Efficient Treatment of Complex Wastewaters at Umicore Precious Metals using Biotechnology

Transcription

1 Efficient Treatment of Complex Wastewaters at Umicore Precious Metals using Biotechnology J. Boonstra, M. Picavet; H. Dijkman and C. Buisman Paques B.V. PO Box AB BALK, Netherlands W. Ghyoot Umicore Research Kasteelstraat Olen, Belgium wouter.ghyoot@umicore.com ABSTRACT Paques and Umicore have demonstrated the feasibility of biological treatment of the highly complex Hoboken wastewaters in a 1 m 3 /h pilot plant at Umicore Precious Metals, Hoboken, Belgium. In the first step arsenic was precipitated from copper smelter effluent as As 2 S 3 using biogenic produced H 2 S. Other metals such as copper, lead and cadmium were also removed as MeS to concentrations well below the discharge levels. Subsequently, sulfate and nitrate were removed to low concentrations (<300 mg/l and < 5 mg/l respectively) in high rate engineered bioreactors. Sulfide produced in the sulfate reduction process was recycled to step 1 for metals precipitation. Selenite/selenate was partly removed in the sulfide precipitation step, whereas the excess was biologically reduced and removed as selenium to concentrations <0.2 mg/l. 1 of 13

2 INTRODUCTION In its striving for sustainable development, Umicore is exploring new ways to treat the wastewater generated at its facilities. In its worldwide operations, a variety of streams are produced. Neutralization with limestone or milk-of-lime is today s most used treatment method. The current effluent legislation figures are met at Umicore. However, biotechniques using sulfur technology open new perspectives for further improvement of the effluent quality and alternatives for gypsum production. Paques specializes in the development, design and realization of high rate biotechnological applications for industry. Technologies for the recovery of metals and for conversion of a broad range of organic and inorganic compounds from water and gas streams are available. Worldwide, some 500 industrial installations using PAQUES technologies have been constructed since For the mining and metallurgical industry THIOPAQ technology is offered. Recovery of valuable base metals using biogenic H 2 S, combined removal of sulfate, nitrate and heavy metals, selenium removal and fluoride removal are examples of applications. To date, some 40 THIOPAQ plants are operated, amongst which 8 large-scale plants in the metallurgical and mining industry [5,6,7,8,9]. Paques has suggested a treatment scenario involving several biotechnological processes to meet Umicore s future water treatment requirements. It was decided to run an extensive demo plant project at Umicore Precious Metals (UPM) in Hoboken. The set-up and results of this project are described in this paper. Current Situation Umicore Precious Metals, a business unit of Umicore, is a world market leader in recycling precious metals and other non-ferrous metals from a complex feed of both recycling and secondary materials. Its facility in Hoboken is one of the world s largest, cleanest and most advanced precious metals recycling and recovery operations. The plant efficiently recycles complex industrial intermediate materials and specific precious-metals-bearing scrap from photographic, electronic and catalytic (oil refining, petro-chemical and automotive) applications. 2 of 13

3 Precious metals, special metals, minor metals as well as base metals are recovered with a unique environmental friendly and metallurgical flow sheet, satisfying the most stringent environmental standards. The currently operated physical/chemical water treatment plant at UPM treats two main streams, process wastewater and site sewer water. Process water This is composed of 2 main streams, Circuit A and B. The mean concentrations of the main compounds as measured during the pilot test are shown in Table I. Compound Table I - Mean Composition of Process Wastewater (dissolved concentrations) Circuit A (35 m 3 /h) Concentration (mg/l) Circuit B (5 m 3 /h) Concentration (mg/l) Cl SO NO NH As Sb 3 1 Cr 0 2 Se 5 0 Pb 9 80 Te 12 1 Ca Tl 1 6 Cu Zn Ni 11 4 Cd F Fe TSS > PH < 2 < 2 Circuit A and B are treated with lime neutralisation and FeCl 3 polishing, which is currently considered Best Available Technology [1]. Neutralisation is achieved with calcium hydroxide (Ca(OH) 2 ). This way, most of the sulfate present 3 of 13

4 is removed as gypsum, most metals precipitate as their respective hydroxides and arsenic is removed as calcium-arsenite/calcium-arsenate. Further arsenic removal is obtained with the addition of FeCl 3 After ph-correction, the treated water is discharged into the river Schelde. Site sewer water Site sewer water is collected and treated before it is reused on-site. A certain bleed of this site sewer water is required to moderate the conductivity. This bleed is treated together with the process waters. Its hydraulic load is about 75 m 3 /h. This stream contains an important amount of nitrate. Furthermore, some metals are present. The characteristics of this stream vary significantly and the mean composition is shown in Table II. Also for this stream, Calcium hydroxide is used for treatment. Table II - Mean composition of site sewer water (dissolved concentrations) Compound Concentration (mg/l) Cl NH NO SO As 3 Se 3 Ca 133 TSS 68 ph 8 The currently operated treatment facility works satisfactory and meets local discharge demands. However, sulfur biotechnology could become an attractive alternative for meeting future effluent permits and waste disposal requirements. Demo Plant The flow sheet that was tested and demonstrated by Paques and Umicore made use of three distinct biotechnological processes and a precipitation unit [10]: 1. Biological nitrate reduction to nitrogen gas 2. Biological sulfate reduction to sulfide 3. Biological sulfide oxidation to elemental sulfur 4. Metals removal through metal sulfide precipitation 4 of 13

5 All processes have been proven on industrial scale. The novelty is the integration of all these technologies into one process. The demonstrated flow sheet can be divided into 3 main process steps: A. Pre-treatment. Arsenic and some other metals present in Circuit A, such as copper, are removed as their respective sulfides using biologically produced H 2 S [4,6,7]. As S 2- As 2 S 3 (1) Me 2+ + S 2- MeS (2) B. Denitrification. Here, nitrate present in Circuit B and the sewer water is biologically reduced to nitrogen gas using ethanol as reductant source. 5 C 2 H 5 OH + 12 NO 3-6 N HCO H 2 O + 2 OH - (3) C. Sulfate reduction. Sulfate is biologically reduced to sulfide with hydrogen gas [5,9]: 4 H 2 + SO H + HS H 2 O (4) Part of the H 2 S is stripped and fed to step A. Excess sulfide is oxidised to solid elemental sulfur (S 0 ), and separated as such. HS O 2 S 0 + OH - (5) The block scheme of the process is shown in Figure 1. 5 of 13

6 Nutrients NaOH C Effluent Air Sulphur H 2 Sewerage Circuit B 3 B NaOH Ethanol Gas recycle A Circuit A As - H 2S contactor 2. As 2S 3 removal 3. Denitrification 4. Mix tank 5. Sulfate reduction 6. Sulfide oxidation 7. Sulfur separation As 2 S 3 Figure 1 - Block Process Diagram Demo Plant A flow of 1 m 3 /h (1% of the actual wastewater flow) was fed to the demo plant. The pilot plant consisted of 3 main units which were completely equipped with bioreactors, mixing tanks, mixers, tilted plate settlers, pumps, mass flow controllers, ph meters, redox meters, temperature meters, gas compressor, air compressor, and heating systems. The bioreactor volumes varied from 2 to 12 m 3. The pilot plant was PLC controlled and equipped with a SCADA system. A picture at the pilot plant is shown below. The main advantages envisaged for this new approach are: Lower residual metal concentrations since they are precipitated as their respective sulfides instead of hydroxides. Very low sulfate effluent concentrations are possible (<300 mg/l). Nitrate and selenium can be removed to low concentrations [2,3,11]. Elemental sulfur is produced as solid waste material. This is 5 to 10 times less voluminous than gypsum. Eliminating the production of heavy metals polluted gypsum. Possibility of reuse of metal sulfides and elemental sulfur. 6 of 13

7 Sulfate reduction As - H 2 S contactor As 2 S 3 separator S ulfide oxydation Denitrification Figure 2 - Thiopaq Demo-plant at UPM The overall objectives of the pilot tests were to demonstrate the feasibility of biological treatment of UPM wastewater and to prove the above mentioned advantages. RESULTS AND DISCUSSION Arsenic Removal Efficiency Arsenic was effectively removed from solution through As 2 S 3 precipitation. Removal efficiencies were usually higher than 99.5%. This resulted in dissolved arsenic overflow concentrations of well below 1 mg/l. Besides arsenic, other metals precipitated as their respective sulfides. Lead and cadmium concentrations were 7 of 13

8 reduced to less than 1 mg/l. Antimony and copper were completely removed. Due to the low ph, zinc, nickel, iron and part of the selenium stayed in solution after this precipitation step, so selective recovery of metal sulfides is possible. By separating the arsenic and cadmium from the zinc and nickel, the last two can in principle be recycled. Sludge Characterisation The arsenic content in the sludge was quite stable at about 20 to 25 wt.% of the sludge. Since arsenic was present as As 2 S 3, As 2 S 3 formed about 40 to 45 wt.% of the sludge. Besides arsenic, copper, lead and cadmium formed substantial parts of the sludge. This was as expected. The sludge settled well in the parallel plate separator, always leaving a clear supernatant. However, not all sludge could be retained at all times. Some small flocks washed out (TSS < 5 mg/l). This washout was mainly due to gas evolution. H 2 S stripped out of solution due to the low ph and the small under pressure created by the ventilation at the top of the sludge separation step. Gas bubbles formed dragged small flocks to the surface. Part of these flocks washed out and part of them formed a layer of foam on the surface. Incorporating a dedicated degassing chamber in this tank will solve this problem and will therefore be incorporated in future designs. Denitrification Nitrate was efficiently removed. Denitrification effluent concentrations of 5 mg/l were standard. Dosing biological produced sulfide for metal precipitation in order to prevent denitrification inhibition worked well. All metals except arsenic precipitated completely. Due to sometimes unexpected variations in influent conditions, the denitrification was exposed to ph peaks as high as 12 and as low as 4. Consequently, the process was automatically stopped and the water was neutralized using either acid or caustic. Re-commissioning was mostly quick and successful. Obviously, future designs will include adequate ph control to prevent these phshocks. Sulfate Reduction Process Parameters: Important parameters for sulfate reduction are ph, temperature, nutrients dosing, type of reductant, CO 2 concentration, and sulfide concentration. The ph determines the strip-ability and toxicity of sulfide. At low ph, H 2 S is easily stripped, but increasingly toxic for the bacteria (reversible toxicity, though). At high ph, H 2 S is less easily stripped. To prevent inhibition and ensure 8 of 13

9 adequate stripping, the ph was set in the near-neutral range, which proved to work well in practice. The temperature was kept between 30 and 35 o C. In this temperature range, good biological activity is guaranteed. For proper bacterial growth and activity, nutrients were fed as a mixture of carbon, nitrogen and phosphor. Furthermore, hydrogen gas was fed to the reactor as reductant. For stable operation it was essential to have a ph-buffer to top off phpeaks. The preferred ph-buffer is bicarbonate (HCO 3 - ). CO 2 and sodium hydroxide were dosed to form this buffer. In industrial practice, CO 2 will be formed as a byproduct in the steam reformer that is utilized to convert methane gas into hydrogen. As a result, H 2 and CO 2 will jointly be dosed to the reactor and no additional CO 2 dosing is required. The sulfide concentration is an important parameter indicating the activity of the bacteria. In the preferred concentration range, sulfide inhibits methanogenic activity (and thereby prevents the unwanted consumption of reductant by methane producing bacteria), is high enough to ensure proper stripping and immediately precipitates all dissolved metals entering the bioreactor (preventing inhibitory effects of the metals). For the bacteria used, 500 to 1000 mg/l is preferred. Above this concentration, sulfide inhibits the sulfate reducing bacteria s metabolism. Efficiency During stable operation, sulfate was efficiently converted to sulfide at a high volumetric loading rate. At those times, sulfate was the limiting compound, i.e. the sulfate effluent concentration was practically zero. Sulfide concentrations higher than 1 g/l were reached without showing any negative effects on the biomass performance. The highest reactor metal influent concentrations were those of nickel, zinc and iron. These were effectively removed without having an inhibiting effect on the bacteria. Arsenic concentrations usually stayed below 1 mg/l, sometimes showing slightly higher concentrations. Selenium Selenium effluent concentrations were continuously lower than 1 mg/l. Literature suggests that selenium precipitation with sulfide occurs in acid conditions [2,3,11]. This was as well observed in the pilot where a part of the selenium was removed in the sulfide precipitation step. This removal is however not sufficient to comply with future legislation. It is known that selenate/selenite can also be reduced biologically [2,3,11]. The pilot results showed that indeed selenium was further removed in the biological processes. Both the denitrification and the sulfate reduction bioreactors contributed to the selenium polishing to values below < 0.1 mg/l. 9 of 13

10 Process Robustness The sulfate reducing bioreactor was exposed to high ph peaks (almost ph 10). The hydrogen using bacteria showed to be resistant to the extreme process conditions. Sulfur Production Elemental sulfur production was evident when the sulfur production was run under oxygen limitation, i.e. just enough air is provided as to only partially oxidize the sulfide to sulfur and not to sulfate. Solid sulfur particles were clearly visible in the process liquid in concentrations as high as 10 g/l. The plant performed very stably at a high volumetric loading rate. Effluent Quality The mean effluent concentrations obtained during a two weeks stable operation period are shown in Table 3 below. Table III - Average effluent Quality (dissolved concentrations) during two weeks stable operation Compound Demand Demo plant Cu 3 < 0.1 Zn 5 < 0.1 Cd 0.2 < 0.1 As Sb Se Pb Ni Co 3 < 0.1 Sn 2 < 0.2 Fe 2 < 2.0 Te 5 < 0.2 Cr 2 < 0.1 Ca Tl 5 < 0.05 Hg 0.05 < 0.01 Cl NO 3 -N 125 < 5 SO of 13

11 Table III indicates that all effluent demands on dissolved concentrations were met. Overall, metals removal through sulfide precipitation was successful. At uncontrolled low ph (<2) arsenic, cadmium, antimony, copper and lead were the main metals removed from Circuit A. In the denitrification reactor, all metals present in sewer water and Circuit B water were precipitated to low residual concentrations. Possibly remaining metals, except arsenic, were efficiently removed in the sulfate-reducing reactor. However, total metal concentrations in the effluent sometimes exceeded the requirements. This was due to overflow of suspended solids from the final parallel plate separator. Arsenic effluent demands could not be met all the time due to overflow of arsenic sulfide from the first precipitation step and redissolution of arsenic sulfide at higher ph. Thus, it is essential to prevent washout of As 2 S 3 from the sulfide precipitation step through better degasification. Besides, arsenic was present in sewer-water and sometimes in Circuit B water. These streams went straight to the denitrifying reactor, bypassing the arsenic precipitation step. Due to the neutral ph in the bioreactors, arsenic from those streams could not be precipitated efficiently as a sulfide. It should be noted that the above table provides the results from a demonstration plant in which some of the physical unit operations, especially the solids separation, are sub-optimal. For guaranteeing deep arsenic removal a final effluent polishing with FeCl 3 addition is required. In order to prevent overflow of suspended solids - and associated solid metal concentrations - from the final decanter, an optimized coagulation/flocculation system prior to the final parallel plate separator, possibly followed by sand filtration, are required. This will guarantee removal of metals to concentrations even lower as in Table 3, as has been demonstrated in comparable industrial plants [5,8,9]. CONCLUSIONS The described process configuration involving 3 bioreactors has proven to be a good fit for Umicore s requirements with regard to wastewater treatment. Effluent concentrations better than required by legislation have been obtained. The highly concentrated and acidic water was treated effectively and the system proved to be robust with regard to peaks. Effective removal of all metals and other relevant compounds was demonstrated. For further arsenic removal some changes to the process design are required. 11 of 13

12 Biological reduction of selenate/selenite to selenium was demonstrated. By separating the formed solid selenium from the water stream, virtual complete removal of this compound was obtained. Umicore and Paques have demonstrated that biotechnology can greatly assist companies in their pursuit of sustainable production. REFERENCES [1] Best available techniques in the Non-Ferrous Metals Industries, 2001, European IPPC bureau. Gent, Academia Press, ISBN X. [2] Mirza A.H. and Ramachandran V.,1996, Removal of Arsenic and Selenium from wastewaters a review, Proceedings of the Second International Symposium on Extraction and Processing for the Treatment and Minimization of Wastes, 1996 TMS Fall Meeting, Scottsdale, Arizona, October [3] G.A. Monteith et al., 2000, Development, testing and full scale operation of a new treatment method for selenium removal from acidic effluents, Lead- Zinc 2000, Eds. J.E. Dutrizac et al., TMS, pp [4] R.W. Peters, Y. Ku, 1985 Batch precipitation studies for heavy metal removal by sulfide precipitation, AIChE Symp. Ser. 81,,pp [5] C.F.M. Copini et al., 2000, Recovery of sulfides from sulfate containing bleed streams using a biological process, Lead-Zinc 2000, eds. J.E. Dutrizac et al, TMS, pp [6] A.S. Peters, 1999, The Selective Removal of Copper and Arsenic from Electrolyte Bleed Development and Design of a Sulfide Precipitation Process, (confidential), nr. PM [7] R. Ruitenberg et al.,2001, Copper electrolyte purification with biogenic sulfide, Electrometallurgy 2001, Eds. J.A. Gonzales et al., MetSoc, 2001, [8] J. Boonstra, H. Dijkman and C.J.N. Buisman, 2001, Novel Technology for the Selective Recovery of Base Metals, Waste Processing and Recycling in 12 of 13

13 Mineral and Metallurgical Industries IV, Eds. S.R. Rao et al., MetSoc, pp [9] H. Dijkman et al., 2002, Optimization of metallurgical processes using high rate biotechnology, Sulfide Smelting 2002, Eds. R.L. Stephens and H.Y. Sohn, TMS, pp [10] M. Picavet, 2002, Biological treatment of Wastewater at Umicore Precious Metals, Hoboken, Belgium, Internal report about demo project, Paques (confidential). [11] L.H. Tidiwell et al., 2000, Technologies and potential technologies for removing selenium form process and mine waste water, Minor Elements 2000, SME, Salt Lake City, UT., pp of 13