Improving Rinsing Operations
What are some methods to improve rinsing operations?
Inadequate rinsing and poor maintenance of rinse tanks between surface preparation stages are likely to result in bare spots, higher disposal costs, increased dross formation and zinc consumption, clogging of filters, and dioxin formation due to the influx of greases and iron into subsequent process tanks. Rinsing after caustic cleaning minimizes carryover of the alkaline cleaner and prevents weakening of the pickling solution from neutralization of free acid. Proper rinsing after caustic cleaning also ensures dirt, oils, and greases on the steel are not carried over to the pickling and fluxing baths where these contaminants are likely to float on the surface and interfere with the performance of these solutions. Rinsing after pickling minimizes carryover of acid and iron salts to the preflux solution and the zinc bath, resulting in reduced dross formation and a lower zinc consumption rate. Additionally, inadequate rinsing after sulfuric acid pickling leads to sulfate contamination of the preflux solution.
Rinsing Tank Maintenance
There is not a recommended time for change-out of rinsing tank water because this interval depends on the quantity of steel being cleaned and the amount of contaminants and oxides on the steel surfaces. Instead, a series of sampling and titration should be performed to determine when change-out should occur for each rinsing tank. For general maintenance, monitor rinsing tanks by sampling periodically, ensuring the pH measurement is near neutral and an oil layer does not develop near the top of the sample. Additionally, skim the rinsing bath and remove bottom contaminants (salts, iron, dirt, etc.) daily at a minimum to prevent carryover to subsequent tanks.
It is possible to neutralize rinsing tanks by adding acid or base to decrease/increase the pH level. However, this is not considered good practice as the added chemical also reacts to produce impurities (salts) at the bottom of the tank. It is usually more efficient to monitor the pH level of the rinsing tanks and repurpose rinse water that has become too basic/acidic to prepare the next caustic/pickling bath.
Improving Rinsing Operations
The most effective way to improve rinsing and tank change out times is to minimize drag-out (i.e. the amount of caustic or pickling solution retained on the work after removal from the bath). Minimizing drag-out results in a longer bath life and reduced spending on chemicals. The amount of drag-out from caustic cleaning and pickling tanks can be decreased by: moving articles twice in and out of the pickling tank, holding the work over the processing tank for a short period to allow maximum drainage, and using wetting agents in the solutions to reduce surface tension and provide better drainage.
After the maximum amount of caustic cleaning or pickling solution has been drained, rinsing performance is further improved by: vigorous agitation in the rinse tank (air or physical movement of the work), using two rinsing tanks after pickling, moving articles twice in and out of the rinsing tank, moving rinse water to the previous process tank (to replace evaporation loss) and replacing with fresh water, spraying water on parts going into the rinse tank, and implementing the use of continuous flow rinse tanks.
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