I
TUHIN K. ROY1 Freeport Sulphur Co., New Orleans, La.
Preparing Nickel and Cobalt Concentrates Here is the new process for recovering nickel and cobalt from lateritic ores adopted at the Moa Bay Plant of the Freeport Sulphur Co.
IN
THE METALLURGICAL industry a concentrate is commonly produced at or near the ore body and then transported from the mill to a refinery where the metals are separated and the primary metals of acceptable purity are produced. Concentration methods such as tabling, froth flotation, or magnetic separation are mechanical and do not use the large quantities of fuel, chemicals, or power generally required for refining. For nickel and cobalt, however, the world's largest potential source of these metals occurs as nickeliferous iron ore or as silicates; the valuable metals are finely dispersed in the ores, impregnating their various other mineral ingredients. The usual mechanical methods of concentration are not useful for these vast but low grade deposits (77). Smelting of the silicate ores in a blast furnace with anthracite or coke, and gypsum or pyrite has been practiced for some time in New Caledonia, Germany, and U.S.S.R. to produce an iron-nickelsulfide matte. However, the cost of transporting the fuel and the sulfur mineral to the mine site, or the ore to the smelter is expensive. Exploitation of the vast nickel and cobalt deposits in Cuba, New Caledonia, Indonesia, and the Philippines would be greatly helped if high grade concentrates could be produced economically near the mines for shipment to convenient refinery sites. The method described in this report (see diagram) has undergone extensive laboratory and pilot investigation and is a key step in a nickel and cobalt leaching and concentration plant built at Moa Bay, Cuba, by Freeport Sulphur Co. The concentrate from this plant was to be shipped to a refinery at Port Nickel where 50,000,000 pounds of nickel metal and 4,500,000 pounds of cobalt metal were to be produced annually. Unfortunately both plants were shut down
Present address, Industrial Consulting Bureau, 5 Parliament Street, New Delhi 1, India.
soon after they went on stream because of political changes in Cuba. The nickel-concentrating plant a t Moa Bay treats nickeliferous iron oxide (laterite) containing 1.3 to 1.4% nickel and 0.12 and 0.14Yo cobalt with dilute sulfuric acid at 450' to 500' F. in steamagitated multistage towers. Sulfuric acid leaching of silicate ores (garnierite or nepouite) has also been found to be commercially attractive by a number of workers (5, 76). The reasons for preferring acid leach to the ammoniaammonium carbonate leach process already established on a commercial scale at Nicaro (2) are: the necessity in the latter process of reducing nickel and cobalt in the ore to the metallic form in a furnace prior to leaching, and relatively poor extraction of cobalt by this method. The applicability of an acid leach to an oxidized nickel ore depends, of ~~~~
course, on the amount of acid consumed per ton of nickel and cobalt extracted. For nickeliferous iron ores, acid leaching at elevated temperatures results in rapid dissolution of valuable metals such as cobalt, nickel, and copper at a relatively low acid-to-ore ratio because the bulk of iron and aluminum remains undissolved. However, even with high temperature acid leach, the pregnant liquor after separation from the tailings is likely to be dilute in valuable metals and relatively high in impurities such as magnesium, aluminum, iron, and chromium. Recovery of nickel and cobalt from such dilute and impure solutions has been a subject of attention to hydrometallurgists for years. One method of recovering nickel and cobalt from acidic sulfate solutions is to neutralize hot leach liquor with limestone or magnesite to a p H of 3.0 to 4.0 and to aerate it so that the bulk of the ~~~~~
~
Proposed Method Cobalt and nickel are not precipitated if hydrogen sulfide gas is bubbled through a solution of fheir salts containing a free mineral acid. However, a very small amount of nickel or iron powder added to an acidic sulfate solution (pH, 7.5) catalyzed the precipitation of cobalt and nickel by hydrogen sulfide at atmospheric pressure ( 8 ) . At higher temperatures and hydrogen sulfide pressure, the precipitation reaction was so rapid that addition of metal powder was not necessary to catalyze the reaction (9). This rapid, selective, and virtually complete precipitation reaction is the basis of this article (diagram). Recycling, where feasible, a part of the barren solution containing sulfuric acid regenerated by the precipitation reaction can reduce sulfuric acid requirements considerably. Also, if is possible to reduce sulfur requirements by reaction of hydrogen with the sulfide precipifate at about 300' C. in a furnace to regenerate hydrogen sulfide and produce a crude nickel-cobalt metal product for shipment to the refinery.
VOL. 53, NO. 7
JULY 1961
559
iron, aluminum, and chromium is precipitated. The filtrate is then neutralized with lime or magnesia until most of the nickel and cobalt values are precipitated as hydroxides. The hydroxide concentrate thus produced contains 15 to 25y0 nickel plus cobalt and is usually contaminated with compounds of manganese, calcium, or aluminum. Another method of producing a nickelcobalt concentrate from dilute acidic liquors (74) is to add lime and pass on HzS-containing gas through the hot liquor to a pH of approximately two. The precipitate is then suspended in water and heavy metal sulfides are separated from gypsum by flotation. Other methods proposed are cementation of nickel metal with granular aluminum or zinc (73) and precipitation of nickel as basic chromate or oxalate (7). These concentration methods were either not selective enough or not economical. Using the proposed method, nickel and cobalt can be selectively and almost completely precipitated from acidic ore leach liquors with compressed hydrogen sulfide. Presence of inert cations help in the conversion through bisulfate formation. The reaction conditions given below and the rapid settling characteristics of the precipitates make the reaction suitable for commercial production of nickel and cobalt concentrates from low grade oxide and silicate ores.
Reaction Conditions
The most important variables in determining precipitation rates are initial pH, temperature, and agitation. The reaction is catalyzed by iron or nickel powder. However, the effect of variations in the seed powder concentration on precipitation rate is minor above 100' C. Between 50 to 150 p.s.i. range, the effect of H2S pressure on the precipitation rate was minor. The initial reaction rates were independent of feed nickel or cobalt concentration and were dependent mainly on initial pH and temperature. Afier the fast initial stage, a linear dependence of reaction rate on nickel or cobalt ion concentrations was noted. Batch precipitation time required for precipitating 99% of nickel and cobalt values from 0.1 molar nickel plus cobalt solutions was only 5 to 15 minutes at 120' C., 100 p.s.i. H2S pressure, and an initial pH of 2 to 3. Equilibrium calculations and rate data showed that a sharp separation of nickel from cobalt is not possible by the H2S precipitation method. The batch precipitation data were successfully used for designing a multistage continuous reactor. Analysis of stage samples obtained during continuous precipitation tests confirmed the picture of the precipitation reac560
of the precipitation test, the charge was filtered, the residue was washed with 200 ml. water, and the dried precipitate samples were analyzed for nickel, cobalt, sulfur, and trace impurities.
tion obtained from the batch data. The sulfide concentrates produced by hydrogen sulfide treatment of dilute liquors obtained by sulfuric acid leach of nickel oxide and silicate ores contained about 6097, nickel plus cobalt. These concentrates were practically free of Mn, Al, Mg. and Ca and contained less than 1% iron.
Thermodynamic Considerations
The following chemical reactions take place when an aqueous solution containing cobalt, nickel and zinc sulfate, and sulfuric acid is subjected to hydrogen sulfide pressure :
Apparatus and Procedure
The batch precipitation tests were carried out in a stainless steel, stirred, 3-gallon autoclave. The stirrer shaft carried a square-pitch marine type impeller, 4.5 inches in diameter, placed 1 inch from the bottom of the vessel. The stirrer was rotated at 643 r.p.m. except as otherwise noted. A 7-liter charge, prepared by dissolving technical grade nickel, cobalt, and other metal sulfates in water, was placed in a flat-bottomed glass jar, 6.75 inches in diameter by 12 inches high, and provided with three, 3/4-inch wide, removable glass baffles. The jar was lowered into the autoclave and water (100 ml.) was added in the annular space between the jar and autoclave to facilitate heat transfer from an external burner. Nickel powder or nickel-cobalt sulfide (-275 mesh) \vas added in most of the runs as seed. When desired reaction temperature was reached, the autoclave was pressurized to the operating pressure with hydrogen sulfide from a gas cylinder. During precipitation. solution samples "ere withdrawn into a sample bomb a t convenient time intervals. At the end
STEAM1
1
OXIDIZED NICKEL OR COBALT ORE
HzS (gas)
$
H2S (as.)
+
H2S (as.) $ H + (aq.)
K
(1)
HS- (as.) ( 2 ) HS- (aq.) e H A(as.) f S-2 (as.) ( 3 ) + 2 (aq.) S-2 (as.)$ NiS (cryst.) ( 4 )
+
Zn+Z (as.)
+ S-2 (as.) -
Co-a (aq.)
+ S-*(as.) e
A
ZnS (cryst.) ( 5 ) COS (cryst.) ( 6 )
(as.)
+ H + (as.) e HSOd- (aq.) (7)
The above solution may also contain impurities such as copper, lead, cadmium. arsenic, or antimony ions. Since the sulfides of these metals have much smaller solubility products than those of nickel and cobalt sulfides, precipitation of these ions will be virtually complete. O n the other hand impurities such as Mg+Z, Ca+2, Fe+2, Al+3, and Mn+2 generally act as inerts, because solubility products of sulfides of these metals are more than 10,000 times those of zinc, cobalt, or nickel (6).
COMPRESSED HYDROGEN
GI
MAKE-UP SULPHUR I CdLEAc I A CiD 7-7SOLI D - L l Q U I D S E PAR AT1ON
I
1SULPHUR
HYDROGEN SULPHIDE GENERATOR
1
I PR EC I P I TAT1ON
TAILINGS I
k
-
i SOLI D L I Q U I D SE PARATlO N
N IC K EL- COBALT SULPHl DE CONCENTRATE
1
BARREN LIQUOR TO WASTE OR SULPHATE RECOVERY
Flow diagram of a concentration plant handling oxidized nickel or cobalt ores
INDUSTRIAL AND ENGINEERING CHEMISTRY
NICKEL AND COBALT CONCENTRATES At equilibrium, the concentration of the various ions may be expressed in terms of HS-, HSOa-, and H2S (aq.) concentrations. Thus,
Where Kz is the equilibrium constant for Reaction 2, Ks is the equilibrium constant for Reaction 3, and K4 is the solubility product of NiS. Let the initial concentration of total sulfate be 6 moles per 1000 grams of water, and let c equivalents per 1000 grams of water denote the sum of the concentrations of the "inert" cations which remain unchanged during H2S precipitation under acid conditions. By balancing the ionic charges in the solution at equilibrium with H2S gas at a given temperature and pressure, we obtain the following equation :
+
+
+
+
2Nif2 2Zn+2 ~ C O + H ~+ c = 2S-' 2b - [HSOd-] HS- ( 9 )
+
+
Expressing equilibrium concentrations of various ions in terms of equilibrium constants or solubility products of Reactions 1 to 7 and designating x for [HS-] and d for the concentration of HzS (as.), we obtain ad
x2 +
K2d
bhd +--=s." + x + 26 x+hd d
- C
(10)
where a =
2Rs -
K,
(Kd f K5 f
K61,
Equilibrium value of [HS-] at any given initial sulfate concentration, inert cation concentrations, temperatures, and H2S partial pressure can be calculated from Equation IO, provided the equilibrium constants of the reactions given in Equations 1 to 7 are known or determined at the precipitation temperatures. Once the values of HS- are known, the equilibrium values of unprecipitated Nif2 can be calculated from Equation 8. Equilibrium concentrations of Cot2 and Zn+2 can be similarly calculated. Equilibrium constants Kz to K7 at precipitation temperatures are calculated from standard free energy and solubility product data available (4, 75). For K1, the H & solubility constant, experimental solubility data were used. Solubility of H2S in acidic sulfate solutions was investigated in the temperature range of 65"-120' C. and hydrogen sulfide pressure of 2-10 atm. Hydrogen sulfide solubility in gram moles per 1000 grams of water at a chosen temperature was plotted against corrected H2S partial pressure, r y c f l p ) ~ . Straight lines
d
I
I
-
32
%I
I
I
'
/
/
l
1
I
~
l
E -
Figure 1. H2S solubility constant K1, as a function of temperature. Solubility of hydrogen sulfide in stripped liquor i s somewhat lower than that in water represented b y the upper curve
were obtained up to H2S partial pressure of about 7 atm. Since
the slopes of these solubility isotherms gave the values of K1 a t corresponding temperatures. Figure 1 shows that H2S solubility in dilute acidic salt solutions is appreciably lower than that in water (77), particularly above 70" C. Since the hydrogen ion and total sulfate concentrations in the end liquors of the Precipitation tests did not differ widely from the synthetic solution (3.5% total sulfate, 1.2% sulfuric acid) used for determining HzS solubility, values of [HZS (aq.)] or d under equilibrium conditions were calculated (Figure 1). Equilibrium concentrations of Ni+2 and C O +within ~ the ranges of temperature, pressure, and concentrations studied were calculated from Equation 10. An IBM digital computer was used to solve for x in Equation 10 and to make sure that the values of x obtained were the only possible real roots. These thermodynamic calculations showed that nickel and cobalt sulfides can be precipitated from dilute solutions to virtual completion at relatively low temperatures and hydrogen sulfide pressures. Thus, for a 0.1 molal nickel sulfate solution containing 0.09 molal sulfuric acid and 0.01 molal cobalt sulcate as the feed, 98.5% of nickel and 98.9y0 of cobalt can be precipitated with hydrogen sulfide at 200" F., and as low a total pressure as 103 p.s.i.g. Approximately the same equilibrium conversion can be reached in a shorter time by raising the temperature to 250" F., and the operating pressure to 142 p.s.i.g. The most important result of these calculations is the demonstration of the effect of the inert cations on equilibrium
precipitation. Thus, if the feed solution described above contained manganese sulfate and magnesium sulfate at 0.1 molal concentration each, equilibrium conversion of nickel at 200" F. and 103 p.s.i.g. would be as high as 99.9670. These calculations showed also that no sharp separation between nickel and cobalt can be expected by selective HzS precipitation, although it may be possible to precipitate pure cobalt from solutions containing relatively low concentrations of nickel. Fortunately, the equilibrium concentrations of cobalt are one tenth to one fifteenth those of nickel. Nickel and cobalt occur in about the same ratio in the Cuban laterite ores. Therefore, HZS precipitation conditions chosen for satisfactory nickel precipitation should also be satisfactory for cobalt recovery from these ores. In a number of batch H2S precipitation tests the run was continued until the liquor samples showed little or no further precipitation of nickel. In Table I, these experimental per cent precipitation figures are compared with those calculated by Equation 10. Unfortunately, the total sulfate or free acid concentrations in feed solutions used in these tests were not determined directly. The free sulfuric acid contents were calculated from the pH and metal ion concentrations as follows: From Equation 7 , [S04-2] =
E H [HS04-] K,
Also, m n = 2JHS04-]
+
+ 2[SOC2j
(11)
(12)
and m = [H+]
+ [HSOa-]
(13)
assuming undissociated HzS04 concentration to be negligible. Since the VOL. 53, NO. 7
JULY 1961
561
Table I.
Run NO.
El E2 E3 E4 E5 E6 E7 E8
Pptn. Conditions H?S presTemp., sure, O C. atm.
Feed Sol. ComD ., G. Equiv./1000 G. Hz0 Inert Total Ni+2 Co+? H?QOp cations sulfate 0.1340 0.0124 0.1136 0.2349 0.4950 0.1340 0.0136 0.1136 0.2286 0.4898 0.1340 0.0136 0.1136 0.2286 0.4898 0.1340 0.0140 0.3788 0.4542 0.9810 0.1520 0.0147 0.1335 0.6003 0.9005 0.1510 0.0147 0.0688 0.5814 0.8159 0.4129 0.0154 0.0998 0.4542 0.9823 0.1680 0.0154 0.4090 0.4290 1.0213
values of n and p H were known, the values of rn, [HSOe-1, and [S04-2] could be calculated from Equations 11, 12, and 13. Because the accuracy of nickel and cobalt analyses at equilibrium concentrations (0.005 to 0.073 grams per liter) is, at best, fair, the agreement of the calculated values kvith the experimental equilibrium data is considered satisfactory.
65 90 90 90 120 90 90 120
Synthetic sulfate solutions used for batch precipitation tests contained the impurities which occur naturally with nickel oxide and silicate ores, and would be expected in the leach liquors resulting from dilute sulfuric acid leach of these
Table II.
NO.
1
2
Feed Soln. Compn., G./L. M n 1.5 Ni 4.4 Mg 0.3 Co 0.4 Zn 0.1 Fe 1.0 A1 4.4 Ni Co
Zn Cu
4.9 0.45 0.1 0.03
A1 Mn Mg Fe
Seed, G./L. 1/2 (Nickel Powder)
3.0 1.5 0.3 0.75
1/2 (Nickel Powder)
5 Recycle
3.0
Zn Cu
A1 Mn M g Be
4
Ni Co Zn Cu
4.9 0.45 0.12 0.04
A1 Mn Mg Fe
3.0 1.5 0.3 0.75
5
Ni Co A1
3.9 0.4 0.4 0.08
M n 1.5 Mg 0.3 Fe 1.0 Cu 0.03
1/2 (Nickel Powder)
Ni 12 Co 0.45 A1 3.0
M n 1.5 Mg 0.3 Be 0.75
1/2 (Nickel Powder)
Ni Co A1
M n 1.5 Mg 0.3 Be 1.0
lj2 (Nickel Powder)
Ni Co
Zn 6
7
562
3.9 0.4 2.6
1.5
at Equil. ObCalcd. by Eq. 10 served 99.54 99.76 99.87 99.64 99.99 99.99 99.88 99.71
99.46 99.46 99.90 98.86 99.85 99.95 99.93 99.00
1.0
+60 mesh
0 0.1 14.5 52.5 47.5
+ 100 mesh +ZOO mesh +325 mesh -325 mesh
NiS-COS
120
120
100
100
yo Pptd. Pptd. Sulfide
Time, min.
Ni
co
6 10
99.3 99.8
97.4 99.2
30
99.84
10 20 30
97.1 98.03 98.14
10
87.3 88.0 88.9
Ni 53.8 A1 0.02 Fe 0.32 Zn 1.23
35.5 M n . 4
3
I
t
i
1
i -i---i---q
21ti+---t-------i
Ll
I
!
VOL. 53, NO. 7
I i_L----
I
JULY 1961
~
565
.I0
TEMP. - 2 5 0 OF: H2S PARTIAL PRESSURE -100 l?S.lA. INITIAL pH ~ 2 . 5 - 3 . 0 I N I T I A L NICKEL 5.0 G.P.L. CONCENTRATION
. PRECIPITATION
-091 .08
.07--
'
PRECIPITATION
TEMP.
- 250 OF.
INITIAL pH -2.5 -3.0 INITIAL COBALT CONCENTRATION =0.63 G.P. L.
.O7
.06
I
I
I
L-
'
.oI
I
2 NUMBER
3
4
L
OF STAGES
5
6
(N1
Figure 8. Y* - Y vs. number of stages from continuous nickel sulfide precipitation tests. These data were obtained with a total solution hold-up time of 16 minutes
Therefore, if the rate of precipitation in the later stages is proportional to C C*, a plot of log (Y* - Y,,,) us. iV should give a straight line. I n Figures 8 and 9, Y* - Y values (calculated from the stage samples obtained during three continuous precipitation tests) are plotted against stage number. While the second, third, and fourth stage samples yield nearly a straight line on the above plot, the first stage samples are out of line as expected. The slope of the straight line portion is a measure of the reaction rate constant during the later part of the reaction.
.olo
I
K
=
= reaction rate constant
equilibrium constant
normality of free sulfuric acid in feed sbln., g. equiv./1000 g. H90 N = stagenumber n = equiv. of total metal ions in feed s o h . per 1000 g. HnO fi = H2S partial pressure, atm. T = absolute temp., K. t = reaction or hold-up time, sec. x = HS- concentration a t equil.: g. molesjl000 g. H20 y = mole fraction of H2S in vapor phase Y = fraction of Ni+* or Co++ pptd. Y" = fraction of Xi+z or C O + pptd. ~ at equil. r = total pressure, atm. =
Nomenclature
A1
constant in Equation 17 a constant in Equation 10 B' constant in Equation 15 b concn. of total sulfate in feed soln., g. moIes/1000 g. H20 .C = concn. of Ni or Co a t time t, g. moles/1000 g . HzO $'* = equil. molal concn. of Ni or Co = equiv. of inert cations per 1000 g. c H2 0 d = concn. of H2S in soln. at equil. moles/1000 g. HzO f/p = fugacity coefficient g = constant in Equation 10 h = constant in Equation 10 = = = =
566
Acknowledgment
The author acknowledges the help of his colleagues at the Chemical Construction Corp. and the Freeport Sulphur Co. in performing experimental work and for numerous suggestions regarding its design, and for the interpretation of results. literature Cited (1) Caddell, J., Hurt, D., Chem. Eng. Progr. 47, 333 (1951).
(2) Dufour, M., Hills, R., Chemical Znds., 57, 621 (1945).
INDUSTRIAL AND ENGINEERING CHEMISTRY
6
7
Figure 9. Y* - Y vs. number of stages for continuous cobalt sulfide precipitation. These data were obtained with total solution hold-up time of 16 minutes
k m
NUMBER 2 3OF STAGES 4 5( N )
(3) Elridge, J., Piret, E., Chem. Eng. Progr. 46,290 (1950). (4) Goates, J., Gordon, M., Faux, N., J.A.C.S. 74, 835 (1952). (5) Guimaraes, C., Molian, T., Assoc. quzm. Brazil 9, 75 (1950). (6) Handbook of Chemistry and Physics, 38th ed., p. 1640, Chemical Publishing Co., Sandusky, Ohio, 1956. 17) \ , Kawamura, K.. Jap. Patent 172.870 (June 10, 1946). (8) Roy: T., U. S.Patent 2,722,480 (Nov. 1, 1955). (9) Roy, T., Schaufelberger, F., Zbid., 2,726,953 (Dec. 13, 1955). (10) Roy, T., Schaufelberger, F., Trans. Inst. Mining and M e t . (London) 64, 375 (1954-55). (11) Selleck, F., Carmichael, L., Sage, B., IND.ENG.CHEM.44, 2224 (1952). (12) Simons, C. S., unpublished report, Cuban American h-ickel Co., New Orleans, La., February, 1957. (13) SOC. anon. Trkfileries et laminoirs du Havre, Ger. Patent 648,543 (.4ug. 3, 1937). (14) Sutterlin, W., Ibid., 720,881 (April 16, 1942). (15) U. S. Bur. of Standards, Selected Values of Chemical Thermodynamic Properties, Circ. No. 500, 1952. (16) Urazov, G. G., Bogatskii, D., Znuest. I
_
Akad. iVauk., S.S.S.R., Otdel Khim Nauk, 1948,194.
(17) L-. S. Bur. of Mines, Materials Survey-Nickel, April 1952, Chap. V ; Rept. of Invest. 5099, p. 5-16, February 1955, Inform. Circ. 7805, p. 25-26, November, 1957. RECEIVED for review May 23, 1960 ACCEPTED December 13, 1960
