Plating Process Systems - Metal Finishing Processes

Troubleshooting *Calcium Removal Treatment*

Symptom

Usual Causes

Correction

How to remove Calcium


	Introduction
	Excessive calcium introduced into Watts type nickel plating solutions can cause difficulties such as roughness, and in air agitated solutions, plugging of air agitation equipment. Excess calcium will precipitate (crystallize) from the solution as calcium sulfate in the form of needle-like crystals. These insoluble crystals cause shelf-type roughness and hole plugging.

	Calcium is most often introduced through the water supply in the form of make up water. The use of deionized (DI) water is recommended where contamination with calcium sulfate is a frequent problem. Where reclaiming of the nickel plating solution is used, the reclaim tank, and all make up water for the nickel plating solution should use DI water. In areas where calcium in the water supply is high (generally > 200 mg/L) consideration should be given to the use of DI water in the rinse preceding the nickel plating solution.

	Reduction of Calcium by Solution Heating
	Calcium sulfate is said to have inverse solubility, that is, the calcium is less soluble at higher temperatures than at lower temperatures. This relationship is approximated in the following chart:


	The chart is only an approximation since many factors will govern the solubility of calcium in a specific plating solution. Nonetheless, it can be seen that as temperature is increased, the solution will hold less calcium sulfate. Therefore, about 40 - 50 % of the calcium can be removed by raising the temperature of the plating solution from 140 °F (60 °C) to 160 - 165 °F (71 - 73 °C), filtered to remove the precipitated calcium sulfate and the solution returned to the plating tank at 140 °F (60 °C). By the same token, if the solution operating at 140 °F (60 °C) has more calcium introduced to it, for example by a large addition of make up water, the calcium may precipitate because the solution can hold no more.

	A temporary remedy to the calcium roughness problem may be obtained by reducing the operating temperature about 5 - 10 °F (3.5 - 7 °C). This will increase the capacity of the solution to hold more calcium, until more calcium is reintroduced. Nonetheless, this may be a good short term answer to a calcium roughness problem until a more permanent answer is determined.

	A more effective procedure is to heat the solution to approx. 165 °F (73 °C) in a separate storage tank. This procedure should not be performed in the plating tank as tank walls, anode bags, and other associated equipment in the plating tank will be coated with insoluble calcium sulfate. (Although the calcium coating will redissolve when the temperature is lowered, the effectiveness of the treatment is reduced significantly.) The treatment procedure is as follows:

	Pump the plating solution to a suitably lined storage tank fitted with heat and mixing capabilities. Heat the plating solution to 160 - 165 °F (71 - 73 °C) and agitate the solution. Maintain the temperature for at least 16 hours, preferably 24 hours, minimum. While still hot, filter the solution back to the previously cleaned plating tank. Clean the filter before re-using in the plating solution. Not performing this step will result in redissolving the calcium sulfate from the filter.

	Removal of Calcium (and Magnesium) by Precipitation as the Fluoride
	An effective procedure for removing most of the calcium makes use of an addition of sodium bifluoride which results in the formation and precipitation of insoluble calcium fluoride. Excess fluoride is not desirable in the plating solution, therefore, the quantity of sodium bifluoride should be less than the amount necessary to completely precipitate the calcium. Sodium bifluoride will also precipitate any magnesium present in the plating solution (magnesium frequently occurs with calcium in most water supplies) and allowance must be made for any magnesium present when determining the amount of sodium bifluoride to use. It is recommended that an accurate analysis of both calcium and magnesium in the plating solution be performed immediately prior to the actual performance of the treatment. The sample used for analysis should be taken at operating temperature so as to represent all of the calcium and magnesium in the plating solution at the time of treatment. An outline of the treatment procedure is as follows:

	At the operating temperature of the plating solution, transfer the solution to a suitably lined storage tank fitted with heat and mixing capabilities. Maintain the temperature of the solution approximately 5 °F (3 °C) lower than the operating temperature. This is to ensure the calcium will remain in solution and allow it to react with the sodium bifluoride. CAUTION: Sodium bifluoride is corrosive to the skin; read all handling information applicable for its use and use appropriate safe handling equipment. (NOTE: Do not use sodium fluoride as this material does not have the solubility of the bifluoride.) Dissolve the appropriate amount of sodium bifluoride (see below for calculations) in a convenient volume of the warm solution. Use about five gallons of plating solution for each two pounds of sodium bifluoride. It is important that the sodium bifluoride be completely dissolved before adding to the treatment tank. Do not add solid sodium bifluoride directly to the plating solution in the treatment tank. Add the sodium bifluoride/ plating solution mixture and agitate thoroughly. Add a slurry of nickel carbonate in water and raise the pH of the solution to at least 4.5. With agitation, add one lb/100 gallons of activated carbon. Agitate for one hour. While maintaining the temperature as in Step 2, allow the solution to settle for at least 16 hours, preferably 24 hours. This step is necessary in order to increase the size of the precipitated calcium and magnesium particles and allow for removal by filtration. Filter the solution back to the previously cleaned plating tank. Adjust the pH and temperature; make any necessary additions that the carbon may have removed, such as anti-pitters. Resume plating.

	Calculation of the Sodium Bifluoride Requirement
	For Gallons
	Calcium (g/L) X 11.5 = lbs/1000 gallons sodium bifluoride Magnesium (g/L) X 19.0 = lbs/1000 gallons sodium bifluoride Total sodium bifluoride required = A + B in lbs/1000 gals

	OR
	For Liters
	Calcium (g/L) X 1.38 = g/L sodium bifluoride Magnesium (g/L) X 2.28 = g/L sodium bifluoride Total sodium bifluoride required = a + b in g/L.

Calculation of the Sodium Bifluoride Requirement

OR