Method and cell for electrolytic oxidation of Ni(OH).sub.2 with stationary bed electrode

A method and cell are provided for anodically oxidizing a metal hydroxide slurry from a state of lower valence to a state of higher valence, the cell comprising an anode in the form of a bed of nickel pellets and a plurality of parallel-connected cathodes extending into said bed of pellets, each of the cathodes being covered by a perforated layer of insulating material to inhibit electrical shorting of said cathodes with said bed of nickel pellets.

This invention relates to the electrolytic oxidation of a metal hydroxide 
slurry from a state of lower valence to a state of higher valence and also 
to an electrolytic cell structure characterized by an anode of high 
surface area. 
STATE OF THE ART 
The electrolytic oxidation of nickel from the nickelous (Ni.sup.+2) to the 
nickelic state (e.g., Ni.sup.+3 and/or Ni.sup.+4) is generally employed as 
a first step in Ni/Co separation, particularly with regard to sulfuric 
acid leach solutions obtained in the sulfuric acid leaching of certain 
nickel ores, such as the limonitic and/or serpentinic nickel ores. 
Nickelous hydroxide is oxidized to the nickelic form, the nickelic 
hydroxide being thereafter used to convert the cobaltous ion in solution 
to the cobaltic state for the subsequent separation thereof from the 
nickel solution. 
The aforementioned method is performed in a conventional parallel plate 
electrolytic cell using Ni(OH).sub.2 slurry containing free sodium 
hydroxide in an amount ranging from about 5 to 20 gpl (grams per liter). 
The current efficiency for nickelous conversion is in the neighborhood of 
15 to 25%, the current per cell being about 10,000 amps, the voltage being 
about 3. 
Some of the disadvantages with the aforementioned methods are as follows: 
(1) high power consumption (high current density) coupled with low current 
efficiencies; (2) a tendency toward anode corrosion, even with low levels 
of chloride ion, which requires costly time consuming effort to maintain 
the cells in usable condition; and (3) in addition, the maximum nickel 
concentration is limited to about 30 gpl due to high slurry viscosities 
and associated poor agitation. 
We have surprisingly found that we can overcome the aforementioned 
difficulties and disadvantages by increasing the surface area of the 
anode, this being achieved by employing a fixed anode bed of nickel 
pellets or shot which enables the use of low current densities and the 
accompanying advantage of lower power consumption by working at the upper 
range of current efficiencies. Thus, any corrosion that occurs at the 
anode can be simply dealt with by the addition of nickel shot to the bed 
or simply replacing the bed with a fresh batch of nickel shot. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide an improved method for 
electrolytically oxidizing a metal hydroxide from a state of lower valence 
to a state of higher valence. 
Another object is to provide a method for efficiently converting nickelous 
hydroxide to the nickelic state by using a fixed bed anode comprising 
nickel shot. 
A further object is to provide an improved electrolytic oxidation cell for 
converting metal hydroxide from a state of lower valence to a state of 
higher valence.

STATEMENT OF THE INVENTION 
One embodiment of the invention resides in a method of oxidizing nickelous 
hydroxide to substantially the nickelic state, the method comprising 
forming an aqueous slurry of nickelous hydroxide containing free sodium 
hydroxide which is then fed to an electrolytic oxidation cell comprising 
an anode in the form of a supported fixed bed of nickel pellets and a 
plurality of parallel-connected cathodes extending into the body of said 
bed and in contact with said pellets. The cathodes are each covered with a 
perforated layer of insulating material. The cell is electrically 
activated and the hydroxide slurry circulated through the anode bed for a 
time sufficient to effect oxidation of the nickelous hydroxide to 
substantially the nickelic state. 
The term "perforated" or "perforation" used herein is intended to cover 
broadly and porous insulating material covering the cathode no matter how 
the pores are produced, whether by piercing very small holes in the 
sheathing (e.g., polyethylene) or by using a woven material, such as woven 
nylon, or any other porous structure, so long as the size of the pores is 
such that shorting does not occur by contact of the coated or sheathed 
cathode with the nickel pellets. 
Another embodiment resides in an improved cell for anodically oxidizing a 
metal hydroxide slurry from a state of lower valence to a state of higher 
valence, the cell comprising an anode in the form of a supported bed of 
nickel pellets with a plurality of parallel-connected cathodes extending 
into and in contact with the bed of pellets, the cathodes each being 
covered by a perforated layer of insulating material to inhibit electrical 
shorting of said cathodes with the bed of nickel pellets, means being 
provided for maintaining circulation of the metal hydroxide slurry 
throughout the bed of nickel pellets during electrical activation of the 
cell. 
Another advantage of the invention is that nickelous hydroxide can be 
oxidized to trivalent and tetravalent nickel in electrolytic cells of the 
invention of virtually any dimension that will permit the introduction of 
a bed of nickel shot (e.g., shot ranging in diameter from about 1/4 inch 
to 3/4 inch). Diaphragmed cathodes of stainless steel have been employed 
spaced 1/2 inch apart. As an example, in testing the concept of the 
invention, a cell 5 inches in diameter has been employed using nickel shot 
of about 3/8 inch diameter. The cathodes may take several forms. For 
example, either cathode rods or plates may be employed in carrying out the 
invention. 
A further advantage of the invention is that the use of an anode bed of 
nickel shot enables the treatment of slurries containing upwards of 60 gpl 
Ni.sup.+2 [as Ni(OH).sub.2 ] or higher as compared to 30 gpl Ni.sup.+2 
employed in the conventional parallel plate system. For example, at a 
current flow corresponding to 12.5 amps per liter of slurry containing 10 
gpl free NaOH, a current efficiency of 20% is obtained. Moreover, 
operating under such conditions reduces the requisite cell volume by a 
factor of about eight over the conventional cell using electrodes as 
parallel plates. An anode bed greatly reduces the size of the anode 
portion of the cell. 
DETAILS OF THE INVENTION 
Tests were conducted using two types of cells, one in which the cathodes 
are in the form of stainless steel rods and the other in which the 
cathodes are in the form of plates. As illustrative of such cells, 
reference is made to the schematics of FIGS. 1 to 4. 
Referring to FIG. 1, a cell 10 is shown partially broken away comprising a 
cylindrical container 11 of plexiglass containing a foraminous partition 
12 of stainless steel mesh (or slotted stainless steel plate) supported 
from the bottom 13 by legs 14, 15, the partition supporting a fixed bed of 
nickel pellets 16 of about 3/8 inch diameter (or other suitable size). The 
bed extends upwardly in the cell to a level 17A which is generally 
slightly lower than the level of the slurry. That is, the slurry should be 
at least sufficient to cover the anode bed. 
A plurality of cathode rods 18 extends downwardly into the bed as shown, 
the cathode rods being attached to an electrically conductive header plate 
19 which covers the cell. The cathode rods are encased in perforated 
polyethylene tubing (note FIG. 5) so as to avoid electrical shorting with 
the contacting nickel pellets 16. 
A nickel hydroxide slurry (Ni.sup.+2) 20 shown at the bottom of the cell 
extends to level 17 which is slightly above level 17A of the anode bed. 
The slurry is continuously pumped via line 21 and pump 22 to the top of 
the cell so as to maintain a uniformly mixed slurry throughout the 
interstices of the nickel pellets which are packed in random self-locating 
relationship with each other, such as billiard balls are packed. The 
slurry is circulated from the bottom to the top of the cell as shown. 
The cell is electrically activated as shown schematically, the cathode 
header being electrically coupled via line 23 to a direct current power 
source 24 (for example, a direct current converter) which in turn is 
coupled via a switch 25 to the foraminous stainless steel partition 12 or 
other convenient location. 
A cross section of the cell is depicted in FIG. 2 which shows the axial 
arrangement of cathode rods 18 throughout the cell cross section. 
In FIG. 3, another embodiment is shown of a rectangular cell using cathode 
stainless steel plates instead of rod. However, the cell can be circular 
as well. Referring to FIG. 3, a cell 30 is shown partially broken away 
comprising a rectangular container 31 of plexiglass or other suitable 
material containing a foraminous partition 32 of stainless steel mesh 
supported from the bottom 33 by legs 34, 35, the partition similarly 
supporting a fixed bed of nickel pellets 36 of about 3/8 inch diameter. 
The bed extends upwardly in the cell to a level 37A which is slightly 
below the level 37 of the slurry. 
A plurality of cathode plates 38 extends downwardly into the bed as shown, 
the cathode plates being attached to an electrically conductive header 
plate 39 which covers the cell. As in FIG. 1, the cathode plates are each 
covered with a nylon screen (40.times.40 thread count) to avoid 
anode-cathode shorting (note FIG. 6). 
A nickelous hydroxide slurry 40 shown at the bottom of the cell extends to 
level 37. The slurry as in FIG. 1 is continuously pumped via line 41 and 
pump 42 to the top of the cell in order to maintain the slurry uniformly 
mixed throughout the bed of nickel pellets. The cell is electrically 
activated as shown schematically, the cathode header 39 being electrically 
coupled via line 43 to a direct current power source 44 which in turn is 
coupled via switch 45 to the foraminous stainless steel partition 32. 
A cross section of the cell is shown in FIG. 4 which illustrates the 
parallel arrangement of cathode plates 38. 
FIG. 5 depicts a cathode rod of stainless steel 46 encased with a 
perforated tubing 47 of polyethylene broken away to show the substrate 
metal. In FIG. 6, a cathode plate is shown made of stainless steel 48 
covered with a nylon screen 49 broken away to show the substrate metal. 
Any metal capable of resisting corrosion may be employed as the 
electrodes. Examples of such metals are lead, titanium, nickel-chromium 
alloys, etc. 
EXAMPLE 
Tests were conducted using three cells of circular configuration. Cell No. 
1 contained nineteen 3/8 inch diameter stainless cathode rods each encased 
in a perforated polyethylene tubing. The total cathode area in this cell 
was approximately 315 cm.sup.2, 30% of which was exposed through the 
perforations. The anode shot which weighed 4000 grams and had an average 
diameter of about 3/8 inch exhibited an anode weight exposed cathode area 
ratio of 42 grams of pellets/cm.sup.2. Converting the 42 grams of pellets 
to surface area, the ratio becomes 30 cm.sup.2 of anode surface per 
cm.sup.2 of exposed cathode area. 
As will clearly appear, the ratio of anode area to cathode area is very 
large. The slurry capacity of Cell No. 1 was 2 liters. 
Cells No. 2 and No. 3 contained five stainless steel plate cathodes, each 
covered with a nylon screen (40.times.40 thread count) as shown in FIG. 6 
to prevent anode-cathode shorting. Approximately 75% of the available 
cathode area was exposed through the membrane. 
Cell No. 2 contained 4000 grams of shot (about 3/8 inch diameter) covering 
8.3 cm of the cathode plates and had a capacity of 2.5 liters of slurry. 
The anode weight to exposed cathode area ratio was 10 grs/cm.sup.2 which 
corresponded to an anode/cathode area ratio of about 7.1 cm.sup.2 of anode 
area per cm.sup.2 of cathode area. 
Cell No. 3 contained 9000 grams of shot covering 17.8 cm of cathode plates, 
the slurry capacity being 2 liters. The anode weight to exposed cathode 
area ratio was 10 grs/cm.sup.2 which corresponded to an anode/cathode area 
ratio of about 7.1 cm.sup.2 of anode area per cm.sup.2 of cathode area. 
In carrying out the tests, stock solutions of NaOH and reagent grade 
NiSO.sub.4 added to de-ionized water were mixed and diluted to yield the 
appropriate Ni.sup.+2 and free NaOH concentrations. The resultant slurry 
was recirculated in each cell for 15 minutes prior to start-up. The tests 
were continued until 100% oxidation was achieved (conversion of divalent 
nickel to the trivalent state), or for 30 hours, whichever occurred first. 
The samples were analyzed for percent oxidation by means of Na.sub.2 
S.sub.2 O.sub.3 /EDTA titration and free NaOH by titration to pH 9.9 with 
0.5N H.sub.2 SO.sub.4. 
The three cells were tested using slurry feeds of either 30 or 60 gpl Ni, 
10 gpl free NaOH with nickel shot of average diameter ranging from about 
0.75 to 1.25 centimeters (about 3/8 inch average diameter). The following 
results were obtained: 
Table 1 
__________________________________________________________________________ 
Test 
Cell Amps/ 
AW/CA 
Ni Vol. 
% OX/Time 
Curr. 
Ox Rate 
No. 
No. 
Amps 
KG Shot 
g/cm.sup.2 
gpl 
L %/Hrs. Eff.,% 
g Ni/Hr 
__________________________________________________________________________ 
1 1 5 1.25 40 30 2.0 
62/29 12 1.3 
2 2 " 1.25 10 " 2.5 
73/29 17 1.9 
3 3 " 0.56 10 " 2.0 
100/25 22 2.4 
4 1 15 3.75 40 30 2.0 
92/20 8.4 2.8 
5 2 " 3.75 10 " 2.5 
100/20 11 3.8 
6 3 " 1.67 10 " 2.0 
100/10 18 6.7 
7 1 25 6.25 40 30 2.0 
87/20 4.7 2.6 
8 2 " 6.25 10 " 2.5 
100/14 9.7 5.4 
9 3 " 2.78 10 " 2.0 
100/9 12 6.7 
10 1 25 6.25 40 60 2.0 
60/23 5.7 3.1 
11 2 " 6.25 10 " 2.5 
100/20 11 6.0 
12 3 " 2.78 10 " 2.0 
100/11 20 11 
13 1 10 2.50 40 60 2.0 
76/24 
17 3.8 
14 2 " 2.50 10 " 2.5 
100/34 20 4.4 
15 3 " 1.11 10 " 2.0 
100/21 26 5.7 
__________________________________________________________________________ 
As will be observed, while the oxidation rate generally increases for Test 
Nos. 1-6 (Cell Nos. 1, 2 and 3) with increasing current, there is an 
accompanying decrease in current efficiency. 
Comparison of results obtained at a given absolute current shows the 
effects of both anode current density (defined in this case as amps/Kg 
nickel shot) and the anode weight to cathode area ratio (AW/CA). For all 
absolute currents, the current efficiency in Cell 2 exceeds that in Cell 1 
in spite of identical anode current densities (compare tests 1 and 2, 4 
and 5, 7 and 8, 10 and 11). This effect is due to the lower AW/CA ratio in 
Cell 2, which results in a greater active anode surface area. In a 
stationary bed cell of this type, the potential at a given anode surface 
point is a strong function of the distance from that point to the nearest 
cathode surface. Since the rate of kinetically controlled electrochemical 
reactions of the type under investigation here are strongly potential 
dependent, it is not surprising that higher efficiencies are obtained at 
low AW/CA ratios, (where bed polarization is minimized.) 
Comparison of results from Cells 2 and 3 (which have equal AW/CA ratios) at 
identical absolute currents, shows decreasing current efficiency with 
increasing current density. (Compare Tests 2 and 3, 5 and 6, 8 and 9, 11 
and 12.) A plot of current efficiency vs current density for Cells 2 and 3 
is shown in FIG. 7. The continuity of this plot verifies that with the 
configurations tested, and at equal AW/CA ratios, substantially the same 
current efficiency-current density relationship is obtained, regardless of 
the cell size. 
The effect of increasing the nickel concentration from 30 to 60 gpl at 
various current densities is shown in Tests 7 and 10, 8 and 11, and 9 and 
12. The increased slurry viscosity obtained at high nickel concentration 
precludes use of 60 gpl Ni in conventional parallel plate cells due to 
poor agitation. However, introduction of recirculating slurry at the top 
of the anode bed (and convection through the tortuous path created by the 
close packed anode shot) provides sufficient slurry mixing to operate at 
60 gpl Ni in the packed bed configuration. At an AW/CA ratio of 10 
g/cm.sup.2, and a current density of 6.25 amps/Kg shot, increasing the 
nickel concentration from 30 to 60 gpl increases the current efficiency 
from 9.7 up to 11.0 percent (compare Tests 8 and 11). 
At the same AW/CA ratio and at a current density of 2.78 amps/Kg shot, the 
current efficiency (and NiOOH production rate) increases from 12 up to 20 
percent when the nickel concentration is increased from 30 to 60 gpl 
(compare Test 9 with 12). This result indicates that as the current 
density increases, the effect of nickel concentration on current 
efficiency decreases. 
The above conclusions are substantiated by the data obtained at 10 amps and 
60 gpl Ni (Tests 13, 14 and 15). For example, comparison of Test 3 with 
Test 15 shows that at low current densities, doubling the current density 
(0.56 vs 1.1 amp/Kg shot) and the nickel concentration more than doubles 
the nickelous oxidation rate (2.4 vs 5.7 g Ni/Hr), and increases the 
relative current efficiency (22 vs 26%). 
In most experiments, the degree of oxidation in Cell 1 did not reach 100 
percent, even after 30 hours. As shown in FIG. 8, percent oxidation vs 
time plots are characterized by an initially near linear portion and a 
tailing portion that asymptotically approaches an upper limit, which is 
apparently determined by the AW/CA ratio. Note that although a lower rate 
of oxidation is observed initially in Cell 2 than Cell 3, the same degree 
of oxidation is finally attained in each, in spite of the higher current 
density in Cell 2. This merging of degree of oxidation is caused by an 
earlier decrease in reaction rate in Cell 3, since the concentration of 
divalent nickel approaches zero as the degree of oxidation approaches 
100%. 
Thus, in order to increase the degree of oxidation, the relatively 
inefficient oxidation of trivalent nickel to tetravalent nickel must 
occur. Under the same conditions in Cell 1, the rate of oxidation drops to 
nearly zero at 80 percent oxidized, indicating that the bed polarization 
is so severe at an AW/CA ratio of 42 that a negligible portion of the bed 
is at the potential required for efficient oxidation of Ni(OH).sub.2 at 
low Ni.sup.+2 concentrations. 
The effect of the free NaOH concentration was investigated in Cell No. 3 
for a slurry containing 60 gpl Ni.sup.+2 at 10 amps, 10 AW/CA and 2 liter 
cell volume and substantially the same results were obtained as compared 
to a conventional parallel plate cell treating a nickel concentration of 
30 gpl. The results obtained are shown in Table 2 below. 
Table 2 
______________________________________ 
Test % OX/Time Current OX Rate, 
No. NaOH %/Hrs. Eff., % g Ni/Hr. 
______________________________________ 
15 10 100/21 26 5.7 
16 5 100/19.5 28 6.2 
17 pH 8 22/32 3.8 0.83 
______________________________________ 
A slight improvement in current efficiency was obtained by decreasing the 
free NaOH concentration from 10 gpl to 5 gpl (Tests 15 and 16, 
respectively). Operation at pH 8 resulted in extremely low current 
efficiency (3.8%) due to the relative enhancement of O.sub.2 evolution 
over Ni.sup.+2 oxidation at that pH. 
To determine the effect of increasing the anode surface area over that 
obtained using shot in the size range 0.75 to 1.24 cm (approximately 5/16 
to 1/2 inch diameter), two tests were performed using shot in the 0.25 to 
0.75 cm range (approximately 3/32 to 5/16 inch diameter). In one test (10 
amps, 30 gpl Ni.sup.+2, 10 gpl free NaOH), the current efficiency for 100 
percent oxidation was 14 percent for the smaller shot size, compared to 20 
percent under identical conditions using larger shot. In similar tests at 
60 gpl Ni, the current efficiency using the smaller shot was 9 percent, 
while the larger shot test yielded 26 percent. These results (and visual 
inspection during the tests) indicate that slurry mixing with the smaller 
shot bed is minimal, leading to stagnant layers around the shot, 
particularly with the more viscous 60 gpl Ni.sup.+2 slurry. 
Thus, for consistent results, the nickel shot should exceed 1/8 inch 
diameter and range from about 1/4 to 1 inch diameter, e.g., from about 3/8 
to 3/4 inch diameter. The range of diameter sizes can be expressed in 
terms of surface area per gram of nickel shot. That is to say, the size of 
the shot whether uniformly spherical or not may be such as to provide a 
surface area per gram of shot ranging from about 0.25 to 2 cm.sup.2 /gram 
(about 1 inch to between 3/8 and 1/4 inch diameter) and preferably from 
about 0.35 to 1.5 cm.sup.2 /gram (this corresponds approximately to an 
average diameter of 3/4 inch to slightly less than 1/4 inch). 
The perforations in the cathode coating should be that amount to provide 
sufficient exposed cathode substrate to assure the desired electrochemical 
properties of the circuit. The amount of area exposed on the insulated 
cathode may range from about 15 to 80% of the total area of the cathode 
and generally from about 20 to 70% of the total area. A preferred range is 
about 30% to 65% of the total area. 
The amount of free NaOH in the nickel slurry may range from about 3 to 20 
gpl and preferably from about 5 to 15 gpl. 
The ratio of anode weight to exposed cathode area should not exceed about 
30 grams/cm.sup.2 and preferably range from about 2 to 25 grams/cm.sup.2, 
for example, from about 5 to 20 grams/cm.sup.2. 
The results show that a relatively high current efficiency of 22% could be 
obtained for 100% Ni(OH).sub.2 oxidation using a fixed bed anode of nickel 
shot at a current input of 2.5 amps per liter or 5 amps per 2 liters (Test 
No. 3). This result compares very favorably with the 15% current 
efficiency obtained with the conventional parallel plate cell at 2 
amps/liter. At 7.5 amps/liter or 15 amps per 2 liters, a very good current 
efficiency of 18% is obtained (Test No. 6). Operation at the foregoing 
conditions can greatly reduce the cell volume in view of the improved 
efficiency obtained with an anode bed of nickel pellets. 
As stated earlier, an important advantage of the invention is that the 
packed bed can treat a slurry containing as much as 60 gpl of Ni.sup.+2 as 
compared to the lower amount of 30 gpl Ni.sup.+2 treated in a conventional 
cell. At a current of 12.5 amps/liter (or 25 amps per 2 liters) with a 
slurry corresponding to 60 gpl Ni.sup.+2 and 10 gpl of free NaOH, the 
current efficiency was 20% (Test No. 12). 
Thus, the electrolytic oxidation cell provided by the invention may handle 
Ni(OH).sub.2 slurries containing about 15 grams Ni.sup.+2 /liter to 100 
grams/liter and generally from about 30 grams Ni.sup.+2 /liter to 80 
grams/liter, for example, 40 to 70 gpl Ni.sup.+2. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations thereto may be resorted to without departing from the spirit 
and scope of the invention as those skilled in the art will readily 
understand. Such modifications and variations are considered to be within 
the purview and scope of the invention and the appended claims.