PROCESS FOR TREATING CAUSTIC CYANIDE METAL WASTES

Abstract: A method for operating a continuous wet oxidation system for destroying cyanide in caustic cyanide and metal wastes which scale upon heating comprises introducing raw waste into the wet oxidation reactor (14) without heating while a mixture of heated dilution water and oxygen containing gas is introduced into the reactor by a separate inlet (12). The reactor (14) is fitted with a pair of valved effluent lines (16,17) and corresponding valved lines (44, 50) for introducing dilute mineral acid into each effluent line at system pressure. Oxidized effluent and offgases exit through a first effluent line (16) while dilute mineral acid flows through the second effluent line (17) into the reactor to prevent scale buildup. Periodically the flows are switched such that oxidized effluent and offgases exit through the second effluent line (17) and dilute mineral acid flows through the first effluent line (16) into the reactor to remove scale. The oxidized effluent and offgases are then cooled and separated.

Description: This invention relates to a process for treating caustic cyanide and metal containing wastes which have a tendency to form an insoluble scale on heating. Caustic cyanide wastes containing metals are generated by the metal plating industry. The wastes are hazardous for both their cyanide and metals content. These wastes have been treated by alkaline chlorination to destroy cyanide but this treatment generally results in generation of solids which still contain appreciable amounts of cyanide. See for example U.S.-A- 4,059,514. Wet air oxidation provides an alternative method for cyanide destruction and waste treatment. The elevated temperature and pressure oxidation provides a high degree of cyanide destruction for these wastes. The high solids content of these caustic cyanide and metal wastes gives rise to scaling problems in wet air oxidation systems which can require frequent shut down to remove scale from the system. Scaling is a problem in feed lines, within the wet oxidation reactor, and in oxidized effluent lines when these wastes are treated by wet air oxidation. Numerous wet air oxidation flow schemes are known but none of them were judged to be applicable to the wet oxidation treatment of such caustic cyanide and metal wastes. U.S.-A- 3,849-536 and U.S.-A- 3,917,460 describe the wet oxidation of soda process pulping liquor to oxidize the organic constitutents therein. The wet oxidation reactor is divided into upper and lower reaction zones with preheated pulping liquor and air introduced into the lower zone where the bulk of the oxidation occurs. The oxidation mixture moves to the top of the upper zone where offgases are vented and the liquor travels downward encountering additional air introduced at the bottom of the upper zone. The highly oxidized liquor exits near the bottom of the upper zone. U.S.-A- 4,070,281 discloses the wet oxidation of cyanogen containing waste using a combination of copper and ammonium salts. Some post wet oxidation treatment is required to remove residual cyanide in the wet oxidation waste. No specific continuous flow scheme is mentioned. U.S.-A- 4,108,680 discloses a process for removing calcium oxalate scale from metal surfaces using a mixture of dilute nitric acid and manganese dioxide. U.S.-A- 4,174,280 describes the wet oxidation of liquid or solid combustible materials which are insoluble, immiscible and difficult to suspend in water. Water and air are preheated and introduced into the oxidation reactor. A suspension of waste is also injected directly into the reactor where oxidation occurs. The oxidized effluent and offgases are removed from the reactor through a heat exchanger used to preheat the incoming water and air. U.S.-A- 4,350,599 discloses wet oxidation of caustic waste liquor where preheated waste and air or oxygen react to generate carbon dioxide which is used to reduce the corrosive nature of the raw waste. U.S.-A- 4,402,837 describes a process for heat treatment of nickel cyanide waste with caustic and steam using conventional heat exchangers and a horizontal reactor with one outlet for treated liquid and another for ammonia and steam removal. U.S.-A- 4,525,283 describes a catalytic wet oxidation of waste at acid pH. A series of wet oxidation reactors is shown where preheated waste and oxygen gas enter the bottom of the reactors and the top of the reactors is provided with an effluent overflow line plus gas transfer, gas circulating and waste gas pipes. The present invention is intended to provide a process for the continuous wet air oxidation of caustic cyanide and metal wastes which provides for uninterrupted waste treatment in which the problem of excessive scaling is overcome, resulting in the continuous production of an effluent with only trace amounts of cyanide remaining therein. To this end, according to the invention there is provided a process for treating by wet oxidation a caustic cyanide and metal containing wastewater showing a potential to form scale upon heating, wherein said raw wastewater is introduced by a first inlet conduit into a pressurized wet oxidation reactor without preheating, and a preheated mixture of oxygen containing gas and dilution water is introduced into said reactor by a second inlet conduit, and wherein said wastewater, dilution water and oxygen containing gas are maintained within said reactor at an elevated temperature and pressure for sufficient time to destroy cyanide and produce an oxidized waste effluent and an oxygen depleted offgas which are removed from said reactor, characterized by the steps of: a) discharging said oxidized effluent and offgas from said reactor through a first effluent conduit while at the same time introducing a dilute mineral acid solution into said reactor through a second effluent conduit to prevent scale and solids buildup therein; and, b) alternating the flow of oxidized effluent and offgas and the flow of dilute mineral acid whereby said oxidized effluent and offgas discharge from the reactor through said second effluent conduit and said dilute mineral acid solution enters the reactor through said first effluent conduit to remove scale and solids therefrom. The reactor temperature may be maintained between about 200°C (392°F) and 370°C (698°F) while the reactor pressure may be maintained between about 300 psig (2,068 KPa) and 3,500 psig (24,129 KPa). Preferably the reactor temperature is maintained between about 200°C (392°F) and 300°C (572°F) and the pressure is maintained between about 300 psig (2,068 KPa) and 2,000 psig (13,788 KPa). For the purpose of carrying out the invention the reactor, which is preferably a vertical bubble column type, may be fitted with a pair of valved effluent lines and corresponding valved lines for introducing dilute mineral acid into each effluent line at system pressure. Initially, oxidized effluent and offgases exit the reactor through the first effluent conduit while dilute mineral acid flows through the second effluent conduit into the reactor to prevent scale buildup therein. After a period of operation the valves are operated to allow oxidized effluent and offgases to exit the reactor through the second effluent conduit and dilute mineral acid to flow through the first effluent conduit and into the reactor to remove scale or solids therein. After a further period of operation the valves are switched back to their initial position, and the process is continued with the valves being switched periodically so that the oxidized effluent and offgas is discharged through the first and second effluent conduits alternatively while diluted mineral acid is supplied to the reactor through the second and first conduits respectively. This mode of operation provides for uninterrupted treatment, overcoming the problem of scale formation by the waste within the wet oxidation system. Preferably said oxidized effluent and offgases discharged from the reactor are cooled by cooling means and said cooled oxidized effluent is separated from said offgases by separating means to produce a separated liquid phase and a separated gas phase. An example of the process in accordance with the invention will now be described with reference to the accompanying diagrammatic drawings, in which:- Figure 1 is a schematic flow diagram of a conventional wet oxidation treatment process; Figure 2 is a schematic flow diagram showing a modified version of the process of Figure 1 as taught by U.S.-A- 4,174,280; Figure 3 is a schematic flow diagram showing a modification of the process of Figure 2 incorporating an acid wash system for the reactor effluent which fails to prevent plugging; and Figure 4 is a schematic flow diagram of an example of the process in accordance with the invention incorporating an acid wash system for the reactor effluent which allows uninterrupted process operation. The caustic cyanide and metals containing wastewater treated by the processes illustrated in the drawings contained high concentrations of cyanide, metals and soluble salts resulting in high total solids for the waste. Analyses of the raw waste (Table 1) showed a Chemical Oxygen Demand (COD) of about 23 g/l, a cyanide content of 34 g/l, copper present at 14 g/l and sodium carbonate at about 340 g/l. Referring to Figure 1, a conventional wet oxidation flow scheme is shown in which the raw waste was diluted with an equal volume of water and placed in a raw waste feed tank 2. The dilute raw waste was brought to system pressure by a high pressure pump 4 and mixed with pressurized air from a compressor 6 in a feed conduit 8. The waste/air mixture was heated to operating temperature by a heater heat exchanger 10 and delivered by a conduit 12 to the lower portion of a wet oxidation reaction 14 of the vertical bubble column type, wherein oxidation occurred. The wet oxidation system operated at 5.5 gal/hr. (24,2 l/h) flow of diluted waste and 1.13 standard cubic feet/minute (SCFM) (32 l/min) compressed air, sufficient to oxidize all cyanide in the waste. The reactor temperature was 280°C (536°F) and 11,720 KPa (1700 psig) pressure. The above flow rate gave a one hour reactor residence time for the heat pressurized waste. The oxidized waste effluent and offgases exited the reactor 14 by an effluent conduit 16 and were cooled by a cooler heat exchange 18. The cool effluent/offgases mixture passed through a pressure control valve 20 and flowed through a conduit 22 to a gas/liquid separator 24. Oxidized effluent exited the separator by a conduit 26 while offgases were routed through a conduit 28 to a caustic scrubber 30 which collected any cyanide carried through the system by the offgases. The scrubbed offgases exited the system by a vent conduit 32. When operated in the above described flow scheme, diluted raw waste plugged the heater heat exchanger 10 with solids after 2 hours, causing process shutdown. The waste feed portion of the system was modified as shown in FIG. 2. This flow scheme has been applied to the wet oxidation of wastes which are insoluble, immiscible and difficult to suspend in water. Undiluted raw waste in the feed tank 2 was delivered by a high pressure pump 4 and an inlet feed conduit 8 directly to the lower portion of the wet oxidation reactor 14 without heating the raw waste. An equal volumetric flow of dilution water from a tank 34 flowed to a high pressure pump 36 where it was brought to system pressure and mixed with compressed air from the compressor 6 in a conduit 38. The water/air mixture was heated to operating temperature by the heater heat exchanger 10 and delivered by the conduit 12 to the lower portion of the wet oxidation reactor 14. The liquid flows were 2.75 gal/hr. (12,1 l/h) each for raw waste and dilution water and 1.13 SCFM (32 l/min) for the compressed air. The reactor temperature was 280°C (536°F) and 11,720 KPa (1700 psig) pressure with a one hour liquid residence as in FIG. 1. The remaining downstream portion of the oxidation system from effluent conduit 16 operated as described for FIG.1. When operated in the flow scheme of FIG. 2, no plugging of feed lines was encountered. However, the reactor top and the effluent conduit 16 plugged after five hours operation causing process shutdown. After removing the plug by mineral acid washing, the wet oxidation system was operated in the FIG. 2 flow scheme at 204°C (400°F) with all other operating parameters the same as described for FIG. 2. After eight hours of operation at the lower temperature the reactor top and the effluent conduit 16 again plugged causing process shutdown. The effluent conduit system at the top of the wet oxidation reactor was modified as shown in FIG. 3 to allow addition of dilute mineral acid to the top of the reactor 14a and to the effluent conduit 16 to control scale formation therein. The raw waste feed and dilution water/air feed to the reactor 14 were operated as described in FIG. 2. A solution of dilute mineral acid from acid supply tank 40 was brought to system pressure by a high pressure pump 42 and delivered by an acid conduit 44 through a block valve 46 to effluent conduit 16 to control scale formation therein. Additional dilute acid was added to the top of the reactor 14a, through a block valve 48 and a conduit 50, to control scale formation at the reactor outlet 14b to the effluent conduit 16. A small open-ended section of piping 52 was attached over the reactor outlet 14b to contain the dilute acid supplied thereto in close proximity to the reactor outlet 14b during operation. The remaining downstream portion of the oxidation system from the effluent conduit 16 operated as described for FIG. 1. The wet oxidation system of FIG. 3 was operated at 232°C (450°F) and 11,720 KPa (1700 psig) with a one hour liquid residence in the reactor as described previously. Dilution water and raw waste volumetric flows were each 2.75 gal/hr. (12,1 l/h) and air was added at 1.13 SCFM (32 l/min). A solution of 18% nitric acid was added at reactor pressure to the effluent conduit 16 and to the volume with the pipe section 52 near the reactor outlet 14b. The total dilute mineral acid flow rate was 0.5 gal/hr. during operation. When operated with the flow scheme of FIG. 3, the effluent line 16a close to the reactor and reactor outlet 14b were plugged with scale after five hours, causing system shutdown. The effluent conduit system at the top of the wet oxidation reactor was modified as shown in FIG 4 to allow addition of dilute mineral acid to each of a pair of parallel effluent lines during system operation. The Figure 4 flow scheme illustrates the process in accordance with the invention. The raw waste feed and dilution water/air feed to the reactor 14, a vertical bubble column type, were operated as described in FIG. 2. Two parallel effluent conduits, 16 and 17, connected the reactor top 14a with cooler heat exchanger 18. The conduits 16 and 17 each contained a block valve, 54 and 56, respectively. Dilute mineral acid from acid tank 40 was brought to system pressure by the pump 42 and the conduit 44 supplied dilute mineral acid through block valve 46 to the effluent conduit 16 at a point between the reactor 14 and the block valve 54. Likewise block valve 48 and conduit 50 supplied dilute mineral acid to the effluent conduit 17 at a point between the reactor 14 and the block valve 56. The remaining downstream portion of the oxidation system from cooler heat exchanger 18 operated as described for FIG. 1. During system operation oxidized effluent and offgases from the reactor 14 exited by first effluent conduit 16 with the valve 54 open. The valve 46 in the acid line 44 was closed to prevent acid flow into said first conduit 16. At the same time valve 56 was closed in second effluent conduit 17 to prevent effluent flow therethrough, and the acid control valve 48 was opened to supply dilute mineral acid to the second effluent conduit 17 and the reactor top 14a which prevented scale formation therein. After a specified period of operation, the effluent valve 54 was closed and effluent valve 56 was opened to allow effluent and offgases to exit the reactor 14 through second effluent conduit 17. Simultaneously acid control valve 48 was closed and acid control valve 46 was opened which allowed dilute mineral acid to flow through conduit 16 into the reactor top 14a which removed scale therefrom. Oxidized effluent and offgases were cooled in cooler heat exchanger 18 as was described in FIG. 1. For economical system operation the heater heat exchanger 10 and the cooler heat exchanger 18 are combined such that incoming dilution water and oxygen containing gas, preferrably air, are heated by oxidized effluent and offgases which, in turn, are cooled before separation. The wet oxidation system of FIG. 4 was operated at 232°C (450°F) and 11,720 KPa (1700 psig) with 1 hr. liquid residence time as previously described. Likewise, dilute mineral acid flow of 18% nitric acid was 0.5 gal/hr (2,2 l/h) from pump 42. The flows of oxidized effluent plus offgases and the dilute acid solution were alternated between conduits 16 and 17 every three hours as described above. This resulted in thirty hours of uninterrupted operation. The time period between alternating the flows between conduits 16 and 17 may be as often as every two hours or as long as every four hours. The concentration of dilute nitric acid employed may be as low as 5% to as high as 20%, depending on the caustic content of the particular waste treated. Analysis of the oxidized effluent shown in Table 1 demonstrated a high degree of cyanide destruction for the invention process. The wet oxidation reactor was opened after the thirty hours of operation. The reactor wall had a adhering scale deposit while a loose, sand-like deposit was found at the bottom of the reactor. Each solid was analyzed for cyanide, metals and ash. The results in Table 2 show the wall scale was mainly sodium, copper, phosphorus and carbonate while the loose scale contained mainly copper, zinc, sodium, phosphorus and carbonate. Cyanide content in each was negligable. Both deposits dissolved readily in dilute mineral acid. These solids account for the apparent removal of copper and zinc from the waste during wet oxidation as seen in Table 1.

Claims: 1. A process for treating by wet oxidation a caustic cyanide and metal containing wastewater showing a potential to form scale upon heating, wherein said raw wastewater is introduced by a first inlet conduit (8) into a pressurized wet oxidation reactor (14) without preheating, and a preheated mixture of oxygen containing gas and dilution water is introduced into said reactor by a second inlet conduit (12), and wherein said wastewater, dilution water and oxygen containing gas are maintained within said reactor (14) at an elevated temperature and pressure for sufficient time to destroy cyanide and produce an oxidized waste effluent and an oxygen depleted offgas which are removed from said reactor, characterized by the steps of: a) discharging said oxidized effluent and offgas from said reactor (14) through a first effluent conduit (16) while at the same time introducing a dilute mineral acid solution into said reactor (14) through a second effluent conduit (17) to prevent scale and solids buildup therein; and, b) alternating the flow of oxidized effluent and offgas and the flow of dilute mineral acid whereby said oxidized effluent and offgas discharge from the reactor (14) through said second effluent conduit (17) and said dilute mineral acid solution enters the reactor (14) through said first effluent conduit (16) to remove scale and solids therefrom. 2. A process according to claim 1, wherein said oxidized effluent and offgases discharged from the reactor (14) are cooled by cooling means (18), and said cooled oxidized effluent is separated from said offgases by separating means (24) to produce a separated liquid phase and a separated gas phase. 3. A process according to claim 2 wherein said cooling means is a cold water heat exchanger (18). 4. A process according to claim 2 wherein said cooling means is a heat exchanger (10) which preheats said mixture of dilution water and oxygen containing gas introduced into said reactor (14). 5. A process according to any one of claims 1 to 3 wherein said dilution water and oxygen containing gas are preheated by an external heat source (10). 6. A process according to any one of the preceding claims wherein said dilute mineral acid introduced into said reactor (14) in steps (a) and (b) is nitric acid. 7. A process according to claim 6 wherein said dilute nitric acid is at a concentration of 5% to 20%. 8. A process according to any one of the preceding claims wherein said elevated temperature is between about 200°C (392°F) and 370°C (698°F) and said elevated pressure is between about 300 psig (2,068 KPa) and 3500 psig (24,129 KPa). 9. A process according to claim 8 wherein said elevated temperature is between about 200°C (392°F) and 300°C (572°F) and said elevated pressure is between about 300 psig (2,068 KPa) and 2000 psig (13,788 KPa). 10. A process according to any one of the preceding claims wherein said wet oxidation reactor (14) is a vertical bubble column. 11. A process according to any one of the preceding claims wherein said oxygen containing gas is air. 12. A process according to any one of the preceding claims wherein said alternating step (b) is performed after two hours to four hours of system operation.

Inventors: BRANDENBURG, BRUCE L., COOLEY, CURTIS D., ELLIS, CLAUDE E.
Applicants: ZIMPRO PASSAVANT ENVIRONMENTAL SYSTEMS, INC.
Application year: 1989
Main classification: C02F11
IPC codes: C02F1, C02F11