Research ‘FINE’ GOLD SLUICING

Sluicing has been used for the concentration of high-density minerals, especially gold, for centuries. It continues to be the predominant mineral processing technique in use for placer gold mining in the Yukon. Now Prof George Poling and his graduate student James Hamilton in the Dept. of Mining and Mineral Process Engineering, University of British Columbia, have shown that modifications to the sluicebox could increase annual gold production in the Yukon by 12% to 112,000 oz. Poling examined how two types of sluicebox riffles and various operating conditions affect recovery of placer gold from different size fractions during sluicing operations. This was accomplished by processing a large sample of screened Yukon placer gravel, seeded with known amounts of sized placer gold, in a series of test runs in a pilot-scale sluicebox. Testing indicates that expanded metal riffles (such as 1-10h) over Nomad matting are superior to 1 1/4 -in (31.35-mm) dredge riffles for recovery of placer gold between 20 and 100 Tyler mesh (850 and 100 microns).

Sluiceboxes, operating at water-flow rates and solid feed rates typical of the Yukon placer mining industry (from 100-700-lb solids per min per ft of sluice width; roughly equivalent to 2-15 cu yds per hr per ft of width) can be an effective method of recovering gold as fine as 150 Tyler mesh (100 microns). The recommended operating procedure is to use 300-400 lb per min of solids per foot of sluice width accompanied by about 200 us gal per min of water at a slope of 1 5/8 -2 in per ft. Gold recoveries as high as 95% of the gold as fine as 100 Tyler mesh placer should be achievable. Gold as fine as 150 Tyler mesh was recoverable at up to 85% in this simple sluice system. In 1986 the Yukon placer operators recovered more than 100,000 oz of gold. It is estimated that an additional 12,000 oz should be recoverable by these optimizing sluice technologies.

Visual observations during test runs and analysis of the data produced the following conclusions:

* Expanded metal riffles are significantly superior to 1 1/4 -in dredge riffles for recovering gold between 20 and 100 Tyler mesh.

* Sluiceboxes operating at water flow rates and solid feed rates from 100-700 lb solids per min per ft of sluice width can be an effective method of recovering gold as fine as 150 Tyler mesh.

* The practice of running “clean” or allowing the gravel feed to stop while water is flowing need not greatly affect recovery.

* Coarsening the upper size of gravel in the feed from 1/4 in to 3/4 in does not significantly influence recovery.

* Normal variations in the solid feed rate can be tolerated in sluicing without excessive gold losses.

* Low water use is beneficial to gold recovery. The best recoveries using expanded metal riffles were obtained using a water to solid ratio by weight of approximately 4:1.

* Nomad matting and Cocoa matting are both effective at retaining gold when exposed during scouring. Nomad matting is much easier to clean up.

The research project was funded by the federal government through the Dept. of Indian Affairs and Northern Development in Whitehorse, the Yukon, via a contract with the Yukon Chamber of Mines. The Klondike Placer Miners Association and Teck Corp. also supported the project.

A sluicebox consists of one or more flumes through which a slurry of water and alluvial gravel is passed. These flumes are rectangular in cross-section and are lined along their base with riffles and a gold-trap “matting” material underneath. A riffle is a device used to affect concentration of heavy minerals, particularly gold. The riffles selected for testing were expanded metal (1-10h) and dredge riffles (1 1/4 in angle iron). Two types of riffles are used in this test work and commonly used in the Yukon. Turbulent eddies are formed in the slurry as it flows over and around the flow obstructions that comprise the riffles. The interaction of these eddies with the particulate material that tends to collect around the riffles forms a dispersed shearing particle bed where particles of gold are concentrated because of their high specific gravities.

To investigate the recovery of fine placer gold in sluiceboxes, a test sluice facility was constructed at Western Canada Hydraulic Laboratory in Port Coquitlam, B.C. An overview of the test sluice facility is shown on page 70. The experimental design required a closed circuit facility capable of processing placer gravel at solid and water mass flow rates as well as sluice gradients typical of modern placer gold mines in the Yukon Territory. The size fractions of placer gold tested were –20+28 Tyler mesh (–850 microns +600 microns), –35+48 Tyler mesh (–425 microns +300 microns), –65 +100 Tyler mesh (–212 microns+150 microns) and –100+150 Tyler mesh. (–150 microns + 105 microns). The photograph on page 68 shows gold particles of various size fractions. A total of 29 test runs was conducted with the expanded metal and dredge riffles and the various operating conditions and gold size fractions.

The test facility consisted of a 30-cm-wide-by-2.4-m-long sluice that received a slurry of gold-bearing test gravel and process water from an entry flume. Gravel was introduced to the entry flume from a 2-cu-yd feed hopper fitted with a well-sealed belt- feeder. Control of solids feed rate was achieved using a variable-speed hydraulic motor on the belt feeder. A closed system was used for the process water to prevent losses of clays and other fine solids.

The gravel used for the test program was obtained from screening 15 tons of gravel from Teck Corp.’s Sulphur Creek placer operation in the Yukon. The gold seeded into this gravel was obtained from this same placer mining operation. The — 1/4 -in fraction of the gravel sample was used for 19 test runs and then the — 3/4 -in fraction was added to this feed gravel during six test runs. The gold-bearing concentrate that remained in the 8-ft-long test sluice, upon completion of each test run, was removed in 2-ft sections. The gold trapped in each section was sized, extracted from the heavy mineral concentrates, dried, and weighed. The entire gravel sample, down to fine silt-sized particles, was recovered at the end of each test. The gold and gravel were remixed in batches for re-use in subsequent tests.

A concise summary of data gathered from 25 pilot-sluicing tests appears in a recent report from the University of British Columbia (Dept. of Mining and Mineral Process Engineering). It lists results of individual tests numbered 4 to 29. Tests numbered 1 to 3 were preliminary to remove the “natural” placer gold present in the — 1/4-in dry-screened fraction of the Sulphur Creek gravel and to develop suitable operating procedures.

Tests 4 through 9 were conducted using solid feed rates representative of operating placer mines in combination with water flow rates, riffle type and slopes considered most effective by Prof Poling and Jim Hamilton. The results demonstrated the high efficiency of sluicing for recovery of gold as fine as 100 mesh. Test 10 was conducted under conditions similar to Test 7 but with 30 second periods of solid feed stoppage for every two minutes of normal solid feed. The purpose of this test was to investigate the effect of sudden stoppages in solid feed to an operating sluice. Tests 11, 12 and 13 were identical to Test 9 except for different solid feed rates. This demonstrated the effect of variation in solid feed rate on gold recovery when all other conditions were held identical. Test 14 was a repeat of Test 6 except that “clear” (no solid feed) water was pumped to the sluice for two hours after all solids had been processed. The common practice, in the Yukon, of pumping water through a sluice without ore feed for long periods was the basis for this test procedure.

Tests 16 through 20 used dredge riffles (1 1/4-in angle iron) in combination with variable settings similar to the previous expanded metal tests (Tests 4 through 14). Test 21 was conducted using expanded metal with a combination of other settings that were not previously used b
ut thought to be effective. The purpose of this was twofold. First, to acquire another data set using 400 us gal per min water flow, 2 3/8 in per ft slope and expanded metal. The second purpose was to process the entire feed using known efficient recovery conditions to determine whether the significant amounts of gold not recovered during the preceeding dredge riffle tests were due to poor recovery or to gold losses from the test facility. The former was found to be true. Test 22 was conducted using expanded metal turned around. Test 23 used Cocoa matting under expanded metal to determine its effectiveness when exposed to severe scour compared to the Nomad matting used in all other test runs.

During Tests 24 through 27, the feed gravel was — 3/4 in due to the inclusion of the — 3/4 in + 1/4 in fraction of the original gravel sample in the test gravel. Conditions selected during these tests were chosen to determine the effects of the larger particles on gold recovery. A variety of conditions representative of the previous series of tests, using the — 1/4-in fraction, were employed.

Test 28 was a scavenger run, conducted at variable settings known to be efficient, to remove virtually all the gold remaining in the test gravel at the conclusion of Test 27. Test 29 used only four barrels of feed gravel seeded with –100+150 mesh gold to determine the recovery of this size fraction under conditions similar to those of the very efficient Test 4.

Results of the data show gold weight distributions in the various sluice sections for each size fraction seeded into the feed. Over the course of the entire series of pilot-scale test runs, nearly 13 g out of 275 g total gold added was lost because of accidental spillages, entrapment in equipment or other unknown causes. In order to calculate total gold recoveries during each test, this loss has been apportioned out equally to each test.

Future research on sluicing would be beneficial if directed toward investigating the recovery of gold when using different riffle types, when processing different gravel types, with or without bedrock fragments, and when using very fine gold ( –150 mesh). REFERENCES Poling, G. W. and Hamilton, J. F. (1987): Fine Gold Recovery of Selected Sluicebox Configurations. University of British Columbia, Dept. of Mining and Mineral Process Engineering, Vancouver, B.C., 77 p. Wenqian, W. and Poling, G. W. (1983): Methods of Recovering Fine Placer Gold, The Canadian Mining and Metallurgical Bulletin, December, 1983, p. 1-10. Joyce Musial is a Toronto-based geologist and freelance writer.