non-ionic, precipitation by ^ ^ H
PRECIPITATION 1 )
Reduction
i J
\ \ByH2 ByCO ByS02 Nickel Silver Copper Cobalt Copper
VOi
Ionio H
\ 1
\ 1
By H2S •
Nis H CoS H
Chapter 6 - Precipitation 155
Figure 6.1 - Precipitation under pressure
Precipitation by hydrogen sulfide, on the other hand is ionic. It is based on the fact that when a H2S is added to a solution containing metal ion, a sulfide is formed whose solubility is very low under these conditions that precipitation takes place immediately:
M2- + 32- -^ MS
While CuS precipitates at ambient conditions, NiS and CoS presipi-tate at high temperature and pressure and in present of a catalyst.
That is why autoclaves are used in this case.
Precipitation of iron oxide by hydrolysis is also an important topic in pressure hydrometallurgy and will be discussed later.
PRECIPITATION BY REDUCTION
Precipitation by hydrogen Nickel and cobalt
Nickel and cobalt are precipitated on industrial scale by this method.
Copper can also be precipitated but not industrially applied. Reduc-tion is usually carried out in horizontal stainless steel autoclaves equipped with agitators, baffles, heating or cooling coils, and the necessary connections for feed and gas inlets and outlets. The prod-uct of this technique is a high-purity powder that can be used as such, or in case of metáis, hot pressed and rolled in form of strips.
Precipitation may be conducted from aqueous as well as from non-aqueous media.
Theoretical basis For the reaction:
the equilibrium constant is given by:
K = [Wf Therefore:
[M2^] • P H ,
log[M2^] = -2pH-(logK + logP^2)
This means that when precipitation is carried out at constant hydro-gen pressure and constant temperature, then at equilibrium there is a linear relation between logfM^*] and the pH of the solution, and the slope of this straight line equals -2. This was confirmed for the precipitation of nickel from NiSO^ solution, as shown in Figure 6.2 and for cobalt as shown in Figure 6.3.
156 Pressure Hydrometallurgy
10.0
1 2 Equilibrium pH
Figure 6.2 - Precipitation of nickel from nickel sulfate solution by hydrogen at 3,500 kPa, [(NH4)2SOJ = 112 g/L, equilibrium conditions
1.0 2.0 3.0 4.0 Equilibrium pH
Figure 6.3 - Precipitation of cobalt and nickel from acid solution, temp.
190°C, H pressure 3500 kPa, [(NHJ^SOJ = 112 g/L
It is olear from the above equation that more metal will be deposited if the hydrogen ions are removed as soon as they are formed. For
Chapter 6 - Precipitation 157
copper, nickel, and cobalt, this is conveniently done by operating in ammoniacal médium:
[M(NH3)J2- ^ nNHg + M 2+
M2^ + H^ ^ M + 2H^
H" + NH, -^ N H /
3 4
It can be seen that increasing the ammonia concentration has two opposing effects:
• Precipitation is favored due to the neutralization of the liberated acid.
• Precipitation is hindered because of the decrease in the reducible metal ions M^* due to the complexing action.
Therefore, there must be an optimum [NH3] / [M^"^] ratio at which these opposing effects are balanced. In the precipitation of nickel, the optimum molar ratio was found to equal two, which agrees with the overall reaction:
W* + 2NH3 + H2 NÍ + 2NH.
^ 40 x : L 7 30
_ i 01
<.-^U lo ce
10
l i l i l í
0
/ ^ 0 ^^">5^ ~
/ ^'"^^
P ^ * \ ^
' / ^^\..
P ^""^^^
/ ^"^^.^
l i l i l í
0 1 2 3 4 5 6
Molar ratio [NH3] / [Co^+j
Figure 6.4 - Effect of the molar ratio [NH3] / [Co^*] on the rate of precipitation;
temp. 200°C, catalyst H^PtCig 5.8x10-5 ^^ |_|^ pressure 3000 kPa
755 Pressure Hydrom etallurgy
The amount of ammonia in solution also influences the rate of pre-cipitation. In the case of cobalt, the rate of precipitation achieves a máximum when [NH3] / [Co^*] = 2, as shown in Figure 6.4. Similar results were also reported for copper.
Another way of removing the hydrogen ions as soon as they are formed during reduction is by reducing hydroxide slurries:
Overall reaction:
M(OH), ^ W^ + 20H-M2^ + H2 - ^ M + 2H"
OH- + H" ^ H,0
M(0H)2 + H2
M + Hp
For nickel and cobalt, it was found that this reaction takes place at 270°C which is a much higher temperature than normally used, but the product is of extremely fine particle size.
Nucleation
In some cases, the presence of a solid surface for precipitation is essential; such a solid is termed a catalyst. Strictly speaking, the process is heterogeneous (contact catalysis) but is different from the heterogeneous process described later.
If no catalyst were provided, the internal surface of the autoclave itself acts as a catalyst and deposition of metáis takes place on the walls or on stirrers. Deposition of metal on the internal surface of the reactor is undesirable because it causes operating difficulties in collecting the metal.
A catalyst may be needed in one médium but not in another. For example, a catalyst is needed for precipitating cobalt and nickel from
Chapter 6 - Precipitation 159
an ammoniacal sulfate but not from an acid médium. Nickel is pre-cipitated catalytically from an ammoniacal sulfate médium but not from an ammoniacal carbonate médium.
Precipitation of a metal may be autocatalytic. Thus, while copper can be precipitated from ammoniacal solution without the need of a catalyst, yet the deposited metal accelerates the process. A differ-ence between the two processes however is that in non-catalyzed reduction, the rate depends on the initial metal ion concentration, while in catalyzed reduction it does not, but depends on the surface área of the catalyst.
The commercial precipitation of nickel from the ammonium sulfate system nucleation is induced by adding a small amount of ferrous sulfate which, upon heating to the reaction temperature, hydrolyzes to ferrous hydroxide thus furnishing the catalytic surface required.
Nickel deposited in the first step acts as a catalyst for the next. After each reduction, the nickel particles are allowed to settle to the bottom of the autoclave, while the spent solution is drawn off and replaced with fresh pregnant solution. In this way, the nickel particles grow to the desired size, at which point the suspensión is discharged and the nickel powder then separated.
There is no need for the ferrous sulfate catalyst in the ammonium carbonate system; as a result, the nickel powder produced in this médium has a lower sulfur and iron impurity level than powder produced from the ammonium sulfate system.
Role of additives
The presence of certain organic or inorganic substances in the aque-ous phase greatly affects the physical nature of metal precipitated. It is possible to precipítate metal powder of certain physical property by simply adding a certain amount of additive. However, when or-ganic additives are used, the carbón content in the powder produced is increased and a special heat treatment is necessary to lower it to 0.01%. Additives may be used for the following purposes:
160 Pressure Hydrometallurgy Chapter 6 - Precipitation 161
Anti-agglomeration
Agglomeration of the precipitated metal partióles may take place, especially at high temperature. This is undesirable because the agglomerated partióles entrap solution causing an impuro produot.
Reagents are therefore added to control the partióle size. These are the same as those commonly used to promote uniform growth of cathodes in the electrowinning of metáis, e.g., ammonium polyao-rylate, arabic gum, gelatin, dextrin, dextrose, and fatty acids suoh as oleio and steario. These additives are adsorbed on the individual partióles, thus preventing their agglomeration.
Smooth surface formation
When anthraquinone or its derivativos is added during reduotion, nickel partióles produoed are smooth and regular because of uniform deposition while in the absenoe of anthraquinone, they are coarse and irregular (Figure 6.5). Anthraquinone has no effect on the precipita-tion of cobalt. The addiprecipita-tion of this additive to ammoniacal nickel sulfate or carbonate Solutions also aooelerates the precipitation, and this effect increases with increasing anthraquinone ooncentration up to a certain valué, beyond which it has no further effect.
Figure 6.5 - Effect of anthraquinone on the shape on nickel powder (320 ji);
(a) no anthraquinone, (b) in presence of anthraquinone, cross section through smooth nickel sphere after 40 successive depositions
,i¿* -í^.;^-?».
'^í'" 'X '-<'^
l'igure 6.6 - Precipitation of nickel by hydrogen. Left: irregular powder. Middle:
course rough powder. Right: Smooth 10 n spheres
Crystalline product
Figure 6.7 shows hexagonal platelets of metallic cobalt precipitated in presence of a suitable additive.
Figure 6.7 - Precipitation of cobalt powder by hydrogen
T
162 Pressure Hydrometallurgy Chapter 6 - Precipitation 163
Industrial application
At the Sherritt-Gordon Plant (Figures 6.8 and 6.9), the purified leach solution obtained by ammoniacal pressure leaching of nickel-cobalt sulfide concéntrate contains 45 g/L Ni, 1 g/L Co, 350 g/L ammonium sulfate, and enough free ammonia to give a [NH3] / [NP"" + Co^""]
molar ratio of 2. The purified solution is reacted with hydrogen at 3500 kPa and 200°C.
on containing 45 g/L Ni Co obtained by ammoniacal ng of Ni-Co sulfide ore
Nickel ' " sulfate recovery
Fe(OH)3
for recycle
Niolíel ammonium - >- sulfate for nickel
recovery
1 Coball 1 ^ powder
Figure 6.9 - Recovery of nickel and cobait by precipitation with hydrogen: the Sherritt-Gordon process
Figure 6.8 - Foil Saskatchewan near Edmonton, Alberta in Canadá
Nickel is precipitated preferentially until its concentration is reduced to about 1 g/L, while all the cobait remains in solution (Figure 6.10).
The spent solution containing 1 g/L Ni and 1 g/L Co is then treated with HjS at 80°C and atmospheric pressure, and the precipitated Ni-Co sulfides are filtered off for recovery. The solution is then evaporated to crystallize ammonium sulfate fertilizer. The mixed sulfides precipitated earlier are leached with H2S0^ at 120°C in presence of air at 7200 kPa; acid is used instead of ammonia to avoid the formation of lower oxidation products of sulfur. The solution is purified from traces of iron by adjusting the pH to 5 and filtering off ferric hydroxide.
Figure 6.10 - Precipitation of cobait and nickel from ammoniacal solution by hydrogen under pressure
164 Pre$sure Hydrometallurgy Chapter 6 - Precipitation 165
The nickel-cobalt separation is carried out by oxidizing Co(II) to Co(III) to 70°C by air at 700 kPa and in presence of excess am-monia:
[Co(NH3),P [Co(NH JJ3- + e-YaO^ + Hp + 2e-
20H-On acidification, nickel ammine complex decomposes and precipi-tates as the double salt nickel ammonium sulfate:
[N¡(NH3)J2" + nH^ -^ NP^ + nNH/
while cobalt remains in solution. The slurry is then filtered to re-cover nickel. The fíltrate, containing cobalt in the cobaltic state, is converted back to the cobaltous state by cobalt powder. This step is essential otherwise a black precipítate of hydrated cobaltic oxide will precipítate during heating. Traces of iron are precipitated by ammonia as ferric hydroxide and separated. Metallic cobalt is then precipitated at 175°C by hydrogen at 2000 kPa in the presence of 25 g/L Co powder as catalyst. After filtration, the solution is then evaporated to crystallize ammonium sulfate. Table 6.1 gives analysis of nickel and cobalt produced by this process.
Table 6.1 - Purity of nickel and cobalt produced by hydrogen reduction
Ni Co Cu Fe
Nickel 99.7-99.85
0.1-0.2 0.01 0.02
Co Ni Cu S
Cobalt 95.7-99.6
0.1-0.5 0-0.02 0.02-0.05
Nickel in Philippines
Nickel was recovered from the laterites in Marinduque (Figure 6.11) by roasting, then leaching the calcines in ammonium carbonate according to the Carón process. The basic nickel carbonate cake obtained was dissolved in ammonium sulfate recycle solution and subjected to pressure precipitation by hydrogen in the same way as Sherritt process. Four autoclaves 2.4 x 9.8 m were used. The plant started in 1974 and shut down in 1986. The reason was the energy crisis of the 1970 because all fuel was imported. The company went bankrupt and was taken over by the Government of Philippines ten years later till it was shut down.
Soutn ^^KyJ <
Chino . ^ ^ B \ . ' ? Sea WtMÍ r '
MARtNOUQUE
jg^^jfiHHM|L' yé^mBBSBj^
MALAYSIA
Pjll Phiiippine H Sea
LUZON
VAS j J l ^ l ^ 4
MINDANAO '\
'V'
1
Figure 6 . 1 1 - Marinduque nickel plant in the Philippines
Copper
The precipitation of copper from CuSO^ solution takes place through the disproportionation of cuprous ion which has been identified in the course of reaction:
166 Pressure Hydrometallurgy
2Cu2^ + H2 ^ 2Cu^ + 2H^
2Cu^ ^ Cu + Cu2^
This leads to low yields oí metal. However, an advantage of this process is that copper can be precipitated from acid solution, i.e., there is no need to add ammonia during precipitation and as a result no ammonium sulfate is produced as the case with nickel and cobalt (Figure 6.12).
Figure 6.12 - Precipitation of copper by hydrogen under pressure in the range 140-170°C
Copper scrap or cement copper is dissolved either in ammoniacal ammonium carbonate at 60°C at atmospheric pressure with continu-ous aeration, or in dilute sulfuric acid. When ammoniacal médium is used, the molar ratio [NH3]/ [Cu^-^] should be equal to 2.4. After filtration to remove insoluble material, a small amount of anti-ag-glomerating agent is added, then solution is heated to 200°C under 6000 kPa hydrogen. The copper powder precipitated is filtered off, washed, and then dried in a reducing atmosphere at 600°C.
Chapter 6 - Precipitation 167
Copper and zinc
A flotation concéntrate of copper-zinc-iron sulfide is leached with ammonia at 90°C under an air pressure of 700 kPa. Copper and zinc pass into solution as ammines, while iron is precipitated as hydrated ferric oxide and filtered. Sulfamates formed during leaching are oxidized and hydrolyzed to sulfate at 230°C and 3500 kPa with air.
The molar ratio of free ammonia to copper is decreased to Vi by adding sulfuric acid to the autoclave prior to reduction by hydrogen to precipítate copper. Small amounts of ammonium polyacrylate are added to permit control of the physical characteristics of the powder produced. The solution is then treated with carbón dioxide under 700 kPa at 37°C to precipítate zinc hydroxy carbonate, which is then ñltered off:
2Zn(NH3)2SO, + CO^ + SH^O ^ Zn(OH)2.ZnC03 + 2(NH,)2SO, The clarified solution is evaporated to crystallize ammonium sulfate which is marketed as a fertilizer.
Precipitation of metáis from non-aqueous médium
Many metal ions are extracted by organic solvents by forming a coordination bond. When this loaded organic phase is treated by hydrogen at high temperature and pressure in an autoclave, the metal precipitates in powder form and the organic phase is regener-ated. The process may be described as precipitation by substitution since no ionic species are taking part in the reaction as compared to precipitation by hydrogen from an aqueous phase. The substitution reaction can be represented as follows:
H 2(g)
where RH is the organic solvent and M is a divalent metal. A typical example is the precipitation of metallic copper powder from hy-droxy-quinoline-kerosene phase containing copper:
168 Pressure Hydrometallurgy Chapter 6 - Precipitation
169
+ H - ^ 2
2(g)
(org)
Uranium oxide from leach solution
+ Cu (s)
(org)
lets and each tower is operated continuously until 10 tons of product has accumulated. The reduction end solution which contains only 3 to 5 mg/L uranium is recycled to the pressure leaching state.
Precipitation by carbón monoxide
Carbón monoxide has been used for precipitating silver from AgNO solution and copper from [Cu(NH3)J^"' solutions obtained by leach-ing brass scrap in ammoniacal ammonium carbonate :
The hydrothermal leaching of certain uranium ones with sodium carbonate:
UO2 -^ UOj^" + 2e-U O / ^ + 3CO32- -. [2e-UOjíCOg)/-
[UOjíCOg)/-YzO^ + HjO + 2e- -^
20H-Uranium dioxide is recovered from the uranyl carbonate leach solu-tion by precipitasolu-tion with hydrogen under pressure:
[UO^ÍCOg)^^ -^ U O / ^ + 3CO32-UO/^ + 2e- ^ UO2
H2 -^ 2W + 2e-Overall reaction:
[UO^ÍCOg)^^ + H2 -^ UO2 + 2HCO3- +
CO32-At Kalna in former Yugoslavia, the reaction is conducted at 150°C and 1500 kPa in vertical autoclaves containing pellets of partly sin-tered UO, as catalyst. The precipítate builds up on the catalyst
pel-[Cu(NH J / - -^ Cu2- + 4NH, Cu2^ + CO + H p ^ Cu + CO2 + 2H"
Precipitation takes place at 150°C with CO partial pressure of 5,000 kPa. The solution is then boiled to precipítate zinc as basic carbon-ate. Precipitation of metáis by CO is much slower than by hydrogen This may be due to the fact that CO first reacts with water to form hydrogen:
CO + up ^ H^ + CO2 Precipitation by sulfur dioxide
On passing SO^ into a solution of copper sulfate at room temperatura copper sulfite will precipítate. However, if precipitation is carried out at 150°C and 350 kPa, metallic copper is precipitated according
lo: ^ C u s o , + SO, + 2H O ^ Cu + 2H SO.
or
Cu2^ + SO.2- + H,0 - ^ Cu + 2W + SO
170 Pressure Hydrometallurgy
PL
%apter 6 - Precipitation 171 The drawback to this process is the low yield of copper as shown inFigure 6.13. The corrosión problems due to the acidic environment, and the presence of small amounts of sulfur in the copper produced.
The low yield may be due to the intermedíate formation of cuprous ion which disproportionates precipitating half of the copper and regenerating the other half as cupric ion:
Cu2^ + e- - ^ Cu*
2Cu* ^ Cu + Cu2*
100
2 3 4 Time, hours
.is formed. The following reaction takes place:
Cu2* + SO + 2NH, + KO -^ Cu + 2 N H / + SO
2-3 3 2 4 4
This resulted in complete precipitation of copper and has the advan-lage of operating under basic conditions thus eliminating corrosión problems. However, it has the inconvenience of producing ammo-iiium sulfate which has to be marketed as fértilizer.
In a similar way, cuprous ion in a copper ammine sulfite solution can be reduced to metallic copper when the pH is adjusted to 3 by sul-furous acid to precipítate the double salt CU2S03.(NH^)2S03 which, when slurried with water and heated at 150°C in an autoclave:
Cu" + e- - ^ Cu
SO32- + Hp -^ S O / - + 2H" +
2e-Overall reaction:
SO32- + 2H* -^ SO2 + Hp
Cu,S03.(NHJ,SO 2Cu + SO, + 2 N H / + S O /
-2 4 4
Figure 6.13 - Precipitation of metallic copper from CuS04 solution by SO^
The decomposition of sulfurous acid at the reaction temperature to HjSO^ and elemental sulfur according to the equation:
Sulfur dioxide generated must be vented during heating and col-lected for recycling. The process, however, was developed up to the pilot stage only at Anaconda Research in Tucson, Arizona.
3H2SO3 -^ 2H2SO, + S + H^O
lONIC PRECIPITATION
accounts for the presence of small amounts of sulfur in the copper produced.
This process was improved by adding an ammoniacal solution of ammonium sulfite instead of SOj, i.e., neutralizing the acid as soon
Precipitation by hydrogen sulfide
Hydrogen sulfide is a poisonous and corrosive gas. In certain con-centrations, it explodes in air. In precipitating metal sulñdes by H2S, the following points should be taken into consideration.
172 Pressure Hydrometallurgy Chapter 6 - Precipitation 173
Acid generation. Acid is generated during precipitation:
M2^ + H^S ^ MS + 2H"
and can be used in the leaching circuit, or must be neutralized before disposal.
Polymorphic precipitates. Cobalt and nickel sulfides exist in several polymorphic forms with different solubilities. The alpha forms, obtained by precipitation from basic solutions, are amorphous and soluble in dilute acids. The beta forms, precipitated from weakly acid solutions, are crystalline and only slightly soluble in 0.1 M HCI.
LIMOR FROM KEUTRAUZATiaít
LIQUOR PREHEATER (LOCATEB IK LEACHING ASEA)
NAKE-UP HjS FHtiH Hj-HgS PLANT
» PS!6 STEAN
OFF H j S COOLER
ÁREA PifODUCT
STMAGE
yL.
BARÜEM Ll TO WASTE PfiflOUCT SULPHIDE Bt TRUCK TO PORT ÁREA ST08ASE
Figure 6.14 - Precipitation of nickel and cobalt sulfides by H^S
Catalysis. Precipitation may be greatly accelerated by the addition of a catalyst. Thus, the precipitation of nickel sulfide from weakly acidic solution is extremely slow, but the addition of small amounts of iron or nickel powder accelerates the reaction greatly especially at high temperature and pressure. A process developed at Moa Plant in Cuba for recovering nickel and cobalt was based on this principie.
The ore containing 1.35 % Ni and 0.15 % Co is leached with sulfuric acid, filtered, and the solution treated with H^S at 120°C and 1000 kPa in the presence of iron powder as catalyst to precipítate NiS and CoS. Precipitation is conducted in pressure reactors. The mixed sulfide is then filtered, dried, and shipped for further processing.