Refining major impurities removed

Table 1. Refining stages of edible oils and the major impurities removed (adapted from...

Refining stage Major impurities removed or reduced... 

Ultra-high-purity Mg has been prepared by either zone refining or vacuum distillation. Zone refining " is a difficult process because of the high volatility and reactivity of the metal. Nevertheless, the process can be carried out in SOj atmospheres where protective films of MgS04 and MgO are formed " ", or in ultra-pure Ar atmospheres "". Zone refining removes a number of major impurities, includ-... 

A major application of liquid phase adsorption is to the removal of relatively small amounts of impurities or color bodies in water treating, sugar refining, and other processes. Both batch and... 

Also of major importance is the content of piecions metals, particularly silver, which contributes significant value, and the presence of the critical impurity elements arsenic, antimony and bismuth, which attract cost penalties. Smelters often have a limited capacity to remove penalty elements within the lead bullion refining operation, and hence need to balance the intake of impurities so as to remain within various capacity limitations. This can often require careful blending of a range of feed concentrates to obtain the optimum feed mix. 

The process is practiced at 220 to 650 F, but most refiners contact diluted cylinder stocks at 475 F and neutral oils at 220 to 320°F. Occasionally lower temperatures are used, but such an operation should be classed as neutralization. The acid-stage oil is mixed with 200 or 200 to 300 mesh clay, and the mixture is heated in a pipestill to the aforementioned temperatures. The hot oil is then allowed to settle for a short time, and the fines (clay dust) are filtered from the oil in Sweetland or similar presses. Sometimes a short percolation filter is used to aid in the removal of the fine clay. R. C. Davidson has made a complete summary of the methods of applying contact treating. The clay is usually not recovered, but Chenault and Miller report the commercial use at 90 to 100 F of a 20 per cent acetone (naphtha) solution for extracting impurities from contact clay. A low contact temperature is employed so that the adsorbed materials will not be too tightly attached to the clay. The reactivated clay has an efficiency of 80 to 85 per cent. Spent clay is mixed with solvent, filtered in a closed rotary-type filter, and washed on the filter with naphtha. The recovery of acetone by distillation from the naphtha and from water constitutes a major part of the process. 

Carbides are binary compounds in which carbon is the more electronegative partner. We shall come across silicon carbide, SiC, again later when we look at the chemistry of silicon this is a very hard material, manufactured in large amounts as an abrasive known as carborundum. Its hardness can be accounted for by the fact that its crystal structure is the same as that of diamond, with alternate carbon atoms replaced by silicon (Figure 10.13). A major industrial use of SiC is in steel refining where its addition to the molten metal removes metal oxide impurities with production of CO and a silicate slag, which is skimmed off. 

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