Chapter 10
Recovery of Valuable Metals from Industrial Wastes 1
S. Natansohn, W. J . Rourke , and W. C. Lai
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GTE Laboratories Incorporated, Waltham, MA 02254
Environmental and economic concerns necessitate the recovery of the heavy and valuable metals present in industrial processing wastes. Fundamental chemical principles were used in the development of a process for the separation and recovery of the variety of metals present in the residues from tungsten ore processing plants. Such ore tailings typically contain residual W and small quantities of Co, Cr, Nb, Ni, Pb, Ta, Th, Zn, lanthanides, and Sc in a matrix of Fe and Mn oxides. The emphasis of this separation path was the early, selective, and quantitative recovery of scandium which is present in concentrations of about 500 ppm. This was accomplished by acid leaching of the waste material in the presence of a reductant and passing the pH-adjusted leachate through a column of a complexing ion exchange resin. The column raffinate was then treated sequentially to separate and recover the other metals. Tungsten, Nb, and Ta were recovered from the solid leaching residue. Waste products from many industrial processes contain at times significant concentrations of metals which are objectionable on environmental grounds and yet constitute an appreciable economic asset. The presence of toxic metals in such wastes constitutes an environmental hazard, particularly because the ever-decreasing pH of the rainwater makes their leachability and contamination of the ground water more likely. Careful and costiy waste-disposal procedures are thus mandatory so as to prevent this from occurring. A preferred alternative is the cost-effective conversion of such waste into useful products. This provides an optimal solution to the wastedisposal problem because (a) it eliminates the need for a safe and costly disposal of the hazardous waste; (b) it maximizes resource utilization and conservation through recycling, and (c) it derives an economic benefit from the sale of the obtained products. 1Current address: Duracell Worldwide Technology Center, 37 A Street, Needham, MA 02194 Current address: Aneptek Corporation, 209 West Central Street, Natick, MA 01760
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0097-6156/92/0509-O129$06.00/0 © 1992 American Chemical Society
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ENVIRONMENTAL REMEDIATION
Consequently, the objective of this study was the development of an industrially viable, cost-effective, environmentally compatible technology for the recovery of metals from tungsten ore tailings. The methodology to be developed was to be sufficiently broad and flexible so as to be applicable to a wide range of metallic constituents present in varying proportions, and for this reason alone, tungsten ore tailings were a very appropriate test system. The following are some of the criteria deemed necessary for the success of such a metal-recovery process, i.e., the process needs to be: •
Waste reducing — the residue of therecoveryprocess should be below 20% of its original mass and be acceptable to a sanitary landfill.
•
Environmentally compatible — in that no hazardous reagents are used in the recovery process nor hazardous products generated.
•
Industrially viable — the processing steps to be well-established and practiced unit operations.
•
Cost effective — the overall processing cost should be significantly exceeded by the product value of the recovered constituents.
•
Flexible — the process developed should be applicable to a broad range of compositions found in tungsten ore tailings.
•
Generic — the technology should have general applicability to therecoveryof metals from secondary sources such as low grade ores, ore tailings, scrap products, and industrial wastes, recognizing, however, that the specific chemical characteristics of each material define the most effective ways of its treatment and utilization.
Tungsten Ore Tailings Origin. The tungsten ore tailings are the solid residue of the digestion process used for the recovery of tungsten from its ores. There are two economically important classes of tungsten ores: scheelite, CaW0 , and wolframite, (Fe . Mn )W0 , which comprises a continuum of Fe and Mn concentrations from ferberite (x = 0) to huebnerite (x = 1). These naturally occurring minerals also contain minor amounts of other elements which fit into their structure either substitutionally or interstitially. The extraction of the tungsten values from wolframite ore concentrates is usually done at 100°C by reaction with strong solutions of NaOH as given by equation 1 (i): 4
(Fe,Mn)W0 + 2 NaOH 4(s)
(aq)
1 x
-» (Fe,Mn)(OH) + Na W0 2(s)
2
x
499% Efficiency
Calcination to Sc 0 Purity: ~ 95% 2
Figure 5. Scandium recovery scheme.
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Recovery of Valuable Metals from Industrial Wastes
10. NATANSOHN ET AL.
hydroxides with the scandium do not precipitate as die oxalates. However, the direct precipitation of scandium oxalate is inhibited in the diglycolic acid eluate because the scandium is complexed with the diglycolate ligand. A quantitative separation of Sc is effected by passing the eluate through a column of a strong cation ion exchange resin in the hydrogen form. As the Sc ions are retained on this ion exchange resin, the replaced H ions protonate the diglycolate anions so that the column effluent is a regenerated diglycolic acid solution which can be used again in eluting a scandium loaded extraction column. The scandium retained on the strong cationic resin is eluted quantitatively with 6 N HC1. In the process, the resin is regenerated by the strong acid and can be used again at undiminished capacity. The scandium is then precipitated by the addition of saturated oxalic acid. The precipitate is filtered, washed, and converted to SC2O3 by calcination at elevated temperatures. The scandium recovery process is very efficient in that 99% of the Sc retained on the initial extraction column is recovered in the sesquioxide product. The overall recovery of the Sc present in the initial feed solution is determined solely by the point at which the extraction process is stopped. The purity of the Sc 0 is about 95%; the impurities are listed in Table VII. +
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Table V u . Impurity Content of Recovered Scandium Oxide Element
wt %
Element
wt %
Ce Th Pb Nd La All others
1.4 1.0 0.85 0.53 0.27 0.62
Pr Gd Ca Fe Mn
0.17 0.16 0.16 0.077 0.044
The major impurities are those whose complexation characteristics are similar to scandium's and would be expected to be carried along in the process. The total of the unlisted impurities is 0.62 wt %, with none of them exceeding 0.1 wt %. The selectivity of the process is demonstrated by the fact that the scandium oxide contains only 0.07 and 0.044 wt % of Fe and Mn, respectively, whereas the starting material had 22 wt % Fe, 20 wt % Mn, and only 0.05 wt % of Sc. Recovery of Metals Concentrate. The Sc-depleted raffinate from the ion exchange process step contains the two major constituents, Fe and Mn, in their divalent state, and other transition and rare metals in small amounts. The recovery of these metals in the presence of large amounts of Fe and Mn is done effectively by selective precipitation in the pH range between 6.5 and 7.5. In this experiment, the pH of the raffinate solution was adjusted with ammonium hydroxide to 7.4, and the resulting precipitate washed and dried. It contains the metals listed in Table VIII in a matrix of hydrated ferric oxide; the precipitation of appreciable amounts of iron, about 14% of the iron content of the raffinate, is primarily due to the partial oxidation of the ferrous
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ENVIRONMENTAL REMEDIATION
to ferric ions during the ion exchange and selective precipitation steps. The concentration of these metals represents a useful upgrading over that in the original tungsten ore tailings and renders the precipitate a viable resource, particularly of the lanthanides. Table VOL Composition of Metal Concentrate Element
wt %
Element
Si Mn Cr Zn Lanthanides (incl. Y & Sc)
5.9 1.9 1.0 0.91 1.8
Al Co Pb Ca Ni
wt % 0.57 0.25 0.23 0.064 0.043
Recovery of Iron. The Fe values are recovered from the filtrate of the preceding process step by oxidative precipitation to yield an easily filterable goethite solid, FeOOH. Filterability is a major technical issue in the removal of iron from industrial streams because of the difficulties encountered in filtering the gelatinous iron hydroxide precipitates. The solution was treated with hydrogen peroxide to oxidize the ferrous to ferric ions, the pH was adjusted to 4, and then digested at 85°C for about 3 hours to precipitate the goethite. The Fe recovery is virtually quantitative, >99%, yielding a high-purity product (Table IX). Table IX. Impurities in Fe Precipitate Element
w/o
Element
w/o
Mn Si Zn Al Co
0.58 0.041 0.020 0.016 0.014
Ni Na Cr Ca Cu
0.0026 0.0024 0.0016 0.0015 0.0009
Recovery of Manganese. The Mn content can be recovered from the filtrate of the goethite precipitation by another oxidative precipitation. The pH of the solution is brought to a value of about 9 by addition of NaOH, and an oxidant such as hydrogen peroxide is added to oxidize all of the divalent Mn. The resulting manganese hydroxide is easily filterable, contains in excess of 99% of the Mn present in solution, and its purity also exceeds 99% (Table X). Table X. Impurities in Manganese Hydroxide Element
wt %
Element
Co Fe Si Ca Zn
0.34 0.16 0.069 0.032 0.026
Ni Na Ce Mg La
wt % 0.023 0.013 0.012 0.011 0.0063
10. NATANSOHN ET AL.
Recovery of Valuable Metals from Industrial Wastes
Composition of Waste Stream. The filtrate from the Mn precipitation is the waste stream resulting from this process. It has a pH of 9 and solid content of 17 g/1 of Na S0 . Its impurity content is given in Table XI, which lists the 10 highest contaminants. The concentrations are given in ppm; no other impurity was detected at a level greater than 50 ppb. The impurity content of this waste stream is quite low and, if not directly disposable, compatible with standard waste water treatment procedures. The low concentration of the transition metal ions, particularly Fe and Mn, which were major constituents of the feed stream attests to the effectiveness of the separation and recovery processes. 2
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Table XL Impurities Content of Waste Stream Element
ppm
Element
ppm
Ca Mg Si Κ Mn
58 20 11 8.5 3.3
CI Fe Ρ Al Sr
0.7 0.5 0.54 0.30 0.17
Recovery of Tungsten. The residue from the initial acid reductant acid treatment of the tungsten ore tailings amounts to about 20% of their weight and contains close to half of the Fe content as well as the major fraction of W, Nb, and Ta. The Fe content is recoverable by treatment of this material with concentrated HC1 at 85°C to obtain a liquid iron chloride concentrate which is separated by filtration. The insoluble fraction is digested in 6 Ν NaOH at 80°C to solubilize the tungsten. The resulting solution has a W concentration of 45 g/1, about 98% of the digested material's W content. The residue of this alkali treatment contains the Ti, Nb, and Ta in a silicate matrix. Summary The preceding discussion describes a sequential methodology for the separation and recovery of various metallic constituents from industrial wastes as exemplified by tungsten ore tailings. It was demonstrated that by exploitation of subtle differences in chemical behavior, it is possible to preferentially dissolve the constituents of the material, separate them selectively, and recover them quantitatively in materials of high purity. In the process, a potentially hazardous industrial waste is converted into useful products, virtually eliminating the disposal requirements and recycling strategic resources, while the effluent of the process contains only traces of the original components. The principles of this technology are applicable to many similar systems. Literature Cited (1)
Mullendone, J.A. Kirk-Othmer Encyclopedia of Chemical Technology; 3rd Edition; John Wiley & Sons: New York, N Y , 1983; Vol. 23, p. 417.
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ENVIRONMENTAL REMEDIATION
(2) Kolthoff, I.M.; Sandell, E.B. Textbook of Quantitative Inorganic Analysis; The Macmillan Company: New York, N Y , 1948; pp. 672-673. (3) Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley Interscience: New York, N Y , 1962; p. 695. (4) Kolthoff, I.M.; Sandell, E.B. loc. cit., pp. 614-619. (5) Ibid; pp. 592-604. (6) Welch, A.J.E. Extraction and Refining of the Rarer Metals; Institution of Mining and Metallurgy, London, 1957, p. 3. (7) Guo Gongyi; Chen Yuli; L i Yu. J. Met. 1988, 40 [7], 28. (8) El-Sweify, F.H.; Shabana, R.; Abdel-Rahman, N.; Aly, H F. Solvent Extraction and Ion Exchange, 1986, 4, 599. (9) Hubicki, Z . Hydrometallurgy, 1986, 16, 361. (10) Diaz, M . ; Mijangos, F. J. Met. 1987, 39 [7], 42. (11) Ringbom, A.J. Complexation in Analytical Chemistry; Interscience Publishers; New York, N Y , 1963; pp. 22-60. (12) Reilley, C.N.; Schmid, R.W. Anal. Chem. 1958, 30, 947. (13) Ringbom, A.J. loc. cit., pp. 332-333. (14) Ibid, p. 351. RECEIVED April 7, 1992
