I/EC
Industrial Wastes
Plating Waste Treatment — No Longer a Necessary Evil Treatment integrated into the processing line minimizes undesirable aspects of required disposal system by Lawrence A. Greenberg,
INDUSTRIAL
WASTE
treatment
has
been viewed by m a n y people as a necessary evil. It is necessary to conserve our remaining w a t e r resources a n d to recover those t h a t have been lost because of excessive pollution. Industrial waste treatment is at the same time a n evil to our industrial economy as it imposes a n increased cost on production. It requires a n initial investment to construct a n d e q u i p the waste treatm e n t plant a n d a n a n n u a l operating expenditure with, in the majority of cases, no possibility of a r e t u r n on this investment. A waste t r e a t m e n t process for cyanide a n d chrome has been developed which minimizes this evil. I n the plating process, the work piece is immersed in a plating b a t h for a specified length of time a n d then removed. As the work piece is removed from the plating b a t h , it brings with it a small volume of plating solution which has a d h e r e d to the p a r t . This solution, usually referred to as drag-out, is the source of cyanide a n d hexavalent c h r o m i u m waste. T r e a t m e n t of this waste as produced in its smallest possible volume can affect considerable economy. T h e process which was developed by Dr. Leslie E. L a n c y ( U . S . Patent 2,725,314) is usually referred to as the " I n t e g r a t e d W a s t e
Fischer & Porter Co.
T r e a t m e n t System." T h e n a m e refers to the fact that the system is integrated into the processing line. Fischer & Porter Co. is licensed to design, engineer, a n d sell the integrated waste t r e a t m e n t system. T h e most popular m e t h o d of t r e a t m e n t of cyanide is alkaline chlorination, which results in complete destruction of the cyanide by oxidation to c a r b o n dioxide a n d nitrogen. T h e reaction takes place in three steps: NaCN + Cl2 — CNC1 + NaCl CNC1 + 2NaOH - * NaCNO + NaCl + H 2 0 2NaCNO + 4NaOH 4- 3C12 -» 6NaCl + 2 C 0 2 + N2 + 2H 2 0 T h e drag-out from cyanide plating operations also contains small a m o u n t s of heavy metal cyanides in addition to the sodium cyanide. These are then broken d o w n a n d the metal salts precipitated in reactions analogous to those with sodium cyanide. Batch Treatment of Plating Solutions
W h e n the is used, the coming out washed with more tanks the plating
b a t c h t r e a t m e n t system p a r t immediately after of the plating b a t h is fresh w a t e r in one or to completely remove solution. This is ex-
tremely i m p o r t a n t since if the solution is allowed to r e m a i n on the part, it will either contaminate subsequent plating solutions, or if this is the final t r e a t m e n t staining m a y result. Insufficient rinsing is the cause of most rejects in the plating line. These free-flowing rinses become c o n t a m i n a t e d with toxic drag-out. In the batch t r e a t m e n t system the entire volume of fresh water rinse must be collected for treatment. Collection is usually in a t r e a t m e n t tank of large enough capacity to hold the waste a c c u m u lated in one shift plus the volume of treatment chemicals. T w o tanks are usually provided so that waste can be collected in one, while t r e a t m e n t is taking place in the second tank. D u r i n g all stages of the t r e a t m e n t the contents are continuously circulated either by a n agitator supplied for each tank or by a circulation p u m p manifolded to serve both tanks. T r e a t m e n t consists of elevating the p H by the addition of caustic, a n d then simultaneously adding chlorine a n d caustic. After t r e a t m e n t the contents of the tank are discharged from the plant. Cost of Batch Systems
T h e initial investment in a treatm e n t system such as this is fairly large. Duplicate t r e a t m e n t tanks must be provided and these are usually of a fairly large size. A single large recirculation p u m p or two agitators must be provided to continuously agitate the reaction mass. I n addition to the actual capital outlay for e q u i p m e n t , the batch system usually requires a fairly large area. O p e r a t i n g costs are also high, owing to the extensive labor requirements of the system. T h e operating cost of such a system would be reduced by the use of instrumentation. T h e batch system m a y be converted to a semiautomatic controlled system by the addition of VOL. 52, NO. 8
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AUGUST 1960 61 A
INDUSTRIAL
WASTES
pH and oxidation reduction potential controls, which will reduce the operating cost, but will substantially increase the initial cost of the plant.
Advantages of the Integrated Treatment System
Operation of Integrated System
The basic concept of the integrated system is the segregation and treatment of the waste at the source. To accomplish this, the liquid film of plating solution which adheres to the part as it is removed from the plating bath is simultaneously treated and removed from the part. The waste treatment is integrated into the processing sequence and no separate treatment is required. The integrated treatment system can be employed following any step which would result in toxic waste carryover, regardless of its position in the processing lines. Although discussion here is limited to treatment of cyanide wastes, the system is applicable to treating drag-out from many other metal finishing operations. In operation the treatment wash tank is substituted for the first rinse tank following the plating operation. The treatment wash solution is continuously recirculated through the treatment wash tank and physically removes the drag-out and at the same time reacts chemically with it. The part is then rinsed with fresh water in the subsequent rinse tank. The effluent from this fresh water rinse tank is now uncontaminated by toxic drag-out and requires no treatment. The wash tank is connected to a larger treatment solution reservoir. The treatment solution is continuously recirculated between the treatment wash tank and the reservoir. The reservoir tank serves three major functions: It neutralizes the shock loading caused by sudden and irregular changes in the quantity of plating solution drag-out ; settles out the insoluble metal hydroxides and oxides formed in the reactions ; and provides adequate time for the second stage of the reaction, oxidation of the cyanate to carbon dioxide and nitrogen, to take place. Only one reservoir tank is required and several wash tanks can actually be served by a common reservoir tank. This tank is usually small compared to the tanks required for batch treatment. There is no theoretical limit on the number of treatment wash 62 A
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Lower initial investment due to lower capital outlay for space and equipment
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Lower operating expenditures resulting from ease of operation and control
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Lower unit production cost because of substantial reduction in product rejects
tanks that can be served by a common reservoir tank. The physical layout of the specific plant is usually the controlling factor. Fischer & Porter recently engineered a cyanide waste treatment system in which parallel proposals were made for a batch and integrated system. In the batch treatment system two 8000-gallon treatment tanks would have been required. The client ultimately selected the integrated system and a 2200-gallon single treatment reservoir tank was provided, with considerable saving in space. The integrated system operates without the need for close instrumented control as high excesses of treatment chemicals are used in a closed loop. Relatively simple paper and spot tests are sufficient for control of the treatment process. These tests, which can be performed by the plater, should not take more than a total of one hour's time over an 8-hour shift. Hence a separate waste treatment operator is not needed. Treatment chemicals arc added either continuously or batchwise as they are consumed by the toxic materials, although continuous addition of chemicals is the more usual procedure. In operation the treatment solution in the reservoir and the treatment wash tank following the plating tank are circulated in a closed system. Chlorine gas is added by pumping the clear supernatant from the reservoir through the ejector of
INDUSTRIAL AND ENGINEERING CHEMISTRY
the chlorinator and diffusing it into the treatment wash tank. The chlorine feed rate is manually set to maintain a solution strength of 300 to 500 p.p.in. available chlorine, although higher concentrations are not harmful. The available concentration of chlorine is checked at least once every 2 hours, using a simple paper test, and at least once a day using the starch iodide test. Caustic addition is made by means of a manually set positive displacement metering pump. The feed rate on the caustic feed pump is set to maintain approximately pH 10 to 11. The pH of the treatment solution is checked at least once every 4 hours. As the integrated system is a closed loop circulation system, the actual requirement for caustic is a function of the relative concentrations of cyanide and caustic in the plating bath. The range of caustic required is approximately 1/3 to 1 pound of caustic per pound of chlorine. Periodically the effluent from the rinse tank immediately following the treatment wash tank is checked for cyanide to make certain that no cyanide is being dragged out of the treatment wash tank. Nontoxic salts, such as sodium chloride and sodium carbonate, slowly accumulate in the treatment solution. Every two to four months, depending on the particular installation, the treatment solution should be discarded and a new treatment solution made up. The most common method of disposal of the treatment solution is to discharge it to a land locked evaporation lagoon. If no lagoon is available a chemical scavenger service can be employed to remove the contents of the solution reservoir. Other methods of disposal, based on the specific requirements of the plant, can be provided if neither of the above is suitable. This is adapted from a paper presented at the Symposium on Industrial Wastes, Division of Water and Waste Chemistry, 1 37th Meeting. ACS. Cleveland. Ohio. April 1960.
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