US3475219A - Bright treatment for workpieces having toxic carryover - Google Patents

Description

United States Patent 3,475,219 BRIGHT TREATMENT FOR WORKPIECES HAVING TOXIC CARRYOVER Leslie E. Laney, Ellwood City, Pa., assignor to Laney Laboratories, Erie, Zelienople, P2,, a corporation of Pennsylvania No Drawing. Filed July 12, 1966, Ser. No. 564,550 Int. Cl. 1508b 7/04, 1/02 U.S. Cl. 13427 4 Claims ABSTRACT OF THE DISCLOSURE An aqueous chemical neutralizing solution containing an alkaline metal hydroxide and permissibly soda ash and a high concentration of chlorine having a relatively high pH of 8 to 14; it also contains .1 to 1 g./l. of benzotriazole. The solution is applied directly to an acid or cyanide carry-over on workpieces having copper or silver surfaces to immediately form a protective coating on such surfaces and to maintain such coating throughout the full neutralization of the toxic carry-over and subsequent water rinsing of the surfaces.

This invention relates to the treatment or conditioning of aqueous solution carry-over, such as adheres to metal workpieces or articles, for removing or neutralizing its toxic material content. A phase of the invention deals with maintaining workpieces having easily tarnished or oxidized metal surfaces in a bright condition or inhibiting their oxidation while quickly and effectively conditioning the aqueous solution carry-over to make it non-toxic or innocuous. Another phase of the invention deals with an improved toxic carry-over conditioning solution.

A problem has been presented from the standpoint of complexity and size of equipment required, cost of materials, the time element and the difficulty of completely conditioning toxic solution carry-over having a cyanide or acid content, without tarnishing or oxidizing copper, copper alloy or silver surfaces of the metal workpieces. The need has been for a procedure or approach to the problem which will enable a conditioning solution to be recirculated and the workpieces to be moved in-line, in the manner shown in US. Patent No. 2,725,314, from a previous processing treatment bath through a toxic carryover treating bath, without the necessity of stopping the movement of the workpieces or greatly increasing the size or length of the bath container, and in such a manner as to fully treat the carried over solution without damaging or tarnishing the highly susceptible surfaces of the workpieces. The difficulty has been to provide some approach to the problem that will meet all the factors involved; that will not adversely effect the removal or neutralizing procedure or cause surface staining of the workpieces, e.g., from the standpoint of adverse reaction with the chemicals or the reaction products of a neutralizing solution conditioning of the toxic carry-over, of making reconditioning-recirculation of the solution impractical, of limiting the concentration of the conditioning chemicals, of requiring an increased time period to complete the removal of the toxic material such as to make continuous in-line treatment impractical, and from the standpoint of surface-oxidizing difficulties during movement of the workpieces to a water washing or rinsing station or at such station.

Copper plated or cuprous alloy, brass or silver plated workpieces which have been subjected to a cyanide-type of plating, etching or treating solution, and workpieces having copper or cuprous alloy surfaces which have been subjected to acid pickling, acid brightening, or acid plating processing, have a toxic chemical solution carry-over that should be chemically treated to render the solution 3,475,219 Patented Oct. 28, 1969 "ice innocuous prior to passing the workpieces through a water rinsing bath or before their processing has been completed. A treatment of copper, copper alloy (including brass) and silver surfaces of workpieces that have been subjected to a cyanide-type plating process is to rinse them in a solution containing an alkaline metal hydroxide, such as caustic soda, to provide a pH of about 8 to 14, with the introduction of chlorine to oxidize the cyanide. However, it has been determined that for a complete treatment of the carried-over aqueous solution (no residual cyanide), it is desirable to maintain a chlorine concentration of a minimum of 500 mg./l. up to a maximum of about 2000 mg./l.

It has been discovered that although a chlorine concentration of about 100 mg./l. up to under 500 mg./l. produces minimum tarnishing or oxidation of copper, cuprous alloy and silver surfaces or plated coatings, that the decomposition of the cyanide complex in the solution is very slow and may involve an hourly period. For example, a 100 mg./l. chlorine concentration produces a decomposition of the silver cyanide complex of only about 50%, using an hour of treatment. A 200 to 300 mg./l. of chlorine concentration will decompose about of the complex within an hour. However, it has been discovered that it is necessary to use a much higher concentration of chlorine to assure decomposition within periods of a few minutes or less. Such rapid decomposition is important from the standpoint of safety of the operation, in that when the treated solution is to be recirculated from the treating tank it may be returned for use again in the treatment tank. In this connection, the change must be rapid enough to avoid cyanide drag-out from the treatment tank and also insure faster flow through the treatment tank and to minimize any delay in flow from the reservoir tank. It is imperature that the cyanides be substantially completely destroyed or removed from the carry-over of the workpieces.

A heretofore limiting or offsetting factor as to the concentration of the chlorine or other treating chemical used, is that a freshly treated or plated copper, cuprous alloy or silver surface is prone to discoloration due to the oxidation as, for example, in a high chlorine concentration solution. It has thus been necessary to limit the chlorine solution concentration in the aqueous toxic removing or neutralizing solution for workpieces having such a type of surface to under 500 mg./l. and to under about mg./l. of chlorine for silver surfaces. Even under such limitations and with some alloys difficulty was encountered, in that a slight tarnishing or darkening of the surfaces resulted with uneven oxidation leading to streakiness, etc.

It was discovered that the solution to the problem involved the provision of a protective, corrosion or oxidation inhibiting coating, layer, or film on the bright surfaces of the workpieces that is immediately or simultaneously applied thereto with the application of the neutralizing or toxic carry-over material removing solution. Contrary to what was expected, it was determined that such proection could be accomplished, employing the neutralizing solution as the carrying agency, irrespective of whether the carry-over on the workpieces is of acyanide or an acid solution type, without adversely effecting the desired reactions of the neutralizing or treating solution chemicals, and without adverse reactions with either the treating solutionchemicals or the reaction products produced with the toxic carry-over. It was also discovered that contrary to expectations, such a protective film will be retained on the workpieces even after they leave the treating solution bath and while they are moved or advanced to a washing water rinse. The latter is highly desirable in that it has been determined that the bright surfaces of the workpieces are sensitive to air oxidation immediately after removal of the toxic carry-over. It was discovered that an organic inhibitor could be employed to electro-chemically form an organo-metallic monomolecular layer of film. Such a film is extremely thin and will tie down and immobilize metal ions that would normally tend to combine with oxidizing elements in the treating solution or the air. Such an inhibitor will be effective even in a treating solution of a relatively high pH and even when the solution is heated to a temperature within a range of about 120 to 212 F.

As to toxic cyanide carry-over of the workpieces, it has been found that a range of chlorine concentration of over about 500 to about 2000 mg./l. or an optimum of about 1000 to 1500 mg./l. may be successfully employed to accomplish a complete reaction with the cyanide, within a period of approximately to minutes, and that the surfaces will be inhibited or protected from streaks, tarnishing or oxidation by employing a stable organic inhibiting chemical, e.g., 1, 2, 3 benzotriazole, C H NHN benzene azimide, in the solution within a concentration range of .01 to 1 g./l. Such a chemical is not only stable in a chlorinated solution, but also in an alkaline solution employed to neutralize work emerging from an acid pickling, brightening, or plating solution, where the workpieces are being subjected to a wash in a neutralizing treatment solution to which caustic soda and soda ash have been added and which solution may be operated at an elevated temperature. The inhibitor enables the full necessary concentrations of chlorine and other chemicals without adversely effecting their reactions or slowing down the processing.

In other words, an inhibitor such as 1, 2, 3 benzotriazole, allows an increased range of treatment solution concentration, and limits or minimizes control requirements by widening the margin for safe operation. It minimizes controls while, at the same time, leads to faster oxidation of cuprous cyanide and silver cyanide solution complexes that are normally slow in their decomposition.

It is well known that cyanide compounds, such as those of sodium, potassium, zinc or cadmium are decomposed in a matter of minutes, in comparison to those of copper, cuprous alloys and silver. It has been discovered that increasing the chlorine concentration in the treatment solution to hasten the oxidation reaction of the cyanide compounds will also effect the tendency to oxidize the metallic surface carrying the toxic chemical film solution that is to be broken down or neutralized. The oxidizing tendency of the chlorine in a chlorinated treatment solution or of the air when the metal surface is subjected to a neutralizing solution of high pH alkaline metal hydroxide, and especially at an elevated temperature, is overcome by the protection provided by the inhibitor and the invisible organo-metallic compound that is formed in view of the inhibiting action described. The chemical break-down of a cyanide solution is as follows:

As intimated, similar difficulties have been encountered in the proneness of copper and various cuprous alloys to oxidize under waste treatment after acid processing. The conventional treatment has been to rinse the workpieces after acid pickling, brightening or plating in a neutralizing solution containing caustic soda and soda ash, using a reducing agent, such as sodium hydrosulfite (Na2S O to limit oxidation of the bright copper surface by air, by the caustic soda or by remaining oxidizing chemicals such as nitrates residual from a pickling process. To insure substantial freedom from oxidation, it has been found necessary to use sodium hydrosulfite at a concentration of about 500 mg./l. and to maintain the caustic soda and soda ash content to yield a pH of about 8 to 11. The copper salts dragged-in from the acid processing are then precipitated as cuprous oxide.

Sodium hydrosulfite is a relatively expensive chemical, costing at the present time in the neighborhood of about twenty-five cents per pound, and it is readily consumed due to the quantity of copper processed, and also due to the fact that it is lost from oxidizing action of nitrate ions and, with air, itself, as the solution is aerated.

Incorporating a chemical inhibitor in accordance with the present invention, enables the complete elimination of sodium hydrosulfite in the make-up of the treating solution, to thereby greatly reduce the cost. An organic inhibitor chemical has been found to be relatively stable and to need only minimized replenishment, even in a continuous in-line process with treating solution recirculation, such as contemplated. Using benzotriazole, caustic soda and soda ash may be made up to yield a pH of 8 to 14 which is a much wider and more desirable range than heretofore possible with sodium hydrosulfite and it is thus easier to control. Benzotriazole is incorporated in the aqueous solution in an effective working range of concentration of .10 to 1 g./ 1. An optimum range of this chemical in a treating solution after toxic cyanide or acid treating or processing has been found to be about .1 to .2 g./ 1. It produces a film on the workpieces that is invisible.

The toxic compounds under consideration, dealing with copper and silver treating solutions, are the cyanides, such as alkali metal cyanides and metal complex cyanides, such as cuprous cyanide, silver cyanide, and cyanides of zinc or nickel when treating copper alloys containing these metals. The cyanide, itself, is oxidized by the chlorination reaction. The copper becomes insoluble in a cuprous hydroxide or cuprous carbonate form. However, it is preferable that it become insoluble in the cupric oxide form. It has been determined that the latter will occur if sufiicient chlorine is available to oxidize, not only the cyanide, but also the copper ion, itself. A high chlorine concentration or a high pH, elevated temperature, caustic soda concentration accomplishes the same results. Cupric oxide is less soluble in water and is more inert when the solid waste is discarded, considering the possible toxic effect of precipitated sludges.

Another consideration is that cuprous cyanide by, itself, is insoluble as is silver cyanide. Such cyanide complexes are made soluble in a solution containing additional alkali metal cyanide compound. This means that for the dissolution of one mole of cuprous cyanide, an additional mole of sodium or potassium cyanide is needed. The same is true with silver cyanide. It has been determined that when oxidizing a chemical solution containing alkali metal cyanides and copper or silver cyanide complexes, the alkali metal cyanide may be decomposed with an insufficient concentration of chlorine; and, in view of the loss of alkali metal cyanide, cuprous cyanide will fall out as an insoluble compound in a precipitate. This same effect will also occur in a similar manner with silver cyanide. In such a case, the cuprous or silver cyanide would remain as an insoluble solid in the sludge. Although reference has been made to insoluble cuprous cyanide and silver cyanide, the condition of insolubility is not an absolute one. Even though less than one mg./l. of cuprous or silver cyanide would be soluble in rain water or in the river where the sludges may be carried, this range of solubility is still suflicient to create a toxic condition. In accordance with the invention, it is now practical to employ a high chlorine concentration in the treatment solution, without oxidizing the surfaces of the work pieces, that is sufiicient to quickly oxidize and break-down the metal cyanide complexes before these can precipitate as sparsely soluble but still toxic metal cyanides.

By employing the principles of the present invention, it is now practical to employ an in-line process, such as set forth in US. Patent No. 3,218,254, for copper, copper a1- loy and silver surfaced workpieces, without spoiling the brightness of the surfaces and even when the treatingsolution is maintained at a relatively high pH and is heated to minimize the time required to accomplish a complete removal of toxic carry-over.

I claim:

1. In an in-line process of treating copper, copper alloy and silver surfaces of workpieces that are sensitive to oxidizing discoloration for removing toxic carry-over in the nature of toxic cyanide in solution, the steps of continuously-progressively advancing the workpieces with toxic carry-over on their surfaces from a toxic chemical solution applying zone into and through a first treating zone for the toxic carry-over and subsequently into and through a water washing second zone; in the first treating zone, providing and maintaining an aqueous toxic material neutralizing chemical solution having a pH of about 8 to 14 and containing a pH promoting alkaline metal compound providing and maintaining chlorine in a concentration of about 500 to 2000 mg./l. in the neutralizing chemical solution of the first treatment zone, applying the aqueous chemical neutralizing solution directly to the toxic carry-over on the workpieces under relatively high oxidizing conditions in the first treating zone during their movement therethrough and substantially completely neutralizing the toxic carryover of the workpieces in such zone, providing about .1 to 1 g./l. of benzotriazole in the aqueous chemical neutralizing solution and maintaining it in a stable condition with respect to the chemical content of the solution and to the content of the carry-over on the workpieces and to reaction through the second zone and while washing-01f their surfaces in the second zone.

2. In a process as defined in claim 1, maintaining the neutralizing chemical solution within a raised temperature of about 120 to 212 F. in the first treating zone during the introduction into and movement of the workpiece therethrough.

3..In a process as defined in claim 1, providing and maintaining the benzotriazole in the neutralizing chemical solution within an optimum range of .1 to .2 g./l.

4. In a process as defined in claim 1, providing and maintaining the aqueous toxic material chemical neutralizing solution with a pH of about 11 to 14.

References Cited UNITED STATES PATENTS 2,257,133 9/1941 Shoemaker.

2,618,606 11/ 1952 Schaeifer 252137 2,725,314 11/1955 Lancy 134-15 XR 2,830,942 4/1958 Elliot 134-29 XR 2,877,188 3/1959 Liddell 252-389 2,941,953 6/1960 Hatch 212.7 XR 3,295,917 1/1967 Cotton 21-2.7 XR

MORRIS O. WOLK, Primary Examiner JOSEPH T. ZATARGA, Assistant Examiner US. Cl. X.R.