Rake classifiers

In the United States boric acid is produced by United States Borax Chemical Corp. in a 103,000 2 3 ric ton per year plant by reacting cmshed kernite ore with sulfuric acid. Coarse gangue is removed in rake classifiers and fine gangue is removed in thickeners. Boric acid is crystallised from strong hquor, nearly saturated in sodium sulfate, in continuous evaporative crystallizers, and the crystals are washed in a multistage countercurrent wash circuit. 

Continuous Percolators Coarse sohds are also leached by percolation in moving-bed equipment, including single-deck and nmlti-deck rake classifiers, bucket-elevator contactors, and horizontal-belt conveyors. 

Wet classifiers mostly are used to make two product size ranges, oversize and undersize, with a break commonly In the range between 28 and 200 mesh. A rake classifier operates at about 9 strokes/mln when making... 

Rake classifiers are inclined, shallow, rectangular troughs, fitted with mechanical rakes at the bottom to rake the deposited solids to the top of the incline (Figure 10.4). Several rake classifiers can be used in series to separate the feed into different size ranges. 

The flowsheet used in this plant (Figure 19.8) consists of a rod mill-ball mill grinding system and a copper rougher-scavenger flotation circuit, followed by two cleaning stages. Initially, the plant used a rake classifier, but now the rake classifiers have been replaced by cyclones. 

Cold-Water Process. The cold-water bitumen separation process has been developed to the point of small-scale continuous pilot plants. The process uses a combination of cold water and solvent. The first step usually involves disintegration of the tar sand charge, which is mixed with water, diluent, and reagents. The diluent may be a petroleum distillate fraction such as kerosene and is added in a ca 1 1 weight ratio to the bitumen in the feed. The pH is maintained at 9-9.5 by addition of wetting agents and ca 0.77 kg of soda ash per ton of tar sand. The effluent is mixed with more water, and in a raked classifier the sand is settled from the bulk of the remaining mixture. The water and oil overflow the classifier and are passed to thickeners, where the oil is concentrated. Clay in the tar sand feed forms emulsions that are hard to break and are wasted with the underflow from the thickeners. 

Figure 11.6. Thickeners for preconcentration of feed to filters or for disposal of solid wastes [see also the rake classifier of Fig. 12.2(e)]. (a) A thickener for concentrating slurries on a large scale. The rakes rotate slowly and move settled solids towards the discharge port at the center. Performance data are in Table 11.11 Brown, Unit Operations, Wiley, New York, 1950). (b) Deep cone thickener developed for the National Coal Board (UK), in a unit about lOftdia the impellers rotate at about 2rpm and a flow rate of 70 m3/scc with a solids content of 6wt%, concentrates to 25-35 wt % (Svarovsky, 1981).

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