Conditioning of laterite pressure leach liquor

A method is provided for conditioning nickel laterite pregnant leach liquor in preparation for the recovery of nickel therefrom in which pregnant leach liquor is mixed in a reaction vessel under atmospheric pressure with an amount of roasted high-magnesium nickeliferous silicate ore to form a pulp thereof, the amount of roasted ore being at least sufficient to lower the free acid content of pregnant liquor entering the thickener to a level corresponding to a pH ranging from about 1.8 to 3. The pregnant liquor is conditioned by stirring the pulp thereof at a temperature not exceeding 100.degree. C. for a time sufficient to lower the free acid content of the resulting conditioned pregnant liquor to the desired level and to effect substantial precipitation of iron as iron hydroxide and dissolved silica in the liquor. The treated pulp is then passed to a thickener to provide an underflow of residual roasted ore and an overflow of conditioned pregnant liquor is passed to metal recovery while withdrawing the residual roasted ore from the thickener, a portion of which is then recycled to the reaction vessel to which freshly roasted ore is also added.

This invention relates to the use of calcined or roasted high magnesium 
nickeliferous silicate ore, such as garnierite for neutralizing and 
purifying (i.e., conditioning) pregnant leach liquor produced in the high 
pressure, high temperature leaching of nickeliferous oxide ores, e.g., 
nickel laterites. 
STATE OF THE ART 
It is known that it is possible to recover nickel and cobalt from lateritic 
ores using hydrometallurgical leaching techniques carried out at high 
pressures and high temperatures. A known method is a leaching process 
developed at Moa Bay, Cuba, in which finely divided nickel laterite ore 
(e.g., 95% passing 325 mesh screen, U.S. Standard) is pulped to 
approximately 40% solids, and the nickel and cobalt selectively leached 
with sulfuric acid at elevated temperature and pressure (e.g., 475.degree. 
F. [245.degree. C.] and 525 psig) to solubilize about 95% each of the 
nickel and cobalt present. The leached pulp is cooled and then washed by 
countercurrent decantation, with the washed pulp going to tailings. The 
acid pH, which is quite low, is then neutralized with coral mud to a pH of 
about 2.5 to 2.8 and the resulting product liquor (containing generally 
about 4 to 6 grams of nickel per liter) is subjected to sulfide 
precipitation by preheating the leach liquor and carrying out the 
precipitation with H.sub.2 S in an autoclave at about 250.degree. F. 
(121.degree. C.) and a pressure of about 150 psig. Usually, nickel sulfide 
seed is added at the feed end to assure substantially complete 
precipitation of the nickel and cobalt. 
After the sulfide precipitate has been washed and thickened to about 65% 
solids, it is oxidized to nickel and cobalt sulfate in an autoclave at 
about 350.degree. F. (177.degree. C.) and a pressure of about 700 psig. 
The solution of dissolved nickel and cobalt is neutralized with ammonia to 
a pH (5.35) sufficient to precipate any iron, aluminum and chromium 
present using air as an oxidant, the precipitate being thereafter 
separated from the solution. The solution of nickel and cobalt is 
thereafter adjusted to a pH of about 1.5 and H.sub.2 S added to 
selectively precipitate any copper, lead and zinc present, which 
precipitate is separated from the solution by filtration. Following 
removal of metal impurities, the nickel is then selectively recovered from 
the solution by various methods, one particular method comprising treating 
the solution in an autoclave with hydrogen at a pressure of about 650 psig 
at a temperature of about 375.degree. F. (245.degree. C.) to recover the 
nickel in metallic form, using nickel powder as seed material. 
Pregnant liquor generated in the aforementioned Moa-Bay type leaching of 
nickel laterite may contain about 30 gpl (grams per liter) of free 
sulfuric acid, 2 gpl of aluminum and 1 gpl iron. A typical Moa-Bay type 
leach is one in which the ore is leached at 240.degree.-260.degree. C. at 
an acid (H.sub.2 SO.sub.4) to ore ratio between about 0.22 and 0.26 and a 
pulp density of 33%. Many of the refining processes available for the 
recovery of nickel from the foregoing solution operate more effectively at 
lower concentrations of acids, iron and aluminum. A typical Moa Bay ore is 
one containing 1.35% nickel, 0.14% Co, 0.9% Mn, 0.02% Cu, and 0.04% Zn, 
47% Fe, 10% Al.sub.2 O.sub.3, 1% MgO and 39.5% of other constituents and 
water of hydration. 
The amount of acid employed to leach the nickel laterite ore is generally 
about twice the stoichiometric amount necessary to neutralize the metal 
values and other acid-consuming constituents in the ore, such as magnesium 
and aluminum. Generally, the pH of the cooled pregnant liquor following 
high pressure, high temperature leaching is quite low (typically 0.5 to 
0.7). At such pH's significant amounts of impurities, such as iron and 
silica are taken into solution. For example, in the sulfuric acid leaching 
of laterites at elevated temperature and pressure, e.g., 270.degree. C. 
and about 800 psig, silica as quartz is generally soluble to a saturation 
level of about 500 to 600 ppm. However, even more silica tends to dissolve 
from amorphous silica liberated from silicate minerals by the acid. Thus, 
at 270.degree. C. this silica may be soluble to a relatively high level, 
such as 1250-1500 ppm. 
In the high-pressure leaching of lateritic ores, the leached pulp is 
subsequently subjected to flashing to bring it to atmospheric conditions 
for the purpose of separating the undissolved solids from the pregnant 
liquor by countercurrent decantation (CCD). 
Under atmospheric conditions, the acid content of the pregnant liquor may 
be such as to effectively retain significant amounts of iron and silica in 
solution. It is known to adjust the pH of a pregnant liquor prior to 
flashing by mixing a magnesium-containing lateritic ore with a leached 
pulp and its pregnant liquor and to subject the mixture to 
high-temperature neutralization (acid kill) at an elevated temperature in 
the range of about 225.degree. C. to 300.degree. C. whereby the pregnant 
liquor of the leached pulp is neutralized and the magnesium-containing ore 
simultaneously leached to form a final pregnant liquor. One embodiment of 
this technique is described in U.S. Pat. No. 3,991,159. In this 
connection, attention is also directed to U.S. Pat. Nos. 3,804,613 and 
4,097,575, the disclosures of the aforementioned three patents being 
incorporated herein by reference. 
In U.S. Pat. No. 4,097,575, mention is made of the use of roasted 
high-magnesium-containing serpentine ores as opposed to the use of raw or 
unroasted ore as disclosed in U.S. Pat. No. 3,991,159, the finding being 
made that roasted ore is more efficient as a neutralizing agent. 
According to U.S. Pat. No. 4,097,575, the high temperature neutralization 
is carried out for a time sufficient to effect neutralization of the mix 
at a pH not exceeding about 1.5, for example, 1.2 or less, atmospheric 
neutralization being also referred to for raising the pH to over 2, for 
example, in the neighborhood of about 3.5. 
It would be desirable to provide a method for efficiently neutralizing 
pregnant leach liquor under atmospheric conditions using roasted 
high-magnesium ore (e.g. garnierite) in which magnesium and nickel 
extractions are maximized and in which the acid of the liquor is 
sufficiently neutralized to provide substantial precipitation of iron and 
silica and to condition the pregnant liquor for the next step in the 
process such as the sulfide precipitation step. 
OBJECT OF THE INVENTION 
It is thus an object of the invention to provide a method for conditioning 
nickel-containing pregnant liquor by employing an atmospheric 
neutralization and purification process using roasted high-magnesium ore 
as the neutralizing agent. 
Another object is to provide a method for the atmospheric treatment of 
pregnant leach liquor, wherein the pregnant liquor is neutralized and 
purified using roasted high-magnesium nickeliferous silicate ore and 
wherein a portion of residual ore is recycled into the neutralization 
process.

STATEMENT OF THE INVENTION 
In its broad aspects the invention provides an atmospheric leach method for 
conditioning pregnant leach liquor following flashing wherein roasted 
high-magnesium nickeliferous silicate ore, e.g., garnierite, is added to 
the pregnant liquor in a mixing or reaction vessel maintained at a 
temperature of about 60.degree. C. to 100.degree. C. The amount of ore is 
sufficient to reduce the acid content to the desirable level. The mixture 
is vigorously stirred for a time sufficient to effect neutralization and 
substantial precipitation of iron as iron hydroxide and also silica, 
following which the leached mix is passed to a thickener where a solids 
underflow is produced (residual ore), a portion of which is recycled to 
the reaction vessel as roasted ore is fed to said vessel. The conditioned 
pregnant liquor overflow after clarification and with the proper pH is 
passed to the next stage of treatment, for example, sulfide precipitation. 
Preferably, the amount of recycle residual roasted ore is approximately 
twice the weight of the roasted ore fed to the reaction vessel, the ratio 
of recycle residual ore to the freshly roasted ore ranging by weight from 
about 1/2 to 1 to 5 to 1 and generally from about 1 to 1 to 3 to 1. 
One embodiment of the invention is directed to a method for conditioning 
nickel laterite pregnant leach liquor in preparation for the recovery of 
nickel therefrom, the method comprising: feeding nickel-containing 
pregnant leach liquor having a free acid concentration of about 10 to 80 
gpl H.sub.2 SO.sub.4 and a temperature ranging from about 60.degree. C. to 
100.degree. C. to a reaction vessel, and mixing an amount of roasted high 
magnesium nickeliferous silicate ore containing over 5% Mg with the 
pregnant liquor to form a pulp thereof, the amount of roasted ore being at 
least sufficient to lower the free acid content of the pregnant liquor 
entering the thickener to a level corresponding to a pH ranging from about 
1.8 to 3. The pregnant liquor is conditioned by vigorously stirring the 
pulp at said temperature for a time sufficient to lower the free acid 
content of the resulting conditioned pregnant liquor and to effect 
substantial precipitation of iron as iron hydroxide and dissolved silica 
in said liquor, and then passing the treated pulp to a thickener to 
provide an underflow of residual ore and an overflow of conditioned 
pregnant liquor. The method is continued by passing the overflow of 
conditioned pregnant liquor to metal recovery, then withdrawing the 
residual ore from the thickener, and recycling a portion of it to the 
mixing vessel into which roasted ore is also added. 
A simple flow sheet for carrying out the invention is illustrated in FIG. 1 
which shows pregnant liquor 10 following flashing from an autoclave being 
fed to reaction vessel 11 having a stirrer 12 therein. Roasted 
high-magnesium ore is added to the pregnant liquor in reaction vessel 11 
and the mix vigorously stirred for a time sufficient to reduce the free 
acid content of the pregnant liquor to the desirable pH (e.g., pH of about 
2.2 to 2.5) and to effect substantial precipitation of iron as iron 
hydroxide and of dissolved silica. 
Following completion of neutralization, the mix or pulp is passed to 
thickener 14 to allow the solids to settle out and form an underflow 
thereof with an overflow of clarified conditioned product liquor 15 going 
to the next treatment step, such as sulfide precipitation. Optionally, a 
flocculating agent 15A is added to flocculate the precipitate to assure 
settling thereof. 
The solids underflow comprising residual roasted ore 16 is removed from the 
thickener with a portion 17 thereof being recycled to reaction vessel 11 
where together with the addition of freshly roasted ore 13 it reacts with 
incoming liquor 10. The rate of flow of pregnant liquor feed and the 
solids is controlled such as to allow sufficient residence time during 
mixing to complete the reaction. With one reaction vessel, the extent of 
the reaction would be necessarily low. Thus, it is preferred to use a 
series of reaction vessels as disclosed in the flow sheets of FIGS. 2 and 
3. 
The embodiment of the invention using a plurality of reaction vessels 
comprises: feeding nickel-containing pregnant leach liquor having a free 
acid concentration of about 10 to 80 gpl H.sub.2 SO.sub.4 and a 
temperature ranging from about 60.degree. C. to 100.degree. C. to a series 
of reaction vessels comprising a first reaction vessel serially connected 
to at least a second reaction vessel including at least one thickener at 
the end of the series of reaction vessels, and adding an amount of roasted 
high magnesium laterite containing over 5% Mg to the pregnant liquor of at 
least one of the subsequent reaction vessels of said series of vessels, 
the amount of roasted ore added to the series of vessels being at least 
sufficient to lower the free acid content of the pregnant liquor entering 
the thickener as a pulp to a level corresponding to a pH range of about 
1.8 to 3. The pregnant liquor in said series of vessels is conditioned by 
vigorously stirring it while at the above-specified temperature for a time 
at least sufficient to lower the free acid content of the pregnant liquor 
entering the thickener and to effect substantial precipitation of iron as 
iron hydroxide and dissolved silica in the liquor, following which the 
treated pulp is passed to the thickener to provide an underflow of 
residual roasted ore and an overflow of conditioned pregnant liquor. The 
overflow of conditioned pregnant liquor is then passed to metal recovery 
and the residual ore withdrawn from the thickener and a portion of it 
recycled, to the first reaction vessel while adding freshly roasted 
high-magnesium ore to at least one of the subsequent reaction vessels. 
The flow sheet of FIG. 2 is illustrative of the foregoing embodiment, the 
flow sheet comprising a train of serially connected reaction vessels V-1, 
V-2 and V-3 containing stirrers 18A, 18B and 18C, respectively. Pregnant 
liquor 18 is fed to reaction vessel V-1 and from there to V-2, V-3 and 
into thickener 19. 
As the pregnant liquor is flowing through the reaction vessels, roasted 
high-magnesium silicate ore 20 is fed to each of vessels V-2 and V-3 and 
the pulp formed vigorously mixed for a residence time sufficient to lower 
the free acid content so that the conditioned pregnant liquor reaching the 
thickener is at the desired pH and substantial precipitation of iron as 
iron hydroxide is obtained. 
To assure settling of the precipitate, a flocculant 22 is optionally 
employed and a clarified product liquor 23 is produced suitable for 
H.sub.2 S precipitation of the metal values present (i.e., nickel and 
cobalt). 
Residence time in the reaction vessels is very important to assure complete 
reaction of the roasted ore with the pregnant liquor. To optimize the 
reaction and to assure a steady state system, the proposed embodiment of 
FIG. 3 may be employed in which four reaction vessels are employed in 
which the roasted ore is added only to the second vessel (V-2A). 
Thus, referring to FIG. 3, pregnant liquor 24 is added to reaction vessel 
V-1A as acid-killed roasted ore is being recycled to it. The vessels V-1A, 
V-2A, V-3A and V-4A each contain stirrers 25A, 25B, 25C and 25D, 
respectively. As the pregnant liquor is circulating throughout the 
vessels, freshly roasted high-magnesium ore is added to the second vessel 
V-2A. The use of reaction vessels V-2A, V-3A and V-4A enables the 
selection of a total residence time to assure complete reaction as well as 
a desirable production rate. With this system a steady state is easily 
obtainable so that the pulp mix entering thickener 26 is substantially 
fully reacted with respect to the pH content (e.g. 2.2 to 2.5) and with 
respect to substantial precipitation of the iron and silica as discussed 
hereinabove such as to provide a clarified product liquor 27 for 
subsequent treatment, e.g. H.sub.2 S precipitation of the metal values 
nickel and cobalt. If necessary, a flocculant 28 may be employed. Where 
the situation is such that the product liquor is not sufficiently 
clarified, two thickeners may be employed series-connected to each other. 
The underflow solids of residual roasted ore 29 is removed with a portion 
30 of it recycled to the first mixing vessel V-1A. As stated earlier, the 
weight ratio of residual roasted ore relative to the amount of freshly 
roasted high-magnesium ore fed to the system may range from about 1/2 to 1 
acid-killed ore to 5 to 1 of roasted ore, or 1 to 3, preferably 
approximately 2 to 1, the ratio depending upon the amount of roasted ore 
employed and its magnesium content. 
DETAILS OF THE INVENTION 
Since it is not uncommon to employ blended ore charges in running a 
commercial plant, various types of ore feeds were prepared and tested, a 
typical combined plant feed being one containing 1.9% nickel and 4.7% 
magnesium. 
The blended ore feed comprises a garnierite fraction (G) high in magnesium 
and a laterite fraction (L) high in iron. The ore feed making up the 
combined plant feed was based on the ratio of G to L+G as disclosed in 
Table 1 below: 
TABLE 1 
______________________________________ 
Ore Ratio Element Content, Weight Percent 
No. G/L + G Ni Co Mg Si Al Fe Cr Mn 
______________________________________ 
1 0.46 1.89 0.057 4.7 6.9 4.3 32.2 1.91 
0.45 
2 0.77 2.45 0.082 9.8 13.9 2.3 19.8 0.89 
0.85 
3 0.56 1.93 0.070 6.8 10.1 3.5 27.4 1.30 
0.52 
4 0.56 2.00 0.090 5.4 8.0 4.8 32.0 1.50 
1.20 
______________________________________ 
Ore No. 1 in which the ratio of G to L+G is 0.46 represents a typical 
combined plant feed. The high limit of the magnesium content in the plant 
ore feed is represented by blended ore feed identified as Ore No. 2. The 
blended ores used for Ore No. 3 and No. 4 are average ores between the 
limits of Ores No. 1 and No. 2. The blends were made in different ratios 
to simulate various compositions in the ore body. 
The total blended ore of each of Nos. 1, 2, 3 and 4 is sieved so that size 
fractions larger than 50 mesh are separated from minus 50 mesh material. 
The minus 50 mesh material, containing most of the L ore fraction, is 
employed in high pressure leach. The plus 50 mesh material which is mostly 
the G fraction is ground to all minus 6 mesh and used for garnierite 
roasting. 
The roasting of such silicate ore is fully described in U.S. Pat. No. 
4,097,575. In order to achieve optimum neutralization the ore should be 
roasted at a temperature below the temperature at which crystalline 
forsterite forms, that is, below 820.degree. C. The ore is preferably 
roasted between about 500.degree. C. to 750.degree. C. to maximize 
magnesium solubility during neutralization of the pregnant liquor. 
The blended ore feed shown in Table 1 was sized to provide a high magnesium 
fraction for each run which was roasted at a temperature of 650.degree. C. 
for 2 hours. The analysis for the roasted high magnesium ores is given in 
Table 2 below: 
TABLE 2 
______________________________________ 
Element Content, Weight Percent 
Ore No. 
Ni Co Mg SiO.sub.2 
Al Fe Cr Mn 
______________________________________ 
1A 2.38 0.13 11.2 35.3 3.0 16.3 1.14 0.84 
2A 1.85 0.07 14.7 37.1 1.9 10.3 0.63 0.73 
3A 1.83 0.10 14.0 34.0 3.0 11.0 1.68 1.13 
4A 2.08 0.18 12.3 32.3 3.1 14.5 1.79 1.91 
______________________________________ 
The pregnant liquor produced from each of the ores of Table 1 is 
neutralized using the corresponding roasted high magnesium ore feed of 
Table 2. The tests correspond to a pregnant liquor feed rate of about 600 
cubic meters/hour at a nickel content of about 5.5 gpl and a free acid 
content of about 35 to 40 gpl H.sub.2 SO.sub.4 by reacting the liquor with 
the roasted ore at a rate corresponding to 43.8 tons/hour and a 
temperature of about 90.degree. C. This would give a neutralized liquor 
containing about 6.1 gpl nickel at a pH level of about 2.0 to 2.5, or a 
free acid concentration of about 0.7 to 1.3 gpl H.sub.2 SO.sub.4. In the 
atmospheric leach process about 52% of the nickel in the roasted ore is 
dissolved. 
Tests have shown that the particle size of the roasted ore is important 
with regard to the speed with which the pregnant liquor is neutralized. 
For example, minus 48 mesh roasted ore effected neutralization of the 
liquor to a pH of 2 in less than 200 minutes as compared to greater than 
400 minutes with a particle size of -8+10 mesh (note FIG. 4). The finer 
size neutralized much more acid than the coarse size over the 
aforementioned time period. This enables the use of four mixing vessels in 
which the residence time of the last three total 180 minutes, or 1 hour 
per vessel. For the four vessels the total liquid residence time would be 
4 hours, although the residence time may range up to about 2 hours per 
vessel. Liquid retention time can be reduced by increasing the solids 
content of the reactors by adjusting the solids recycle. 
In one test four agitated atmospheric leach tanks were employed with a 
working volume of 4200 liters, except for run 4A in which two 2-hour tanks 
were used as described. The residence time for each tank averaged 2 hours. 
The roasted ore corresponded to Ore Nos. 1A, 2A, 3A and 4A. The slurries 
of the pregnant liquor and the aforementioned ore ranged in temperature 
from about 72.degree. C. to 87.degree. C. The slurry flows by gravity 
through bottom discharge lines from tank to tank. The leach tank levels 
were maintained by a vertical riser after the last tank. The leached 
slurry from the last leach tank was discharged to a thickener. The first 
and third leach tanks were equipped with steam heating coils to bring the 
slurry to operating temperature as required. 
The pulp of solids from the underflow of the thickener after the fourth 
leach tank was split, one part being sent to high pressure leach, the 
other part being recycled to the first leach tank and fed along with 
pregnant liquor. Leach circuit operation for the four runs is summarized 
in Table 3. Mass flow rates in Table 3 were taken for the most part from 
daily data sheets, or based on a careful analysis of the data to select 
the best values consistent with both mass and component balances. 
Specifically, pregnant liquor, recycle residue and leach residue were 
estimated on a best fit basis. Other flows were, for the most part, 
calculated from the daily data sheets. 
The four runs operated with the following recycle ratio [residual roasted 
ore to new roasted ore]: 
______________________________________ 
Standard 
Average Ratio 
Deviation 
______________________________________ 
1A 1.9 0.2 
2A 1.6 0.2 
3A 2.2 0.5 
4A 2.2 0.05 
______________________________________ 
Without solids recycle, roasted garnierite would have to be increased 25 
percent to neutralize the pregnant liquor. 
The neutralized liquor produced in all four runs (1A, 2A, 3A and 4A) was 
essentially the same with a pH of 2.3 to 2.4 and a ferric iron level of 
less than 0.4 gpl. Magnesium extraction from the roasted ore for the four 
runs was 70 to 80 percent. 
The results are summarized in the following tables: 
TABLE 
__________________________________________________________________________ 
Summary of Atmospheric Leach 
Operating Conditions 
Test 
Test 
Test 
Test 
Prototype 
Run Run Run Run 
Quantity Units 
Design 
1A 2A 3A 4A 
__________________________________________________________________________ 
Number of Leach Tanks 
-- 4 4 4 4 2 
Residence Time 
hours 
1 2 2 
2 2 
Per Tank 
Leach Temperature 
.degree.C. 
87 72 73 
74 76 
Pregnant Liquor 
Flow Rate kg/hr 
4424 1720 
1600 
1410* 
1384 
Flow Rate l/hr 
3912 1584 
1427 
1271 
1234 
Recycle Slurry 
Solids Flow kg/hr 
-- 268* 
125 
273* 
398 
Solution Flow 
kg/hr 
-- 366 233 
426 333 
Slurry Flow kg/hr 
-- 634 358 
699 631 
Slurry Flow l/hr 
-- 402 253 
468 401 
Garnierite Feed 
Solids Flow kg/hr 
126 136 99 
111 124 
Solution Flow 
kg/hr 
126 132 100 
112 125 
Slurry Flow kg/hr 
252 268 199 
223 249 
Slurry Flow l/hr 
174 179 132 
147 164 
Neutralized Liquor 
Flow Rate kg/hr 
4479 1724 
1598 
1435 
1438 
Flow Rate l/hr 
3939 1570 
1427 
1289 
1278 
__________________________________________________________________________ 
*Adjusted to suit best balance. Primary measurement or analysis not 
compatible within 10% of observed results. 
TABLE 4 
__________________________________________________________________________ 
Summary of Atmospheric Leach 
Operating Conditions 
Test Test 
Test Test 
Prototype 
Run Run Run Run 
Quantity 
Units 
Design 
1A 2A 3A 4A 
__________________________________________________________________________ 
Leach Residue 
Solids Flow 
kg/hr 
111 110 78 88 100 
Solution Flow 
kg/hr 
111 136 130 123 99 
Slurry Flow 
kg/hr 
222 246 208 211 199 
Slurry Flow 
l/hr 125 176 158 250 135 
4/11-19 
5/6-20 
6/16-22 
Roasted Garnierite [A-A]* weight percent 
Ni 2.38 1.85 
1.83 2.08 
Co 0.13 0.07 
0.10 0.18 
Fe 16.30 
10.30 
11.00 
14.50 
Al 3.00 1.85 
3.00 3.10 
Mg 11.20 
14.70 
14.00 
12.30 
Mn 0.84 0.73 
1.13 1.91 
Cr 1.14 0.63 
1.68 1.79 
SiO.sub.2 35.30 
37.10 
34.00 
32.30 
Loss of Ignition (LOI) Ball Mill Feed 1.5 to 2.4% typical 
Loss of Ignition (LOI) Ball Mill Discharge 8 to 12% typical 
__________________________________________________________________________ 
*Analyzed by Atomic Absorption 
TABLE 5 
______________________________________ 
Summary of Atmospheric Leach 
Operating Conditions 
Test Test Test Test 
Run Run Run Run 
Quantity Units 1A 2A 3A 4A 
______________________________________ 
Leach Residue [A-A] weight percent 
Ni 1.57 1.31 0.91 1.14 
Co 0.06 0.05 0.04 0.09 
Fe 21.50 17.00 16.00 18.30 
Al 3.09 3.40 3.20 3.50 
Mg 3.74 4.30 3.80 4.04 
Mn 0.19 0.22 0.16 0.64 
Cr 1.60 1.50 2.00 2.10 
SiO.sub.2 41.00 45.50 49.70 41.00 
Percent of feed solids remain- 
84 79 79 81 
ing after leaching 
Pregnant Liquor 
Ni gpl 4.55 4.95 5.40 5.70 
Co gpl 0.12 0.18 0.17 0.22 
Fe gpl 3.10 1.40 2.60 2.40 
Al gpl 1.40 1.30 1.90 2.20 
Mg gpl 9.70 15.10 17.00 12.07 
Mn gpl 0.90 1.55 1.20 1.90 
SiO.sub.2 gpl N/A N/A 0.80 N/A 
H.sub.2 SO.sub.4 
gpl** 28.60 30.70 37.80 41.30 
pH pH N/A N/A N/A 
______________________________________ 
**Actual value calculated from titrated free acid minus acid equivalent o 
ferric sulfate 
TABLE 6 
__________________________________________________________________________ 
Summary of Atmospheric Leach 
Operating Conditions 
Test 
Test 
Test 
Test 
Run Run Run Run 
Quantity Units 1A 2A 3A 4A 
__________________________________________________________________________ 
Neutralized Liquor 
Ni gpl 5.20 
5.17 
5.96 
6.10 
Co gpl 0.18 
0.19 
N/A 0.28 
Fe gpl 1.40* 
0.14 
0.38 
1.00 
Al gpl 1.50 
1.20 
2.10 
2.40 
Mg gpl 14.70 
19.70 
25.30 
20.20 
Mn gpl 1.30 
1.70 
1.90 
2.65 
SiO.sub.2 gpl -- -- 0.60 
-- 
H.sub.2 SO.sub.4 *** 
gpl 0.9 1.10 
1.10 
1.90 
pH pH 2.40 
2.35 
2.35 
2.13 
Neutralizing Power 0.36 
0.47 
0.48 
0.45 
[kg acid/kg roasted garnierite] 
Neutralizing Efficiency 
70 
77 
78 
73 
##STR1## 
__________________________________________________________________________ 
*High iron due to ferrous iron concentration of 1.0 gpl. 
***Estimated from plotted data. 
TABLE 7 
______________________________________ 
Summary of Atmospheric Leach 
Operating Conditions 
Test Test Test Test 
Run Run Run Run 
Quantity Units 1A 2A 3A 4A 
______________________________________ 
Metal Extraction from Garnierite 
Ni percent 45 44 61 56 
Co percent 62 47 67 61 
Fe percent precipitation 
Al percent 19 27 16 23 
Mg percent 72 77 79 73 
Mn percent 81 76 89 73 
Cr percent 3* 3* 3* 3* 
SiO.sub.2 
percent precipitation 
______________________________________ 
*Estimated value only. Data deviation is too large to calculate actual 
value. 
The neutralizing power of the garnierite ranged between 0.36 to 0.48 kg of 
H.sub.2 SO.sub.4 per kg of roasted garnierite. There is a relationship 
between the free acid in the pregnant liquor to be neutralized and the 
neutralization power of the roasted garnierite. That is, the higher the 
free acid is initially, the more effectively the roasted garnierite is 
used. 
In run 4A only two 2-hour tanks or vessels were used. This was done in an 
attempt to demonstrate that less residence time would provide as effective 
a performance as the residence time obtained with four 2-hour tanks used 
in the previous runs. For comparison it is estimated that the two 2-hour 
tanks retain 90 percent of the feed for 60 minutes, compared with 200 
minutes for the four 2-hour tanks. 
An average neutralized liquor pH of 2.13 with iron levels over 1 gpl 
resulted for the entire run from operation with the two-tank 
configuration. The average results were due primarily to insufficient 
garnierite addition during the early part of the run. Table 8 shows that 
better results were obtained when the garnierite feed rate was increased. 
TABLE 8 
______________________________________ 
Garnierite Feed Rate vs Neutralized Liquor pH (Run 4A) 
Pregnant Roasted Garnierite 
Neutralized 
Liquor Flow Feed Rate Liquor 
kg/hr kg/hr pH [field log] 
______________________________________ 
Variable garnierite magnesium content, pregnant liquor 
flow and recycle rate. 
1438 115 2.05 
1374 121 2.09 
1378 124 2.11 
1352 128 2.22 
1315 140 2.70 
1364 138 2.61 
______________________________________ 
Special atmospheric leaching tests conducted, in addition to the 
aforementioned runs, demonstrated that adequate neutralization could be 
accomplished in a one-hour vessel provided sufficient garnierite was used. 
In run 4A, it was noted that when the neutralized liquor was maintained in 
a storage tank, iron precipitation continued to occur which resulted in a 
drop in pH from 2.0 to 1.5. This occurred frequently in some laboratory 
tests. However, this may be avoided by increasing the residence time 
and/or by increasing the number of vessels. 
While only one thickener is shown in the flow sheets of FIGS. 1 to 3, two 
may be employed wherein the overflow from the first thickener enters the 
second thickener which serves as a clarifier. As stated earlier, it may be 
desirable to use a flocculant to speed up the settling rate of the 
precipitate in the thickener. A typical flocculant is one referred to by 
the trademark SEAN MG-200 which is a slightly anionic polyacrylamide 
high polymer. 
The thickener underflow generally has a pulp density ranging from about 35% 
to 40% of solids and may range up to about 50% solids. 
Roasted garnierite ore may also be used as a neutralizing agent in the 
treatment of barren liquor resulting from the sulfide precipitation of 
metal values from the pregnant liquor. Since the acidity of the barren 
liquor is quite high, it presents a problem of disposal into environmental 
surface waters. The pH of the barren liquor which may range from about 1.8 
to 3, or more generally about 2 to 2.8, should be raised to a level of 
about 5 to 7 before it is disposed of. For convenience, the term "barren 
liquor neutralization" is referred to as BLN. 
Referring to the general flow sheet of FIG. 3A, the pregnant liquor 31 
enters the acid-kill section 32 (for example, a train of vessels as shown 
in FIG. 3) where the high acidity is reduced to a pH range of about 1.8 to 
3, generally from about 2 to 2.8 to prepare it for sulfide precipitation 
at station 33, using a sulfide-producing agent, e.g. H.sub.2 S. 
Following precipitation of NiS and any contained cobalt as CoS, the 
precipitate formed is removed as sulfide product at 34 and barren liquor 
35 is passed to station 36 where it is treated in a single or a series of 
tanks. Roasted garnierite 37 containing magnesium in excess of 5% may be 
employed as the neutralizing agent, the amount employed being sufficient 
to raise the pH to a level of about 5 to 7 (e.g., 5.5 to 7). The 
neutralized barren liquor 38 can be safely disposed into environmental 
surface water. 
The amount of garnierite added depends upon the free acid content of the 
barren liquor which can range from about 10 to 20 grams/liter. During the 
treatment manganese, iron, nickel and aluminum, among other elements, may 
be precipitated. 
In one experiment, barren liquor composition resulting from the treatment 
of a 1:1 Tiegabhi garnierite/laterite ore blend was as follows: 
Free H.sub.2 SO.sub.4 : 14 gpl; Mn: 2.3 gpl; Co: 
7 ppm; Al: 1.0 gpl; Mg: 25.0 gpl; Cr: 50 ppm; Fe: 0.7 gpl; Ni: 50 ppm. 
According to the experiment, 75 grams of roasted garnierite per liter of 
the foregoing barren liquor was required to neutralize it and precipitate 
cations to the following levels: 
Ni: 10 to 30 ppm; Al: &lt;1 ppm; Co: 1 to 3 ppm; Cr: &lt;1 ppm; Fe: 3 to 20 ppm; 
Mn: About 50% precipitated. pH: 5.5 to 7. 
The above results were obtained at a retention time of 3 hours at 
90.degree. C. using three stages in a countercurrent circuit. 
In a cocurrent circuit, a retention time of 4 hours at 90.degree. C. in 
four stages was required with a 200% thickener underflow recycle to the 
first vessel. Operationally, the cocurrent circuit is easier to control. 
The results of experiments indicated that the magnesium utilization from 
the roasted garnierite is 40% to 50% in both the cocurrent and 
countercurrent operations. As will be appreciated, any change in the 
barren liquor composition will influence the roasted garnierite 
requirements as illustrated below: 
______________________________________ 
Change In Roasted 
Change In Cation Level 
Garnierite Consumption 
______________________________________ 
1 gpl Al 18 gpl 
1 gpl Fe 5.5 gpl 
1 gpl Mn 5.5 gpl 
1 gpl Free Acid 3 gpl 
______________________________________ 
It is important to keep the aluminum concentration as low as possible. 
Injection of sodium sulfate in the leach train will produce a low aluminum 
level. 
As has been stated herein, the roasted high magnesium nickeliferous 
silicate ore (e.g., garnierite) should contain over 5% magnesium. 
Preferably, the ore should contain more than 8% magnesium. 
In carrying out the invention, the pregnant liquor to be treated in the 
atmospheric leach is generally produced by leaching at elevated 
temperature and pressure a nickeliferous oxide ore having the following 
composition by weight: about 0.8 to 3.5% Ni (e.g., 1.25 to 2.5% Ni), about 
0.005% to 1% Co, about 0.25% to 10% Mn, about 0.3% to 10% Cr, about 0.2% 
to 10% Al, up to about 30% Mg, about 2% to 45% SiO.sub.2 and up to about 
55% iron, with oxygen making up the balance, the foregoing metal values 
being present as oxides. 
The high-magnesium ores which may be employed in the roasted form for both 
the pregnant and barren liquors include those having the following 
composition by weight: 0.8% to 5% Ni (e.g., 1.5% to 5%), about 0.005% to 
1% Co, about 0.25% to 5% Mn, about 0.3% to 10% Cr, about 0.2% to 10% Al, 
about 5% to 30% Mg (e.g., 8% to 30%), up to 45% SiO.sub.2 and less than 
45% Fe, the foregoing metal values being present as oxides. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations 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.