Scums treatment

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Septic Tank Untreated liquid household wastes (sewage) will quickly clog your absorption field if not properly treated. The septic tank is a holding tank in which this treatment can take place. When sewage enters the septic tank, the heavy solids settle to the bottom of the tank the lighter solids, fats and greases partially decompose and rise to the surface and form a layer of scum. The solids that have settled to the bottom are attacked by bacteria and form sludge. 

Figure 6. Comparison of scum occurrence after RIE treatment, 3jum line and space (a) X-8000K2 (1.2pm thick) on PIQ ( m thick), (b) X-8000K2 (1.2pm thick), (c) conventional positive photoresist (2pm thick) on PIQ (0.5pm thick), (d) conventional positive photoresist (2pm thick) on PIX (0.5pm thick).

Table II. Comparison of Scum Occurrence after RIE Treatment...

In order to remove the resist after the RIE treatment, wafers were immersed in phenolic-type resist stripper, but the resist as well as the scum could not be stripped. Since a thin silicon oxide layer is formed on the resist surface, and the composition of scum is thought to be silicon oxide as discussed above, it is necessary to remove this silicon oxide layer (scum) prior to the resist removal. Therefore, resist stripping was done in two steps. In a first step the wafer was immersed in buffered hydrofluoric acid solution to remove the silicon oxide and was then treated with conventional resist stripper. 

Therefore it is very common to find islands of solids floating in the anaerobic ponds. This often results in dead spots and short-circuiting in the ponds, which reduces the treatment efficiency of the system. Obviously it is very labor intensive and expensive to maintain the ponding system in very satisfactory conditions. It is also imperative to ensure that as little oil as possible is allowed to get into the anaerobic pond. Otherwise the oil will agglomerate with the rising solids brought up by the biogas and form a sticky scum that is difficult to remove. It is not advisable to allow excessive accumulation of the scum so that the effectiveness of the system is not adversely affected. 

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