Sink—float method

Table I. Average Organic Affinity of Some Metals Determined by Float-Sink Methods...

Determination of the washability characteristics of coal by the float and sink (float-sink) method can be applied to coal of any particle size provided suitably large vessels to hold the larger lumps are available. Air-dried coal, not dry coal, should be used since the separation depends partly on the difference in specific gravity of the clean coal and dirt particles, and the specific gravity, in turn, is dependent on the moisture content of the coal. If the coal is dried before the test is carried out, the conditions will then differ from those in commercial washers, and the results will be at variance with those obtained in practice. 

All of the samples prepared by centrifugation and the float-sink method were demineralized and milled to three micron diameter. The separations were conducted in the presence of a surface active agent, polyoxyethylene(23) lauryl ether, in cesium chloride solutions (14). The macerals were washed thoroughly to reduce the quantity of surfactant in the maceral to less than 500 ppm and the quantity of chloride ion in the wash water to below the limit detectable by precipitation with silver ion (14). 

The maceral concentrates obtained by float-sink methods from PSOC-828 and 1103 were somewhat less pure than the macerals obtained by hand or by centrifugation. For convenient comparison, the data obtained in the exchange reactions have been normalized to 100% maceral purity on the basis of the petrographic information presented in Table II using simultaneous linear equations. The normalized results are summarized in Table IV. 

After the wash, the material moves to the separation stage. Two most common methods for separating plastics by resin type are float-sink and hydrocyclone. The float-sink method is less costly but requires a relatively large tank size in order to realize volume effectiveness. A hydrocyclone is more costly, but uses less water, has no moving parts, and generally takes up less space than the older float-sink method. 

Two bituminous coals of moderate ash content were chosen for this paper to illustrate this method of determining coal-mineral association. The first sample was an Upper Freeport coal with 1.3% moisture, 9.88% ash, and 1.56% total sulfur. The second sample was an Indiana No. 3 coal having 10.5% moisture, 7.35% ash, and 4.26% total sulfur. Both coals had been precleaned at a coarse particle size, ground to minus 325 mesh (44 ym), and then separate samples were cleaned by float-sink and by froth flotation techniques, as described elsewhere [5]. Analyses of the feed coals are included in Table I. 

Samples Studied. From British Seams. The suite of samples was collected and separated by the Coal Survey of the National Coal Board under the direction of Dr. G. W. Fenton, and the authors are indebted to Dr. Fenton for the gift of the samples. The suite consisted of the vitrinites, spore-rich exinites, fusinites and, in one case, the micrinite from five British coal seams of Carboniferous age together with the vitrinites from two coals of higher rank from which it was impracticable to separate the other macerals. The exinites were separated by float-and-sink methods from selected black durain bands, as also... 

As far as the measurements on micrinite are concerned, might not the term micrinite as used here in fact refer to several inert macerals, namely, semifusinite, sclerotinite, and micrinite This would seem likely in view of the float and sink methods used to concentrate the vitrinite. 

Another common method is sorting by density in a float-sink tank or hydrocyclone. This method is, however, difficult for polyolefins as these have very similar density. Air classification uses a combination of density and shape to achieve the separation, e.g. for separating plastic films and paper residues. 

Note (1) Where the ash content of coal is too high to allow classification according to the present systems, it must be reduced by laboratory float-and-sink method (or any other appropriate means). The specific gravity selected for flotation should allow amaximum yield of coal with 5-10% of ash. (2) 332a. .. > 14-16%VM. 332b. .. > 16-20%VM. 

Float-Sink or Suspension Method This method involves placing a solid sample into a liquid with known and adjustable density. The density of liquid is incrementally adjusted until the sample begins to sink-float (ASTM C729-75 [25]), or is suspended at neutral density in the liquid (ASTM C693-93 [26]). At the point of equilibrium the density of the sample is equal to the density of the liquid. 

Density methods the density of commonly-used plastics varies between 0,9 and 1,7 g/cm [5], but the most common contaminants, such as paper and metal, have densities outside this range. Density-related methods can thus be used for both purifying and sorting plastics. Commonly-used techniques are air classifiers, hydrocyclones, and float/sink baths. Often, air classifiers are used to separate light contaminants such as paper, while hydrocyclones or float/sink baths take care of the heavier fractions. 

Conventional float and sink methods for the derivation of the partition curve are expensive and time-consuming. In the diamond and iron ore industries, the density tracer technique has been developed to evaluate the separation efficiency. This technique has also been adopted for studies of coal separation. 

Before smelting, zinc sulfide is usually concentrated by a float and sink method. The zinc sulfide is then roasted to form zinc oxide, followed by reduction with carbon and/or carbon monoxide. Zinc metal is distilled off. 

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. 

Flotation. The method of mineral separation in which a froth created in water by a variety of reagents floats some finely cmshed minerals, whereas other minerals sink. 

The more dense, more mature peas sink and are drawn off near the bottom of the separator, while the less dense, less mature peas float and are carried off at the top. During the process the peas absorb salt, and unless it is removed by thorough washing enough may be retained to make the product objectionable to the taste. In a packing specification it is necessary to state the amount of salt that will be tolerated. Available methods for the determination of salt are not applicable to production-line control. One which is sufficiently simple and rapid for this purpose is definitely needed. 

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