Equipment for Gold Gravity Concentration
There are in the market several equipments for gold gravity concentration (e.g. sluices, spirals, cones, shaking table, jigs, and centrifuges). Some of them should be used at small scale and other at full scale. For example, one of the important innovations in the forties was the development of the first wet spiral concentrator by Humphreys, which gave an upgrade in just a single pass. This is a modified sluice that achieves a better stratification length than the Cone concentrator by incorporating a curved through spinning multiple turns. Nevertheless, it has been appreciated more recently the difficulty in distribution and control are inherent problems with large, all spiral installations and a more balanced design with cone concentrators as primary separators and spirals in the cleaning stage offer advantages in installation space, capital cost and operation.
The original spiral concentrator has suffered several changes in construction material and geometrical design. As a result, a model called MC700 was introduced in the market some time ago and was coupled with shaking tables at Round Mountain gold operation in Nevada to recover free gold and auriferous pyrite. The operation of this unit allowed Round Mountain to upgrade its concentrate and reduce the size of the cyanidation circuit. The gravity circuit was designed to treat approximately 8,900 t/d and the material was treated on three stages, rougher, and two cleaner stages using spirals. The spiral concentrate fed a shaking table circuit and the average gold recovery was 65-71%. Other new spiral design was called HC1800 and has groves close to the concentrate port. These grooves running at an angle across the trough from near the outer wall to a position upstream of the ports are employed to improve recovery of fine gold.
Other typical gravimetric equipment is the shaking table, which has experienced several changes since the introduction of the classical Wilfley table. For example, some time ago, Budin Consulting developed a new shaking table, which offered some advantages over the traditional table. According to the manufacturer, the concentration table is lightweight, has low energy consumption, simple operation and its inclination may be adjusted in any direction without stopping the operation. The table’s collection boxes for concentrate, tailings and intermediate products provide for the dry recovery of materials, if required.
Other traditional equipment is the jig, which has been used to treat particles of coarse size range. Improvements in the design of the operating chambers, combined with careful attention to the pulse cycles have reduced the bottom size that can be handled. It is important to mention that there is nothing new about the idea of a moveable screen within a jig. A new development is called in-line pressure jig, which includes a central feed location from which slurry flows radially across the circular bed. Rather than water pulsing through the screen, the screen itself is pulsed vertically with a saw-tooth motion. With treatment capacity of up to 58 t/h and upgrade ratios of 148:1 on feeds with coarser sizes in the 6-20 mm range. Apparently, the consumption of water is lower and also the operating costs.
Probably, the introduction of centrifugal equipment has been the last innovation of the last 25 years. These units substitute gravity by a centrifugal force driven by a need to extend the technology available to recover fine gold particles. One of the best examples has been the operation of Placer Dome’s Campbell mine in northwestern Ontario where gold gravity concentration has been very important. Prior to modernize the plant with the installation of Knelson Concentrators, the jib-based circuit recovered 44-47% of gold. An important improvement in the operation was achieved with the installation of Knelson concentrator, the gold gravity recovery was improved to 60-62%. The introduction of this centrifugal unit optimized the operation costs.