When to Discard Solution, Recharge a Parts Washer

How can we determine the ideal time to discard the cleaning solution and recharge these washers? Chemetall’s David Gotoff answers this question.


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Q. We have several parts washers on our production floor that use an alkaline cleaner to remove the metalworking fluids prior to subsequent operations. How can we determine the ideal time to discard the cleaning solution and recharge these washers? 

A. Determining the ideal time to recharge a parts washer can be challenging, as the answer is not universal to all applications. First and foremost are two very important points that must be considered: the question of how clean the parts being washed “need to be” and the cost impact of a failure. Good process control, including maintaining the concentration and product parameters, will greatly help achieve ideal performance. Luckily, there are evaluations you can perform within your facility to help you make an educated decision about when to discard and recharge the washer fluid that is based on facts and supported by data.

Knowing the volume of soils building up in a cleaner stage is critical. I highly recommend continuous soil removal for all cleaning processes whenever feasible, using cleaning products formulated to float the soils to aid in the removal process. Filtering is effective in removing particulate soils only.

A test that will provide valuable data is the “free-to-total-alkalinity” titration ratio. This requires an alkaline cleaner with sufficient alkalinity to be controlled using a “free-alkalinity” titration. Typically, this is a phenolphthalein indicator titration in which the solution turns from red color to clear when titrated with a standard acid solution. The free-alkalinity level should remain constant as you add product to maintain the required concentration. After completing the free-alkalinity titration, add a second indicator to the same flask (typically methyl orange or bromocresol green) and continue to titrate with the same standard acid solution until the appropriate color change for the indicator. This value is the “total alkalinity” of your cleaner solution. You can then divide the total alkalinity by the free alkalinity, and determine your “alkalinity ratio.” A fresh charge of the cleaner will have the lowest ratio. Over time, as soils enter the cleaner, this ratio value will increase as the total alkalinity value increases, and cleaning performance will be compromised. Relating the ratio value to the cleaning performance of your process will allow you to establish an upper limit for the ratio and define a time to discard the bath.

The emulsified soil load present in the cleaning solution at any given time can be determined by performing an oil split test. To run this test, you need to obtain a representative sample of the bath and transfer it to a graduated glass vessel (a stoppered graduate cylinder, Cassia flask or Babcock bottle are ideal). To a 100-ml sample, carefully add either hydrochloric acid or table salt in sufficient amounts to separate the emulsified soils, which will rise to the top. Be cautious, as this can generate heat or gas; vent the stopper during the process and add reagents slowly. The “soil load” is the volume of floating material, in percent by volume, when a 100-ml sample is used. Be sure to sample the washer solution in a representative way, avoiding capturing floating oils when collecting the sample (liquid being pumped to the spray risers would be an ideal sampling spot). Always sample the cleaner tank the same way each time. This test should be done on a routine basis—daily or weekly depending on the production volumes—and the values recorded in a log book. Over time, relate the level of soil load with cleaning performance and determine the maximum level of soil load tolerable for your process. 

As cleaning solutions near their end of life, they require increasingly higher volumes of concentrate additions to keep the product concentration within specification, as controlled by titration (very much the recommended method of solution control for cleaners). The volume of concentrate added per day or per week will be lowest when the solution is freshest and highest just prior to recharging. This is due to carbonate salts forming in alkaline solutions caused by the absorption of carbon dioxide from the atmosphere. The carbonates and other alkaline salts in the product “buffer” the solution, making additions of alkali less effective on increasing the pH. Often, it becomes cost-effective to dump and recharge the cleaner sooner rather than later to reduce product usage and overall cost. Factors such as the cost to dispose the used solution must be taken into consideration to make an educated decision. By plotting the cleaner additions (consumption) over time and monitoring this increase as the cleaning cycle progresses, you can determine the ideal time to take action. 

These best practices, along with appropriate cleanliness testing, will help you manage your cleaning.  

David Gotoff is a product manager for Chemetall US, Inc. Visit chemetallna.com.