Phosphating solutions contain either zinc, iron or manganese phosphate and phosphoric acid with suitable accelerators. During the process, the clean steel parts are immersed in (or sprayed with) the metal phosphate-phosphoric acid bath, iron is dissolved at the surface and a phosphate coating is formed. The operation of these baths requires an even application of solution and good chemical control. Sludge accumulation results in poor quality, lost production and need for frequent maintenance. The sludge generated will not harm the phosphate process as such, but is detrimental to the operation since it tends to foul and plug heat exchangers, circulating pumps and strainers, spray nozzles, etc. The sludge must therefore be removed, preferably with an automatic separation system, such as the continuous roll filter shown in Figure 4.36 .
Zinc phosphate baths require acid resistant materials, such as 316 stainless steel. They generate more sludge than the iron phosphate baths, which can be handled with carbon steel and cast iron. The filtration of two or more tank turnovers per day using a filter medium with moderate porosity (25–40m) has proved effective. A tapered bottom phosphate tank aids in conveying the sludge to the pump. Either in-tank or out-of-tank pumps may be used to transfer the sludge-laden solutions to the filter. Filtrate is returned to the phosphating tank by gravity or a sump pump in the clean reservoir of the filter.
The continuous belt filter, shown in Figure 4.37, is a type that is particularly wellsuited for the continuous cleaning of phosphate bath solutions. The framework of the filter is constructed in unstressed steel, the outer faces being tied together by diagonal struts. The collecting sump has an outer flange connection (8) for the removal of the filtrate; there is a slurry scraper (9) and waste chute (10) for the slurry. The band filter bed is a perforated plastic sheet (4), with a continuously circulated filter band (3) above it in plastic material. The solution inlet (2) has an adjustable deflector plate and an inlet
connection (1) for the unfiltered phosphate solution. A spray installation for water (6) and compressed air (7) consists of two oscillating nozzle jets, and two magnetic control valves for controlling the operation. Band transport is through a drive motor (5).
In operation, the unfiltered solution flows through the inlet (1), to the inlet chamber (2), through the plastic filter band (3), into the collecting sump and through the outlet (8) back to the bath. The impurities build up on the bed of the filter band (3) into a sludge layer. The layer thickness depends on the bonderizing process being used. The layer formation reduces the porosity and so raises the quality of filtration. As the filter band (3) moves forward, the filter sludge is dropped out through the waste chute (10). At the same time, the forward movement of the band opens two magnetic valves for water and compressed air; two nozzle jets (6 and 7) spray the returning conveyor band from the reverse side and clean off any slurry residue.