Sink-float separations

The current process of heavy media separation (HMS sometimes called dense media separation) is the most widely used sink-float process today and is very efficient at reducing the acid-insoluble content of magnesite however, it is not as effective at reducing the lime content. This means that magnesite undergoing HMS can have a much higher insoluble content than that of lime and still achieve a chemical purity equivalent to that of direct-haul ore. The practical limit for the lime content is approximately 6wt%, while the limit for acid-insoluble material can be as high as 20 wt%. 

The ore fed into the HMS plant is first sized before entering the cone separator, the fines being rejected from the system to ensure that the suspension media is not contaminated with this material. The floats and sinks are removed from the cone and onto separate screen belts and are thoroughly washed using overhead sprays to remove any adhering suspension media. 

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Thus, the above results for the Upper Freeport coal indicate that density-based processes (e.g., float-sink separation) should be able to remove significant amounts of mineral matter, while surface-based processes (e.g., froth flotation) will likely be unable to reduce the mineral content significantly. Results of cleaning tests reported elsewhere [5] have generally verified these predictions. 

Analyses of float-sink separates of coal reveal a systematic variation of the minor elements with the organic matter which can be arranged into an organic affinity series. This series appears to be related to the chelating properties of the metals. Deviations in this series may be explained by the chemical nature of the depositional environment. 

Float/sink separations were carried out on the milled products using aqueous solutions of zinc chloride (1.30 specific gravity). Twenty gram samples of the dried coals were thoroughly mixed with 150 ml of aqueous ZnCl2 before centrifuging at 1700 rpm for 45 minutes with a model K, size 2, centrifuge... 

ASTM analytical data presented in Table I indicate that the float-sink separation achieved an 84% and a 56% removal of ash from... 

Altland et al [12] proposed a process in which near-and super - critical liquids are used. The process consists of five elements air classifier removes foamed materials float / sink separation using water separates PP, LDPE, and HDPE from PS, PVC and PET. 

Table 3 Quality of float / sink separation [12].(Reprinted from Brian L. Altland,...

It has to be noted that 1% of PVC in PET would not have been commercially acceptable. The estimated cost of float / sink separation using near - or super - critical fluids is 0.08 per pound for CO2 and 0.27 per pound for SF6. 

For certain gasification processes it may be necessary to limit the ash fraction in the coal to a certain value. Because the mineral matter has in general a higher density than the organic fraction of the coal, a density or float-sink separation can be carried out to remove material of increased mineral matter. This process is suitable for higher-rank coals and is accomplished using a liquid with an adjusted density. Therefore, it is also called washing of coal. 

There are other complications in using float-sink separations. The adhesion of a small bit of label made of a different material alters the apparent density of a chip. Fillers and dirt may increase the apparent density. The method is obviously of greatest use when the overall composition of the mixed plastic stream remains relatively uniform. 

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