Engineering & Mining Journal

AUG 2018

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

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Page 31 of 107

MINERAL PROCESSING 30 E&MJ • AUGUST 2018 In the mid-1970s, dry concentration of gold and tungsten minerals was investi- gated and a dry process pilot plant de- signed for a North African government for gold and tungsten deposits in the Sa- hara desert. A 5-gram-per-metric-ton (g/ mt) gold deposit was selected as the lo- cation to operate the dry pilot plant for one year. Dry tables were selected as the concentration device with air being the fluid medium. Treating five particle size ranges coarser than 200 mesh, those ta- bles achieved gold recoveries greater than 90%. The dry process fatal flaws were a low unit area of production capacity for the dry air tables. And, the amount of vi- bration those tables generated would have required a substantial building structure. These flaws outweighed the economics. Fast forward to 2004, a dry pilot plant design engineer (the author) was seeking a challenging retirement hobby. The hob- by selected was to devise a dry concen- tration method that eliminated the fatal flaws of the North African experience. The process equipment requirements would be a high unit area of production capacity, no vibration and low energy requirements. Two possible designs were considered. The first design was for a hindered settler based on wet counter flow sizing. With five years of dry silica milling expe- rience, it was felt that low-pressure air could replace water as the fluid medium. This design would have a high unit area of capacity, require no vibration and would have low energy requirements. The second design for consideration was to adapt dry air-gravity conveyors. With this type of conveyor, fine dry particles are fluidized with low-pressure air, which forms a flowing film. The challenge was to determine how these conveyors could be modified to segregate, concentrate and recover heavy minerals. The flowing film aspect could be a limiting production rate factor. But, it was felt that limitation would be overcome by processing a bed of material instead of a flowing film. The second design was ultimately se- lected for investigation. The redeeming feature being the unit could be built out of plywood in a garage. And the stain- less-steel medium could be purchased and fitted to the prototype profile. The first design was beyond the designer's ability, capacity and retirement budget to build such a prototype unit in a garage. The particle size range selected for the modified air-gravity conveyor proto- type was 100% passing 50 mesh (300 microns) down to 96% retained on 270 mesh (53 microns). Within this particle size range, air gravity conveyors are able to convey minus 50 mesh particles. Removal of low-terminal velocity minus 270 mesh particles would ensure a relatively dust free operation. The basic, length to width geometry, for air-gravity conveyors was fol- lowed for all of the dry gravity separator prototypes. Briefly, this is a generalized diagram of the dry separator prototype. In 2005, prototype fabrication was started for a junior mining company. That company held a Barite deposit where a source of water was not available. By the end of one year, the company had run out of research funds. TM Engineering of Burnaby, British Columbia, had fabricat- ed the SS medium and feed bin for the dry separator prototype. They saw a min- ing industry niche for a dry process and offered to support development by provid- ing shop space and modification support. Development continued on Barite until an oil industry drilling mud specification (4.1 SG) could be produced. Then work was switched to base metal ores. During this time, the price of gold increased sig- nificantly, so the objective was switched to gold ores. Since then, development work has focused entirely on producing a dry gravity gold separator. Two binary standards were employed for prototype development. The standards were made up of No. 70 silica sand, with various concentrations of magnetite (5.2 SG) to simulate base metal ores; and with tung- sten-carbide/cobalt (14.6 SG) to simulate gold ores. Because magnetite and tung- sten-carbide/cobalt have magnetic proper- ties, process samples could be separated, the fractions weighed and metallurgical re- sults calculated. No assaying was required, which reduced development costs. Development Program A multiyear development program (this was after all a retirement project) went through four generations of dry gravity separator types. In each case, the basic operations of feeding, segregation, con- centration and recovery remained the same. Along the way, many technical and mechanical problems were resolved. For example, small changes in fluidization air pressure proved to be sensitive to the op- erating requirements. Initially, dry feeding was difficult to deal with. Once the minus 50 mesh fines were aerated and fluidized, it became a difficult fluid to meter. This was resolved with a guillotine-type feed gate. The height of sands in the aeration feed bin was in effect a hydraulic head, where the feed rate varied according to the depth of material in the bin. Since gravity separations work best with constant feeds, this was resolved by converting the aera- tion feed bin to a constant head bin. The segregator, concentrator and recov- ery sections went through several geomet- ric, mechanical and slope changes. More than 200 runs were required to establish a viable prototype separator. The most chal- lenging of all problems was to establish an effective mineral/metal recovery method. In a number of cases, what seemed to be the correct way often turned out to be the reverse of that thought process. At the end of development work, low weight base metal and gold rougher con- Development of Dry Gravity Separator A retirement hobby may lead to mineral processing breakthrough By George Rodger The Dry Gravity Separator processes a bed of material instead of a flowing film.

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