Ind. Eng. Chem. Process Des. Dev. 1881, 20, 229-232
229
Production of Nickel by Hydrogen Reduction of Nickel-Loaded Organic Acid Solutions Danlel A. Boateng, Tammy C. Ng, Colln R. Phllllps,' and Atlln S. Tombalaklan' Department of Chemical Engineering and Applled Chemistry, University of Toronto, Toronto, Ontario, Canade
M5S lA4
Several organic acids were investigated for the extraction of nickel from aqueous solution and its subsequent reduction by hydrogen in the organic phase. The system di(2-ethylhexyl) phosphoric acid in Shellsol 715 was found to be the most suitable. Hydrogen reduction is very slow in the absence of a catalyst but is markedly accelerated by nickel powder or anthraquinone, the former resulting in a better yield. The effects of temperature, hydrogen pressure, nickel concentration,and reduction time were examined. The reduction process depends on the degrees of polymerization and solvation of the nickel-organic complex and the free solvent to diluent ratio in the solution. The reduction rate is limited by a mass transport step.
Introduction High purity metal powders may be produced by hydrogen reduction of the metal ions from aqueous solutions of their salts (Ipatieff and Werchowski, 1909; Schaufelberger and Roy, 1955; MacGregor and Halpern, 1958; Hahn and Peters, 1965; Evans, 1968; and Burkin and Burgess, 1969). The theoretical aspects of these hydrometallurgical processes have been reviewed from a physicochemical point of view (Meddings and Mackiw, 1964). Hydrogen reduction of metal ions is favored by a high pH of the aqueous solutions of the metal salts. This view has been substantiated from an electrochemical approach (Wadsworth, 1969). A limit to the pH range which may be used with any particular ion, however, is set by the solubility product of its hydroxide (Burkin, 1966). The limitation set by precipitation of metal ions as their hydroxides in aqueous solutions may be avoided by the use of a solvent other than water, in which case recovery from nonaqueous media must be preceded by solvent extraction to transfer the metal to the nonaqueous media. Reported studies (Burkin and Burgess, 1969) indicate that the choice of a suitable organic solvent is critical to the success of metal reduction by hydrogen. The present work forms part of a study of nickel recovery by hydrometallurgicalprocesses (Boateng, 1979) in which solvent extraction with organic acids is used for the solution purification step. Although most carboxylic acids are themselves reduced by hydrogen under the conditions necessary to reduce their salts of copper, nickel, and cobalt, organic acids which are highly branched in the a-position are not (Burkin, 1973). Principles The reduction of nickel by hydrogen is a heterogeneous process and requires the presence of an active surface, the reduction rate being proportional to the area of this active surface. Colloidal carbon or nickel powder may serve as seed particles, and anthraquinone may be used to help provide active surface. The hydrogen reduction of nickel may be represented as (RZ-Nih, + Hz Ni + 2(R-H), (1)
-
where the subscript o refers to the organic phase. A rate equation for the reduction may be written as School of Engineering, Laurentian University, Sudbury, Ont. 0196-4305/81/1120-0229$01.25/0
d [R2-Ni] = k'a[R2-NiIoM dt where k'is a modified rate constant which is a function of the hydrogen pressure and a is the specific interfacial area. The hydrogen reduction reaction would be influenced by the preceding solvent extraction process. A detailed discussion of the various equilibria involved in the extraction process has been given elsewhere (Boateng, 1979). In the organic phase the equilibria involving the extracted metal species MR, may be written as -
J'(MRz)o + ((MRz)j)o
(3)
for polymerization for which the equilibrium constant, Kp, is given by (4)
where j is the degree of polymerization. Solvation of the complex occurs in accordance with ((MRz)j)o+ x(RH)o * ((MRz)j.(RH)x)o
(5)
where x is the number of acid molecules solvating the complex, and the solvation constant, K,, is given by The values of both j and x depend upon the degree of loading and affect the reduction reaction. Experimental Procedure Two methods were used to prepare the nickel-loaded organic solution for hydrogen reduction. Method I. Nickel was loaded into the organic solution from an aqueous sulfate solution whose pH was adjusted to 6 by the addition of sodium carbonate. The aqueous phase was agitated with an organic phase consisting of the organic acid in a diluent (kerosene or Shellsol 715). The organic phase was then separated and used for the hydrogen reduction step. Method 11. The organic phase was pre-equilibrated with caustic soda solution, and the treated organic phase was then agitated with the aqueous nickel solution. The loaded organic was diluted to the desired concentrations by adding (a) more Shellsol 715 or (b) more DEHPAShellsol 715 (33% by volume). An autoclave equipped with a glass liner, a heater, a constant speed agitator, a pressure gauge, and a thermo0 1981 American Chemical Society
230
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
Table I. Reduction of Nickel in Various Organic Solutionsa by Hydrogen [No Catalyst; Temperature = 1 0 0 "C; Pressure = 860 k%; Reduction Time = 30 min]
a
organic solution
% nickel recoverv
naphthenic acid/kerosene naphthenic acid/Shellsol 715 DEHPA/ kerosene DEHPA/Shellsol715
1.65 0.65 2.50 3.40
TIP'E ( H I N )
20
20
40
60
80
120
100
Solutions prepared by method I.
Table 11. Reduction of Nickel b y Hydrogen in DEHPA/Shellso1715a a t Various Temperatures and Times (Pressure = 860 kPa)
a
concn of Ni org. phase, g/L
temp, "C
time, min
Ni recovery, 76
3.25 4.50 4.50 4.00 3.55 7.80 4.55 2.45 9.30 6.35 7.80 4.25
25 25 70 70 100 100 140 140 180 180 200 200
30 45 30 45 30 45 30 45 30 45 30 45
0 0 1.7 2.0 3.4 5.0 7.3 9.5 11.7 14.5 15.5 18.7
Results and Discussion From preliminary experiments, naphthenic acid and di(2-ethylhexyl) phosphoric acid (DEHPA) were selected as solvents on the basis of their low solubilities in water ( G O mL/ lo6 mL water at pH 6 and room temperature) and their efficient nickel extraction from an aqueous phase. The results of hydrogen reduction using these solvents in two diluents, kerosene and Shellsol 715, are given in Table I. Absolute recoveries are poor, but DEHPA/Shellsol715 appears to be the most efficient organic system. Given the possibility of Ni-Co separation (Burkin and Burgew, 1972), DEHPA/Shellsol715 was selected for the rest of the study. The effects of reduction time and temperature are shown in Table I1 for reduction a t 860 kPa hydrogen pressure. Nickel recovery, which was low in the absence of catalyst, increased with temperature. However, at temperatures in the range of 180 to 200 "C, a green-black paste residual was observed, perhaps indicating decomposition of the organic acid. A temperature of 140 " C was found to be
COC
900
HYDROGEN ORESSURE K P P
Figure 1. Effect of hydrogen pressure and reduction time on the nickel recovery. Table 111. Reduction of Nickel in DEHPA/Shellsol 715a in the Presence of Catalysts [Temperature = 1 4 0 OC; Hydrogen Pressure = 860 kPa]
Solutions prepared by method I.
couple well for temperature measurement was used for the reduction experiments. The temperature could be controlled manually to within f 5 "C. The glass liner could be removed after each run to obtain the nickel deposit for analysis. To initiate an experiment, the autoclave without the glass liner was heated to about 10 "C above the desired operating temperature. The glass liner, containing 1L of the nickel-loaded solution, was then inserted into the autoclave. The stirrer was started and hydrogen introduced into the reaction vessel at the desired pressure. Zero time was taken to be the moment of introduction of hydrogen. At the end of the run, hydrogen was vented and the contents of the reaction vessel quickly removed and filtered. The precipitates were then dried in an oven and weighed. The nickel content was determined by a gravimetric method using dimethylglyoxime (Young, 1953).
700
600
500
40C
catalyst none nickel powder anthraquinone a
quantity/L of soln. (0.1 g) (0.1 8)
Ni recovery, %
9.5 86 49
Solution prepared by method I.
Table IV. Hydrogen Reduction of Solutions Prepared by Method II(aIa l0.100 L! of Ni as CatalystlL of Solution1
temp, "C
pressure, kPa
80 140 140 140
860 791 1136 1480
initial Ni Ni concn, reduction recovery M time, min % 8.8 8.8 8.8 8.8
80.0 80.0 80.0 80.0
0.22 4.1 3.1 4.8
a Original nickel solution of 0.176 M was prepared by extraction with 33.4% by volume DEHPA in Shellsol 715.
a practical upper limit for producing nickel powder without production of such a residue. The effects of hydrogen pressure and reduction time on nickel recovery at 140 "C in the absence of any catalyst are depicted in Figure 1. While increases in both hydrogen pressure and reduction time improved the recovery, the actual recoveries were still low. The relative constancy (-20% decrease) of the nickel concentration in the organic solution and the availability of the additional reaction surface of the nickel metal produced may explain why the reduction rate decreased only slightly with time. Recovery was markedly improved through use of the catalysts anthraquinone and nickel powder (Table 111),nickel powder appearing to be the better catalyst. With nickel loaded by method I1 (preequilibration of the organic phase with sodium hydroxide), a very concentrated nickel organic solution was produced. Dimerization or even polymerization of the nickel-organic complex, MR,, is
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
possible under such conditions (eq 3). When organic solutions prepared by diluting concentrated solutions with Shellsol 715 were reduced, very little nickel was produced, as shown in Table IV, even in the presence of nickel powder catalyst. However, when the dilution was effected by adding 33.4% DEHPA-Shellsol715 solution, a nickel recovery of 97.7% was obtained from a solution of 4.4 X M Ni, after 100 min, at 140 "C and 1480 kPa hydrogen pressure. These observations indicate that the hydrogen reduction is affected by the free solvent to diluent ratio in the solution, which influences the nature of the solvated complex to be reduced. The results obtained for a wide range of conditions using a nickel powder catalyst are presented in Table V. There is no definite trend with hydrogen pressure, in contrast to the slight dependence of recovery on hydrogen pressure observed a t lower pressures for reduction in the absence of a catalyst (Figure 1). At the highest pressure of 1825 kPa, a large amount of a green precipitate was produced, apparently indicating some conversion of the organic phase (other than reduction to give nickel metal). The recovery also decreased with increase in initial nickel concentration, which may be explained in terms of the much more difficult reduction of polymerized nickel-organic complex at high nickel loadings. In the rate eq 2, the specific interfacial area, a, is a function of the conversion since substantial catalytic action of the nickel powder is due t o freshly formed nickel. If for simplicity a is given by a = Ci{1
+f
Table V. Hydrogen Reduction of Nickel from the DEHPA/Shellsol 715 Systema Using Nickel Powder Catalyst (0.1 g/Lof Solution) reductemp, pressure, "C kPa
140 120 100 80 140 140 140 140 140 140 140 140 140 100 100 100
1480 1480 1480 1480 1825 1136 791 1480 1480 1480 1480 1480 1480 1480 1480 1480
g/L
2.58 2.58 2.58 2.58 2.58 2.58 2.58 5.16 7.74 10.32 2.58 2.58 2.58 2.58 2.58 2.58
M 4.4 4.4 4.4 4.4 4.4 4.4 4.4 8.8 13.2 17.6 4.4 4.4 4.4 4.4 4.4 4.4
time, min
80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0
100.0 60.0 40.0 100.0 60.0 40.0
Ni recovery, %
87.2 94.0 70.2 21.7 67.8b 65.5 69.8 35.5 31.6 23.4b 97.7 62.4 42.2 93.8 37.6 26.4
M=l. 0
'9 0
..., , .
(7)
where f is a constant representing the ratio of potential weight of nickel deposited by reduction to weight of added nickel powder, and Ci is the initial specific interfacial area, then by substituting eq 7 into eq 2 and integrating, there results
initial Ni concn
a Method II(b), dilute solutions made from 0.176 M Ni solution (organic phase for extraction: 33.4% by volume DEHPA in Shellsol 715). A green residue was produced and filtration was difficult.
140'C 100'C
t+
q
231
,,
Regression l i n e s
"i 1
/
/
for M = 1, and
(1
+ flMCM-1
In
(1 + flc, -
c
(1 + f)co- c,
(9)
for M # 1. If the right hand side of eq 8 or 9 is called RH and plotted against time, t, the actual value of M is that which gives a straight line of slope Cik! If all the available surface area is assumed to be active and if all nickel particles are assumed to be spherical and of the same size, then Ci may be estimated from
where m is the mass of nickel added of size dpand density pp, and V is the volume of the reaction mixture, Variation of a with conversion is accounted for through eq 7, and Figure 2 shows that the data can only be poorly fitted to a straight line when M = 1.0. The results from the hydrogen reduction experiments could be reproduced to within 10%. By allowing M to vary, best fits were obtained for the data at M = 0.4 as shown in Figure 3. Variation of temperature over the range 100-140 O C affects the yield but had no significant effect on the rate
0. DO
Figure 2. Kinetics of the hydrogen reduction of Ni loaded organic solution (eq 8).
constant. The calculated slopes of the lines in Figure 3 are 1.0 X and 8.9 X 10" for 140 and 100 OC,respectively. Since these are not significantly different at a confidence level of 0.95, the rate-controlling mechanism may be a mass transfer step. The value of M depends on the values off and C,. I f f is small and Cois large, M would be zero in a well-stirred reactor. It would seem that transport of nickel-organic complex molecules to the reaction surface is the rate-controlling mechanism. Conclusions The system DEHPA-Shellsol 715 was found to be
Ind. Eng. Chem. Process Des. Dev. 1981, 20, 232-239
232
A
.......,
M Ni organic solution could be recovered after 100 min. A temperature of 140 O C was found to be suitable for the heterogeneous reduction process. The reduction process depends on the state of the complex equilibria existing in the organic phase-the relative amounts of polymerized and single molecule organic complex, the degree of solvation, and the free solvent to diluent ratio in the solution-and is sensitive to the procedure for loading the nickel into the organic phase. The reduction rate is limited by a mass transfer step, probably the transport of nickel-organic complex molecules to the reaction surface.
loo’c Regression l i n e s
at
Literature Cited
I
1 0
TO.00
50.00
&.OD
7b.00
Bb.00
3i.00
TIME (MINI
!
00
Boateng, D. A. D. Ph. D. Thesis, University of Toronto, 1979. Burkln, A. R. “The Chemistry of Hydrometallurgical Processes”. E. and F. M. Spon Ltd.: London, 1966; Chapter 7. Burkin, A. R . Trans. Inst. Min. Metall. Sect. C 1073, 44. Burkln, A. R.; Burgess, J. E. A. Powder Metall. 1060, 12, 51. Burkin, A. R.; Burgess, J. E. A. “Production of Metal Powders by Reduction of Loaded Extractants wtth Hydrogen”, International Symposium on Solvent Extraction in Metallurgical Processes, Antwerp, May 1972; Antwerp Technogisch IWitnut K. viv Genwtschap Metallurgy, 1972; pp 51-58. Evans, D. J. I. Trans. Inst. Min. Metall. Sect. C 1068, 831. Ipatleff, V. N.; Werchowski, W. feritchte 1000, 42, 2078. MacGregor, E. R.; Halpern, J. Trans. AIME 1058, 212, 244. Meddlngs, B.; Mackiw, V. N. “Unit Processes in Hydrometallurgy”, Gordon and Breach: New York, 1964; p 345. Schaufelberger, F. A.; Roy, T. K. Trans. Inst. Min. Metall. 1055, 64, 375. Von Hahn, E. A.; Peters, E. J . phys. Chem. iS65. 69, 547. Wadsworth, M. E. Trans. Met. SOC.AIMEIO80, 245, 1381. Young, R. S. “Industrial Inorganic Analysis”, Chapman and Hall Ltd.: London, 1953; p 186.
Figure 3. Kinetics of the hydrogen reduction of Ni loaded organic solution (eq 9).
suitable for nickel production by direct hydrogen reduction of the nickel-loaded organic solution. With nickel powder as catalyst, up to 97.7% of the nickel content of 4.4 X lo-*
Received for review June 11, 1979 Accepted October 25, 1980 This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada.
Selection of the Disperse Phase in Spray or Plate Extraction Columns A. H. P. Skelland’ and N. Chadha Chemical Engineering Department, University of Kentucky, Lexington, Kentucky 40506
Criteria are established for deciding whether the controlling phase should be dispersed or continuous in perforated plate or spray extraction columns. Relevant factors include the plate spacing or height of free rise or fall of the drops before coalescence, and the presence of surface active impurities. The findings are qualitatively similar for both high and low interfacial tension systems. For the latter it is shown that nonionic, anionic, and cationic surfactants induce minima in the continuous- and disperse-phase overall capacity coefficients at intermediate concentrations of surfactant. This parallels previous findings for systems with high interfacial tension.
The purpose of this study is to establish criteria for selecting the phase to be dispersed in perforated plate or spray extraction columns. The conventional criterion in this regard has been to disperse the phase present in larger volume, so as to provide the greater total interface.
* Chemical Engineering Department, Georgia Institute of Technology, Atlanta, GA 30332. 0196-4305/81/1120-0232$01.25/0
However, Skelland (1967, 1974) has suggested that, where comparable amounts of each phase are present, it may be better to disperse the phase offering the smaller resistance to mass transfer, as indicated by the magnitude of the distribution coefficient. In this case, even if trace quantities of surface-active impurities should stop internal circulation currents within the drops, this will have a less serious effect on the rate of mass transfer, because the bulk of the transfer resistance now lies in the continuous phase. 0 1981 American Chemical Society
