper, iron and nickel are prevalent in the Hemlo tailings pond. A typical CIP tailings solution analysis is given in TABLE 6.1.

The tailings solids settle in the pond and supernatant effluents currently cover an area of about 40 ha. During the non-winter months, aging significantly reduces the cyanide and heavy metal content of the tailings reclaim water. Figure 6.1 shows the reduction of the cyanide level and seasonal variation during a typical year (1987).

Several mechanisms (2) contribute to the natural degradation process. Absorption of carbon dioxide lowers the tailings solution pH from 9.5-10.0 to 8.3-8.6, thereby promoting the formation and subsequent volatilization of HCN. Ultraviolet radiation plays an important role in the breakdown of iron cyanide complexes. The warmer, sunnier days of spring and summer have a dramatic impact on the iron content of the tailings solution with a corresponding drop in cyanide levels.

Metals liberated by the breakdown of metallocyanide complexes precipitate readily as metal hydroxides due to the alkaline conditions in the pond. A notable exception has been nickel. Concentrations of nickel have not dropped as quickly in the tailings pond during the summer months as those of copper and iron. It is likely that as nickel cyanide complexes dissociate, nickel precipitates more slowly than the other metals since then pH in the pond is lower than that required for optimal nickel precipitation.

The natural cyanide degradation shown in Figure 6.1 is very substantial and significant. However, the natural degradation during the first three years of operation was not nearly as complete as at the neighboring Williams and David Bell mines, owed by Corona Corp. and Teck Corp. (3).

With solution chemistry of the three mill tailings being similar and with the Golden Giant having a tailings solution retention time approximately equal to the two neighboring operations, it was postulated that perhaps the reason for less complete cyanide degradation was a relatively small tailings pond surface area at the Golden Giant operation.

Through operations changes, it was possible to almost double the free water surface area in 1988 over that of the previous year.

This move resulted in significantly lower minimum cyanide and metal levels in the tailings pond solution. Table 2.2 shows a comparison between 1987 and 1988 and how the minimum contaminant levels are much lower in 1988. This seems to confirm that the greater surface area and shallower depth increases exposure to ultraviolet rays and favors absorption of carbon dioxide, leading to enhanced natural degradation.

It is a given that successful water management and effluent treatment must be achieved in today’s operating environment. Finding the optimum solution requires scrutiny of the conditions at hand with due consideration given to all aspects of the operation.

REFERENCES 1. Goodwin, E; Canadian Patent Application “Process for the Removal of Cyanide and Other Impurities from Solution.”, 1987.

2. Mines and Mill Wastewater Treatment, Environment Canada Report No. EPS 2/MM/3.

3. Personal communication with K. Meyer, Teck-Corona and C. Patel, Corona Corp.

The authors acknowledge the many contributions made by Hemlo employees to the development of the Hemlo effluent treatment scheme. The successful application of the Process was a group effort requiring the diligence, patience, and determination of many individuals.

The authors thank the Management and Board of Directors of Hemlo Gold Mines Inc., for permission to publish this paper.