A HYDROMETALLURGICAL PROCESS for the TREATMENT of INDUSTRIAL WASTES by Harold P

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A HYDROMETALLURGICAL PROCESS FOR THE TREATMENT OF INDUSTRIAL WASTES by Harold P. Rajcevic Vice President, Recontek, Inc. 11770 Bernard0 Plaza Ct., Suite 351 San Diego, CA 92128

Undoubtedly, members of the American Electroplatere and Surface Finishers (AESF) Society will agree that our nations'rp growing waste problem is monumental. The 160 million tons of garbage we generate each year are rapidly filling up our landfills. In addition to the 14,000 landfills that shut down during the last decade, nearly a third of the U.S.'s 6,500 existing landfills are expected tu close within the next five years. This has prompted exportation of waste over state lines, precipitating an unprecedented "waste war" between the states. Among the actions and protest between states have been these unsettling examples: This past October, Pennsylvania Governor Robert Casey issued an executive order freezing the amount of out-of-state waste Pennsylvania landfills can accept. Last March, New Mexico Governot Carrey Caruthers declared a year long moratorium on new landfills. And most recently, Louisiana citizens turned out to protest when a train carrying Baltimore sludge (dubbed the @#pooh-poohchoo-chooI8) tried to unload. I While metal wastes make up only about 9%, by weight, of the nations's wastes, a solution must be found to handle these toxic materials that is acceptable to the public and industry alike. Up to now, the solutions have included incineration, landfilling, and deep well injection. Critics contend, however, that authorizing more of these types of disposal sites would only be treating the symptom and that new sites of this nature will themselves become environmental hazards. Clearly, this is an unacceptable solution, since the most recent EPA data shows that of the approximately 1200 sites on or proposed for the Superfundls National Priority List, cleanups have been completed at only 41 sites and begun at another 204. A significant answer to America's waste problem lies in recyclinq. For the past three years, we have been actively involved in developing a hydrometallurgical process to treat industriql wastes typically generated by the plating, finishing, galvanizing, and pickling industries. The three main objectives of the R h D work were as follows: 1. To recover the contained metal values in the wastes: 2. To develop a process with zero discharge to the atmosphere; and

1 3. To eliminate all land disposal for any waste. The pilot work has been completed and we are now in the process of building a commercial plant that will treat about 50,000 tons/year of industrial wastes in Newman, Illinois, with other facilities in various stages of planning around the U.S.

The hazardous wastes to be treated by the process, all of them inorganics, are classified as follows: 1. Alkaline liquid wastes 2. Cyanide liquid wastes 3. Mixed acids liquid wastes 4. Wet sludges - F006 wastes 5. Dried sludges - F006 wastes Each of these materials may contain the following elements at different concentrations: copper, nickel, tin, lead, iron, chromium, zinc, cadmium, and precious metals; in addition, salts of calcium, sodium, and barium as chlorides, sulfates, nitrates, and phosphates may be present as well as inert materials such as silica, magnesia, alumina, and others. Because of the extreme variability of the materials to be processed, the flow sheet was developed as a batch-type process. scriati on of Wastes - Feed Mater ials The materials to be processed include: Class 1 - Alkaline Solutions: Typically produced from boiler blowdowns, these solutions contain small amounts of metallics and some sodium salts--the pH is over 9; the ammonium, fluorides and nitrates are usually less than 100 PPm. Class 2 - Cyanide Solutions: These solutions are generated by the metal finishing industry and contain both sodium hydroxide and sodium cyanide, plus cyanide salts of various metals. The pH is over 9 and the ammonium, fluorides, and nitrates are all less than 100 ppm. Class 3 - Mixed Acids: These solutions are also typically produced by the metal finishing industry. They contain metallic salts as well as free acids (HC1, H,SO,, HNO,, etc.) from such industrial processes as pickling, galvanizing and others. The pH should run in the 1-4 range.

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Class 4 - Wet Sludges: Generally produced from FOO6 wastes by alkaline precipitation (neutralization), these materials are usually slurries containing varied amounts of solids in suspension. The free water content may be as high as 75080%. Class 5 - Dry Sludges: Dry sludges are very similar to wet sludges, except that the generator has removed most of the free water by filtration prior to shipment. This brief description of the above materials shows the tremendous variability of the feed materials and at the same time, the incompatibility of mixing these wastes in their original state. For these reasons, the flow sheet was developed as a combination of several individual processes or systems in which pre-treatments take place before combining flows for actual metal recoveries. An over-all view of each system is described below. These systems are carried out in their own individual areas with tanks, reactors, pumps, etc. exclusively dedicated to them to prevent cross-contamination.

Svstem 1 - Treatment of Alkaline Wastes Alkaline solutions contain small amounts of metallics and typically sodium hydroxide and have a free water content of about 90-95%. Average assay of these wastes is as follows: Water 90-95% Sodium Sulfate 2-4% Sodium Chloride 1-2% Metals < 0.1% Sodium Hydroxide 1-2% The basic step in the pre-treatment of alkaline solutions consists of eliminating the free water while concentrating the metals and salt content left behind. This is accomplished in a glass lined reactor where water is evaporated and collected for use throughout the plant. The volume in the original wastes is reduced by 75-80% and the sludge left behind is collected and transferred to the caustic system along with sludges coming in from diiferent sources.

Svstem 2 - Treatment of Cvanide Wastes Cyanide wastes are generated by the metal finishing industries and the typical composition is as follows:

3 Water 80% Metals 10% Sodium Cyanide 6% Sodium Hydroxide 42 The preliminary basic step in the pre-treatment of these wastes is the destruction of cyanides by a thermal and oxidation process. The solutions are fed to a reactor where the temperature is raised to the boiling point, while air or oxygen is continuously introduced to the solution. The reactions taking place are represented by the following chemical equations:

1. 4NaCN + 2H,O + 50, + 4C0, + 2N2 + 4Na(OH)

2. 4KCN + 2H,O + 50, + 4C0, + 2N, + 4K(OH) These equations are typical for any alkaline cyanide. Metal cyanides are decomposed by the same thermal oxidation, producing metal hydroxides as shown below:

1. 2NaCu(CN), + H,O + 50, -* 2Cu(OH) + 2NaOH + 4C0, + 2N,

2. 4Cu(OH) + 2H20 + 0, + ~CU(OH)~

3. 4NaCu(CN), + 6H,O + 110, -* 4Cu(OH), + 4NaOH + 8C02 + 4N, These reactions are very slow and require long refluxing periods in the reactor to reduce the cyanide concentration in the sludges to less than 0.1 ppm. While the destruction of cyanides is taking place, the metals which were in solution precipitate out as metal hydroxides and are separated by filtration: the hydroxides are reprocessed along with the incoming sludges and the alkaline solutions are fed to the caustic process. The CO, and N generated in the reactor is exhausted through a caustic scrubber and finally vented out of the stack.

System 3 - Mixed Acid Wastes These metal finishing wastes are typically solutions containing the following components: Metals (Cu,Ni,Cr,Zn) 3-5% Water 65-70% Chlorides 5-10% Sulfates 1-3% 1 Free HC1 3-6% Some solids in suspension are usually present. The first step in the pre-treatment process consists of an anion interchange displacement of HC1 by H,SO,. This reaction is carried out in a glass lined reactor connected to a reflux column, condenser and receiver; the reflux column contains berl saddles that provide 4 the desired fractionation. The solution is heated to 228'F (HC1 distillation temperature) and enough H,S04 is added to produce the following reaction:

2 Na C1 + H2S04 4 2HC1 + Na,SO, or M C1, + H,SO, -* 2HC1 + M SO, The chlorides are displaced and condensed as concentrated HCl; any amounts of HNO present will be decomposed to N gas and H,O. The HC1 produced 2s sold back to the original waste generators or in the open market. The contents in the reactor, now converted to sulfates, are filtered to remove any solids present; these solids are fed back to the system treating sludges while the clear sulfate solutions are collected for further treatment. If the chrome content in the acid filtrate is low, then the solution is transferred to the copper electro-winning system; if chrome is present in significant amounts (valence 6) then SO, is introduced to reduce the Cr to Cr++*. When this has been accomplished, the Cr++* is precipitated as Cr(0H) using sodium hydroxide. After filtration, the chromium hydroxide is dried and sold in the open market. Systems 4 and 5 - Treatment of Slugaes Sludges being processed in these systems are F006 sludges generated from alkaline neutralization; they are either !'wet sludgestg containing 75-85% water and shipped to treatment facilities as such or "dry sludges" which are essentially wet sludges which the generator has filtered and dried to reduce both transportation and incoming weight charges. In the new process, wet and dry sludges are classified into two systems: a) sludges containing zinc, and b) sludges without zinc. Typical analyses in both systems are shown below: A. Wastes with Z inc B. Waste s without Zinq CU 8-10% Ni = 0.5-1% Sn = 0.5% Pb = 4-5% Fe = 2-3% Cr = 1-2% Zn = 8-10% Cd = 0.1% Wastes grAgtcontainina zinc are digested in a sodium hydroxide circuit dissolving zinc, lead, and cadmium, while any copper, nickel, iron, and chromium remain in suspension. Any caustic solution or residue set aside in the treatment of alkaline or cyanide wastes is also fed to the system treating wastes containing zinc.

5 When the reaction is completed, the slurry is filtered to remove the insolubles which are fed to system v@B1utreating wastes without zinc. The filtrate is transferred to a cementation system where zinc powder is added to cement out any element more electro-negative than zinc in the electro-motive force series. The lead and cadmium, after precipitating out, are removed by filtration, leaving only zinc in solution as sodium zincate. The Pb-Cd residue or concentrate is dried and sold to cadmium or lead producers: the zinc solution is fed to an electro-winning system to recover the zinc as a metallic product. Wastes withput Zinc fB1, along with any residues generated in the previous system, are blended and leached in a strong sulfuric acid solution dissolving any copper, nickel, chromium, and other metals. Aeration, temperature and agitation are used to speed up the reaction. Insolubles, such as lead sulfate, calcium sulfate and precious metals remain in suspension during leaching. At the end of the leaching period, copper powder is added to cement out any precious metals, especially silver that may have been dissolved during digestion. After filtration, the insolubles are dried, bagged, and sold to precious metal refiners for the recovery of precious metals and lead. The filtrates will have in solution the copper, nickel, some iron, and chrome. Depending on the level of the chromium content the solution may be processed through the chromium recovery system already described. Also, depending on the nature and origin of the sludges, HC1 and HNO, may also be part of the solution generated. In this case, the solution is treated by anion exchange as previously described in the mixed acid system. If no HC1 or HN03 are present, the solution is pumped directly to the copper electro-winning circuit for copper recovery. Electro-recoverv of Metal Values During the treatment of alkalines, cyanides, mixed acids, and sludges, solutions were produced in which metals such as copper, zinc, nickel, and other minor elements were solubilized either in an acid media (H2S04)or in an alkaline solution (NaOH). The next step in the process recover these metals in a saleable form, either as metallic products or as salt compounds. The recovery of copper and zinc is accomplished by electro- winning, while nickel is recovered as nickel sulfate by evaporation and crystallization. The electro-winning recovery of copper and zinc are very well known technologies practiced in many operating plants in the U.S. and around the world. Nickel, which is present in the copper electrolyte because of the relatively low concentration of H2S04 and the high water content is recovered by evaporation and crystallization. Nickel sulfate becomes less soluble as the concentration of H2S04

6 increases and the recovery of nickel i6 based on this principle. The treatment consists of evaporating a large portion of the water contained in the tail copper elactrolyte resulting in an increase in H2S0 concentration and forcing the NiSOz to crystallize out. hhis evaporation proceso could be continued to such a high concentration of H,SO, that all the NiSO, would be crystallized, but in order to avoid slower settling rate with increasing densities of the solution, this process is considered finished when a density of the atolution of approximately 58-60 Be' corrected to standard conditions is obtained. At this point some NiSO, will remain in solution. The evaporated water is then collected and reused in the different process of the plant; the nickel sulfate salt is decanted, filtered, washed and shipped out of the plant to nickel producers. The percent nickel in the salt varies between 23 and 25%. The liquor from filtration, by now free of metallic elements, is sulfuric acid with a concentration of about 751. This acid is recycled back through the system to treat new wastes. Any surplus of acid can be sold back to the waste generators or on the open market. Sodium hydroxide solution collected after the electro-winning of zinc is also recycled back to the system: any excess water can be distilled off with the water being reused. Due to the variety of industrial wastes being treated, some elements such as sodium and iron will build up in the solutions in process. Secondary operations have been developed to isolate impurities build-up and their recovery. production of Sodium Sulfate Wa2S0,) Sodium contained in the industrial wastes arriving at the plant will either be eliminated in the residues containing the lead and precious metals or it will build up in the solutions used to recover copper, zinc, nickel, etc. A bleed is necessary to maintain the balance in the different systems. Sodium can be isolated in a usable end product as sodium sulfate. The recovery is accomplished in the same manner as nickel sulfate by evaporation and crystallization, The water eliminated is collected and used in the process while the sodium sulfate is filtered, dried, and shipped out in 2000 lb. bags. If any build up of iron take place, a secondary process is also available to bleed this element out of the system. Ninety-nine percent of the iron arriving at the plant in the industrial wastes is contained in sludges carrying zinc. Since very little iron will dissolve during caustic digestion, most of it will report in residues to be fed to the acid

7 digestion system. Some iron will remain insoluble, but mom8 will dissolve in the acid circuit as Fe2(S0J3. By the addition of NaOH to the solution in a holding tank, the iron can be precipitated out as jarosite. After filtration, the jarosite can either be sold or converted to ferrous mulfate in solution by dissolving with HS04 and reducing with SO,. After adjusting the pH to the proper fevel, ferrous sulfate rolutions can be sold on the open market. This is a brief description of our new process to treat hazardous industrial wastes. The flow sheet clearly demonstrates that treatment and recycling is the long term alternative to landfilling, especially when generators consider the increased liability associated with the disposal of these wastes. The process will eliminate the liability for the generators once the shipment is accepted at the plant; it will also recover the metal values and share the credits from their recovery with the producer, making the over all cost for waste disposal much lower than landfill disposal. Society will no longer permit us to contaminate the ground, air, and water resources: it is industry's responsibility to look for acceptable alternatives, and with our new process, we will be one of the leaders in this new field.