The present invention relates to a novel process for the treatment of aqueous streams such as waste streams and metal ion-containing or other precipitatable material aqueous streams, with a minimum of treatment water by utilizing electrodialytic water splitting, and in some instances, reverse osmosis in sequence.
At the present time, given the ever-increasing cost of energy and the increased demands for the control of environmental pollution on the one hand, and the need to conserve important metal resources on the other, there arises a serious need for new and cost-effective processes to treat industrial process streams some of which are dilute in metals. Sometimes this is done for the purpose of recovering metal values; other times it is for the purpose of avoiding environmental pollution.
Typical metals which are sources of environmental pollution and which are valuable as minerals are copper, lead and cadmium, which are found in a wide range of waste waters resulting from plating and finishing processes such as metal finishing, from various rinses from pickling operations, from acid mine drainage and also from the extraction of metals as from depleted ore tailings by dump leaching. In certain processes, the rinse solutions are so dilute that a simple process of neutralization, precipitation and collection of an insoluble precipitate suffices. However, in the case of cadmium and lead as examples, the allowable toxic metal levels in effluents are so low that removal by precipitation often does not suffice. In the case of the copper recovery from the acid leach dumps, the residual concentration of acid is so high as to require a substantial amount of base for its neutralization.
In many industrial processes which formerly were able to employ lime for the precipitation of heavy metals and for the neutralization of acid such as sulfuric acid, with the formation of insoluble calcium sulfate, these are not longer environmentally acceptable because even the low level of contamination of heavy metals causes the solid waste to be classified as a toxic waste, requiring transportation to special dumps at considerable expense.
Electrodialysis, including especially electrodialytic water-splitting, processes are well known and of considerable value in industry. In electrodialysis, electrolytes are removed by the electric current from a feed compartment and selectively transported across ion-permeable membranes into a concentrate stream, with the feed stream being thus converted into a deionate stream. In water splitting, a feed stream contains anions and cations, wherein the anions of the feed stream plus hydrogen ions from the water pass by the current from a bipolar membrane into an acid compartment, while cations from the feed stream plus hydroxide ions from the water pass from the bipolar membrane into a base compartment.
The electrodialysis process is highly developed and is described in several texts including the chapter by Shaffer and Mintz in "Principles of Desalination, Part A" edited by Spigler and Laird, Academic Press, New York 1980. Bipolar membranes and the process of water splitting are also well known. Bipolar membranes have been articles of commerce for several years, with those described by Dege, Chlanda et al., U.S. Pat. Nos. 4,116,889 and 4,253,900 as recent examples. Water-splitting processes are also described in several recent patents including U.S. Pat. Nos. 3,787,304, 4,219,396, 4,238,305 and 4,107,015.
The problems which arise from the occurrence of precipitation on the surfaces of monopolar and bipolar ion exchange membranes are well known. They result in an increase in ohmic resistance and, upon continued passage of current, a significant deterioration of the membrane takes place. It is known to occur under ordinary conditions of electrodialysis where unwanted water splitting occurs with a pH shift at the surface of the membrane. For example, when a calcium bicarbonate solution is being demineralized, the pH shift occurring at the surface of the anion-permeable membrane acts to produce base in the feed compartment with the conversion of soluble bicarbonate to insoluble carbonate. This effect can be so severe that a periodic acid wash is required to prevent build-up of insoluble salts. Sometimes current reversal is employed for the same purposes.
Similarly, in the concentration of salts under conditions of electrodialysis, concentration polarization taking place at the surface of the membranes in the concentrate can result in the solubility product of sparingly soluble compounds being exceeded, with the formation of insoluble salts and a harmful precipitate forming on the surface of the membrane. An example is to be found in the patent of Chlanda, Gregor and Liu, U.S. Pat. No. 3,752,749, wherein, during the concentration of a solution of the sparingly soluble compound fumaric acid, a precipitate of fumaric acid forms on the anion-permeable membrane in the concentrate cell; it can also form to an extent so high that it blocks the cell ports. Deterioration of the process and eventual destruction of the cell can result. Possible solutions entailed the use of an elevated temperature to increase solubility or the need to lower the temperature of the external, circulating concentrate stream to precipitate and settle out the solid so as to maintain a cell concentration below that defined by the solubility product constant of fumaric acid.
All of these harmful precipitation effects are particularly troublesome in water splitting processes. Here, traces of calcium and magnesium in feed solutions of sodium or potassium salts to be converted into the base can result in the formation also of the insoluble hydroxides or carbonates of calcium and magnesium, being formed usually on the face of the cation-permeable membrane facing the base compartment. Even traces of such metals can substantially inhibit the water splitting process for producing sodium hydroxide or sodium carbonate.
Similarly, the conversion of soluble salts from the feed compartment to form insoluble metal hydroxides by water splitting is not practical in conventional water splitting because of the substantial formation of the insoluble precipitate, usually on the surface of the cation-permeable membrane in the base compartment. Similarly, a leak of hydroxide ions across the cation-permeable membrane from the base compartment into the feed compartment in the case of water splitting can produce a precipitate of insoluble calcium or magnesium salts on the surface of that membrane in the feed compartment. Since trace amounts of these cations are present in most sodium feeds, one finds this additional, deleterious effect, which may require for its solution a prior precipitation of the calcium and magnesium salts from the sodium chloride feed prior to conversion by water splitting, as by the addition of carbonate prior to its passage into the cell.
The harmful effects of precipitation are also observed in two-compartment water splitting devices of a kind described in U.S. Pat. No. 4,107,105 where a feed of a mixture of a sodium sulfate and sodium bisulfite is fed into the acid compartment of a water splitting cell containing only cation-permeable membranes separating the bipolar membranes, where in the acid compartment the feed becomes acid from the acid arising from the bipolar membrane, liberating the gases sulfur dioxide and sulfur trioxide. The sodium ions move across the cation-permeable membrane into the base compartment. In the base compartment one finds that traces of divalent metals such as calcium and magnesium can form insoluble precipitates, usually on the side of the cation-permeable membrane facing the bipolar membrane; the harmful effects described earlier will ensue.
Likewise, the technique of reverse osmosis (hereinafter "RO") is well known and has been utilized for various applications. For example, RO has been employed in the automobile industry to treat the rinse water emanating from various painting operations.
Yet, these technologies have not been heretofore utilized cojointly in a fashion to deal with the several and diverse problems associated with the problem streams described herein.
It is accordingly an object of the invention to provide a process which will handle all these manifold problem streams, one which does not require elaborate chemical processing and one which can be operated on the very large scale of acid leach operations on the one hand as for copper recovery and also on a small scale for battery manufacture, smelters, electroplating, metal finishing and pickling operations on the other.
Another general object of this invention provides a process which can eliminate the disposal problems associated with toxic wastes. A related and more specific object is to provide a process in which the toxic wastes that would otherwise have to be disposed of are rendered useful so as to be capable of being returned in some fashion to either the manufacturing process involved or as an aid in the treatment of the stream resulting from such manufacturing process.
It is also an object of this invention to prevent adhesion to the membrane of insoluble precipitates which form during electrodialysis, particularly in such processes as those used to treat Kraft Black Liquors in which lignin acids precipitate, without and/or apart from the conventional measures such as intermittent treatment by acid or base to dissolve harmful precipitates, or increasing the temperature of the stream or cooling to precipitate outside the cell.