CA1055711A - Method of enhancing the atmospheric leaching of nickeliferous matte - Google Patents

Abstract

METHOD OF ENHANCING THE ATMOSPHERIC
LEACHING OF NICKELIFEROUS MATTE
Abstract of Disclosure A method is disclosed for preparing matte for the atmospheric acid leaching thereof. Such mattes include nickeli-ferous sulfide mattes (e.g. nickel sulfide and nickel-copper sulfide mattes) containing 30% to 75% nickel, up to about 50%
copper, up to about 10% Co, 4% to 25% sulfur and up to about 5%
iron, the nickel (cobalt), copper and sulfur content making up at least about 90% by weight of the matte. The amount of sul-fur contained in the matte is less than that required stoichio-metrically to combine with the metal values therein and does not substantially exceed that amount required to combine with nickel as Ni3S2. The method resides in establishing a bath of said molten matte of predetermined composition, the temperature of the molten matte being at least about 100°C higher than its liquidus-solidus temperature, following which the temperature of the molten matte is lowered to a granulating temperature rela-tive to said liquidus-solidus temperature so that the granulating temperature falls within the range of about 50°C above and about 100°C below said liquidus-solidus temperature. The mol-ten matte is thereafter quenched in the presence of a quenching fluid from said granulating temperature, and then comminuted to the desired size for leaching, thus providing a feed pro-duct characterized by improved dissolution characteristics with respect to the nickel therein when subjected to atmospheric leaching in a hot acid solution.

Description

1~557~
This invention relates to a method for enhancing the atmospheric leaching of nickeliferous matte, such as nickel sulfide and nickel-copper sulfide mattes.
By "atmospheric leaching" is meant the leaching of nickeliferous matte in a hot acid solution at atmospheric pres-sure as opposed to high pressure leaching.
State of the Art Methods have been proposed for the selective leaching of nickel from nickeliferous sulfide mattes, such as nickel sulfide and nickel-copper sulfide mattes.
For example in U.S. Patent ~o. 940,292 (dated ~ovem-ber 16, 1909), a method is disclosed for selectively leaching nickel from nickel-copper matte containing about 40% nickelr 40'~ copper and the balance substantially sulfur. The matte is lS pulverized and agitated with a hydrochloric acid solution at a temperature of about 180F (83C). According to the patent, a substantial amount of nickel is selectively dissolved from ~he matte with the copper remaining in the residue.
In U.S. Patent ~o. 967,072 (August 9, 1910), a nickel-copper matte is leached with hot sulfuric acld to dissolve selectively ~he nickel, the process relying on the~formation ~
of H2S in situ to prevent the copper from being dissolved from the matte. ~ ~

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A method is disclosed in U.S. Patent No. 1,756,092 (April 29, 1930) for selectively extracting nickel from nickel-copper sulfide matte. In order to increase the solution rate of nickel into the acid, the matte is melted and then rapidly cooled by granulation in water. The leaching i5 car-ried out in sulfuric acid under atmospheric pressure and a temperature of about 80C to lOO~C (176~F to 212F) U.S. Patents No. 2,223,239 (November 16, 1940), No. 2,239,626 (April 22, 1941) and ~o. 2,753,259 (July 3, 1956) disclose the selective leaching of nickel from nickel-copper matte with acid, such as HCl or H2S04. The patents point up the importance of employing matte in which the sulfur content is stoichiometrically less than the amount required to combine with all the metal values present. The less the amount of com-bined sulfur, the higher is the nickel recovery following leach-ing.
However, a problem in the atmospheric leaching of nickeliferous matte is that the leaching characteristics of matte tend to vary from composition to composition, whether in the as-cast state or in the conventionally produced granu-i lated state. Such characteristics are generally reflected in ¦ prolonged leaching times which adversely affect the economics I ~ of the process. It would be desirable to provide a method of i pretreating a matta independent of its history 50 as to assurea consistent feed product having substantially uniform leaching characteristics, particularly with respect to shorter leaching times and higher extraction efficiencies.

l~SS7~1 Objects of the Invention It is thus the object of the invention to provide a nickeliferous sulfide matte in a form capable of being econo-mically leached in hot acid at atmospheric pressure to extract selectively therefrom a substantial portion of the contained nickel.
Another object is to providé a method of pro~ucing a nickeliferous matte having improved leaching characteristics wherein a plurality of mattes of different histories are blen-ded together, with or without the addition of metal scrap, such as nickel, cobalt, copper-containing scrap and alloys containing these metals.
These and other objects will more clearly appear when taken in conjunction with the following disclosure and the appended claims.
The Invention According to the lnvention, a method is provlded for the preparation of sulfide matte for atmospheric acid leachlng, such as nickeliferous sulfide mattes containing by weight about 30~O to 75/O nickel (e.g. about 40% to 70~/O)~ up to about 50~O
copper (e.g. about 2% to 40%), up to about 10% Co, about 4% to 25% sulfur ~eOg~ about 5% to 20~/o) and up to about 5% iron, the nickel, copper and sulfur content making up at least ab~ut 90~/O
by weight of the matte, the balance being gangue, other me~als, ;. : ~ .

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--~ 16~5S7~1 ~, and the like. It is important that the amount of sul~ur con-tained in the matte be less than that required stoichiometrical-ly to combine with the metal values therein and not exceed that amount required to combine with nickel as Ni3S2. In order to provide a matte capable of being economically leached at at-mospheric pressure and at temperatures ranging up to àbout 100C
to extract selectively a substantial portion of nickel there-from, a method is employed which comprises establishing a bath .
of molten matte of predetermined composition having the afore-mentioned characteristics, the temperature of said molten matte being at least about 100C higher than the li~uidus-solidus tem-perature thereof, lowering the temperature of the molten matte to a granulating temperature relative to said liquidus-solidus temperature so that the granulating temperature falls within the range of about 50C above to about 100C below said liquidus-solidus temperature, granulating the matte and quenching it in the presence of a quenching fluid, e.g. water or a cool gas, from said granulating temperature, and then comminuting~sald~
granulated matte to produce a feed product characterized by im-proved dissolution characterlstics with respect to the nickel therein when subjected to atmospheric leaching in a hot acid solution.
Tests have indicated that matte pretreated and pro-duced in the foregoing manner exhibits superior d~issolutlon

2~ characteristics compared to mattes of similar composltion~in either the as-cast state or in the conventionally produced granu-lated state. Mattes produced in the as-cast state generally in-: : ~

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volve the slow cooling of a large molten mass of nickeliferous sulfide which usually results in a segregated structure. Such mattes in the comminuted state do not behave uniformly during atmospheric leaching as such mattes generally have different melting and cooling histories. convehtionally granulated mattes are those mattes which are granulated by pouring said matte fr~m a temperature substantially above the liquidus-solidus tempera-ture of said matte, that is, more than 100F above said liquidus-solidus temperature.
While conventionally granulated mattes in the commi-nuted state generally exhibit superior leaching characteristics over as-cast mattes, such mattes vary in characteristics from lot to lot or composition to composition and do not always pro-vide uniform leaching results.
Time is an important economic factor in atmospheric leaching. Thus, by shortening the leaching time in the selec-; tive dissolution of nickel from ~he matte, the residue remain-ing can then be processed much sooner by high pressure leaching to recover therefrom the remaining metal values, such as nickel and copper.
We have found that we can obtain uniform and~markedly improved leaching results by controlling the granulation tem-_ perature within a range that does not exceed 50C~above the~
solidus-liquidus temperature of the matte composit~ n~ and whLch . ~ .' ~55'7~L~

extends to about 100C below said solidus-liquidus temperature, so long as a temperature is chosen within the range tnat en-ables the molten matte to be easily poured and granulated in a quenching medium, e.g. water, water`spray, cool gas, etc.
The thermal characteristics of a particular matte composition are determined by heating the matte to a superheat temperature above its solidus-liquidus temperature and then allowing it to cool slowly to and through the solidus~ uidus - temperature range using an immersion thermocouple to detect the one or more thermal arrests indicative of the transitory change of state of the matte during cooling. Haviny determined that thermal arrest of the liquidus-solidus temperature for a specified matte composition, this temperature is then utilized to determine the correct granulation temperature defined herein-above.
Details of the Invention As stated hereinabove, matte grades of different his-~
tories can be treated in accordance with the invention to pro-vide comminuted ma~te having the desired dissolution rate in hot sulfuric acid. The amount of nickel of the total nickel pre-sent available for leaching is dependent on the composition of the matte. For example, the lower the amount of sulfur in the matte, the higher is the amount of available leachable nicXel (usually in the form of finely dispersed metal nickel), and as the compound Ni3S2, the nickel combined as ~iS being not rea-dily acid soluble within the time period of intsrest.

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As illustrative of the foregoing, analyses of var-; ious~grades of mattes are given in both the as-cast condition and the granulated condition (the invention) as follows:
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Available Nickel _ Percent Elemen t % of Total Type Matte~i Cu S Fe Nickel in Matte*

lA (As-Cast) 40.039.6 16.4 0.23 61 . 2A (Granulated) 40.8 38.8 16.6 0.40 61 (Invention) -.
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lB (As-Cast) 68.03.4 21~0 3.4 50 2B (Granulated) 68.0 6.8 18.6 2.88 50 (Invention) .
lC (Granulated) 51.6 38.4 5.0 1.84 92**
(Low S) 2C (Low S) 52.0 38.8 5~4 1.28 95 (Granulated~
(Invention) * - Calculated from ~i-Cu-S Ternary Diagram ** - Received as-granulated from Outokumpu (Finland) : :: : ., Each of the mattes lA, lB and lC in the table is~ in - 10 the as-received condition, mattes lA and lB being in the~:as~
cast state while matte lC was received from Finland in the con-ventionally granulated state.
: The foregoing mattes were then converted to the :
granulated mattes of the invention, to wit, 2A, 2B and 2C.

In the case of matte lA, a charge was produced:by . , blending with the as-received:matte such materials as nickel powder, copper shot and element sulfur in percentage amounts .

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corresponding to the initial composition of matte lA. The ; ~ charge was melted in a 15KW induction furnace, and the melt raised to 2500F (1371C). An immersion thermocouple was inser-ted into the molten bath and the bath turned off. A cooling curve was obtained which gave a rest point at the liquidus-solidus temperature of about 1760F (960C).
Using the rest point temperature as a guide, the foregoing charge was reheated to substantially above the liquidus-solidus temperature (2500F or 1371C) and then tapped into a preheated clay-graphite crucible (preheated to 2000F or 1093C) and the charge allowed to cool to the desired granulating tem-perature of about 1800F (982C) before being poured into a preheated pouring basin having a quarter-inch opening or ori-fice in the bottom thereof, the basin being disposed within a granulating apparatus comprising a rotating disc having radially disposed carbon vanes fixed thereto. A perforated copper ring is located within the apparatus surrounding the pouring basin through which water is sprayed as the stream of molten matte is fed gravimetrically at about 1800F (982C) to impinge on the surface of the rotatlng disc for deflection by the carbon vanes, The temperature of the matta during the pouring step is substan-tially at the liquidus-solldus temperature or may be slightly above it, e.g. 20C to 50C above. Thus, as the molten matte lS thrown by centrifugal force to the sides of the apparatus as fin~ly divided globules, it is immediately quenched to form . , .

,; ~ ' , frozen granules which drop to the bottom of the granulation apparatus where they are collected.
After granulation, the product is ground to 50~/0 minus 400 mesh, with about 75% of the comminuted product passing through 270 mesh.
Matte 2B was produced by granulating matte lB and matte 2C produced by granulating matte lC, the granulated mattes being thereafter ground as stated hereinabove.
Following production of the comminuted mattes of the invention, each of the mattes was subjected to a leaching program using a leaching solution containing 22 grams per liter (gpl) o~ Cu+~ ions and 46 gpl of H2S04. The presence of copper ions in the solution corresponds to recycle spent electrolyte from copper electrolytic cells to be discussed later. Each of the tests was conducted in 450 milliliters (ml) of solution into which air is vigorously bubbled at a rate of 520 ml/min, the solution being stirred at a rate of 420 RPM with the temperature \ maintained at 75C (167~F). The solution with the foregoing acid concentration has a starting pH of less than about 0.5 and gen-erally a pH in the neighborhood of about 0.3.
The matte granulated according to the invention is compared to the corresponding as-received matte by leaching ~ the respective mattes under the foregoing conditions until the pregnant solution reaches a pH of about 5.3, the foregoing pH
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` 1~557~L1 being taken as indicative of the completion of dissolution for practical purposes. The time to reach the pH is measured and used in comparing the leaching characteristics of the various mattes.
The results obtained in leaching 164 grams each of mattes lA and 2A in 450 ml of solution (about 27% solids by weight) are given in Table 2 as follows:
Tabl,e_ ?

Type IMelting~Granulation ¦Time to pH ~ % Ni Matte F ¦ F = 5.3 min. qpl Extraction ___ 1A As-Cast 83 0.001 18.1 2A ¦ 2500 ¦ 1800 ~ 65 0.001 26.1 The solution following leaching of 164 grams reached a pH of 5.3 much sooner with the granulated matte than with the as-cast matte and larger amounts of soluble nickel were dis-~
solved. In both tests, the copper was cemented out of~solution by the free nickel in the matte to the very low level of 0.001 gpl 1 part per million (ppm).
, However, when a lower amount of 91 grams of the same matte samples were leached in the same amount of solution (that is, at a lower pulp density), approximately 80'~o of~the avail-able nickel was extracted,;the-granulated matte (2A3 being lea-ched 25 minutes sooner than the as-cast matte (lA), Su~s~an-' ~ -12-~ , tially all of the copper was cemented from the leaching solu-tion by the free nickel in both mattes as shown in Table 3.

Table 3 Type ~ on j Time to pH Cu~+ ¦ % Ni Matte F ¦ F = 5.3 min. qpl Extraction lA As-Cast 145 0 . 002 47 ~ 6 _ 2A 2 5 0 0 ¦ 18 0 0 _12 0 0 . 0 02 _4 7 3 _ I
Similar tests were conducted on mattes lB and 2B.
These results are given as follows:
Table 4 Type Melting ¦Granulation Time to pH Cu ¦ % Ni Matte _ F ¦ F = 5-3 min._ qpl Extrac-tion lB As-Cast ~? 2 50 7 ~ 2 0 2 5 . 9 2B 2500 ¦ 1800 110 0.001 41~1 The leaching of granulated matte 2B was completed after 110 minutes; about 41% of the nickel being extracted which corresponds approximately to about 80~/o of the avallable sol~~ble nickel. Substantially all of the copper was rejected from solu-tion. The cast matte, on the other hand, leached very~poorly and after more than 250 minutes, only 25~/o was dissolved. The solution (lB) still contained relatively large amounts of copper ~ (7~2 gpl).
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, I ~ . - , - ", - . . - - . , -1~55'7~1 Tests have indicated that the temperature of granu-.
lation has an effect on the leaching activity of the material.
For example, matte 2A granulated from temperatures of 2300~F
~-, to 2500F (1260C to 1371C) tended'to be less reactive than the matte granulated in the neighborhood of 1800F.
¦ ~o batches of low sulfur matte lC were melted at J
2500F (1371C) and then granulated at 2100F tll50C) and 2300F (1260C), respectively. A cooling curve showed that .
this matte had a solidus-liquidus temperature of about 2155F
'A 10 tll80C). Thus, one granulation temperature (1150C) was 30 below the solidus-liquidus temperature, while the other (1260C) was 85 above this temperature.
In a leaching program involving 88 grams of each of .
the two differently granulated mattes in 450 ml (about 16~5%
solids by weight) of the aforementioned solution, the following results were obtained:
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Table 5 Melting Granulation Time to pE Cu++ /O Ni -.3 F Temp~ F _ = 5.3 min. qPl Extracted j~ 2G 2500 _2300 185 _0 001 38.3 2500 2100 _ 165 _ 0.001 , 37O4~ __ ~ As will be noted, while substantially the same amount .~ .
of nickel was dissolved, the matte granulated at 2100F (1150C) wasimore active in that the solution reached a pH of 5.3 about 7 ~ :

, -~557~1 20 minutes sooner. In other words, atmospheric leaching is - completed sooner with the matte granulated in accordance with the invention.
When 49 grams of the same matte were similarly leached in ~he same amount of solution as in Table 5, the following re-sults were obtained:
Table 6 Melting Granulation Total Leach Final Cu~+ I % ~i F F _ Time PH qpl Extracted 2500 2300 460 4.5 4 0 ~9 2 . . .
2500 2100 380 4.~5_ -7 _ 51.8 Much faster leaching was obtained with the material granulated at 2100F (1150C) as compared to a granulation tem-perature of 2300E ~1260C). As will be noted, the matte quen-ched at 2100F (1150C) leached 80 minutes sooner and, moreover, rejected most of the copper from solution, The atmospheric leaching in general is carried out for a period of time until the pH increases to substantially over 4, e.t. 4.5 and above and, preferably, at least about 5.~ ~
It appears that nicXeliferous mattes granulated in accordance with the invention produce a very fine grain struc-- ture which is almost homogeneous when examined optically. In contrast, the cast matte contains much larger grains with segre-gated phases. In the cast matte, the area of contac~ among the different phases is greatly reduced and, as a result, the rate of galvanic type reactions due to the presence of free nickel during leaching is also reduced. ~- ~

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The granulation temperature has a beneficial effect on the leaching time. As stated above, the temperature may range from about 50C above the solidus-liquidus temperature to 100C below this temperature, with the granulation tempera-ture preferably ranging to below the iiquidus-solidus tempera-ture, for example, 15C to 75C below this temperature.
Generally speaking, for mattes having the same Ni/Cu weight ratio, the liquidus-solidus temperature increases with decreasing sulfur content. For example, in the case of low-sulfur matte lC (51.6% Ni, 38.4% Cu, 5% S and 1.84% Fe), the liquidus-solidus temperature determined ~rom the Ni-Cu-S ter-nary diagram is about 2180~F (1200C). An experimental cool-ing curve placed the temperature at approximately 2200F. The foregoing matte corresponds to the mattes referred to in Tables 5 and 6.
As stated hereinbefore, the granulation treatment of the invention is important in providing a eed product exhibit-ing enhanced leaching rates. This is important as it enables the setting up of a unit operation in which a plurality of pro~
cesses can be coordinated together to provide recovery of such metal values as nickel, cobalt, copper, among others.
~ The granulation treatment may constitute the flrst _step of the process. Thus, following granulation, the matte is comminuted to provide a~finely divided feed product, such :
as a feed product in which at least 50% by weight passes through .,:
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~L~557~1 270 mesh (U.S. Standard Screen). The matte may be a blend of conventionally prepared matte compositions to which scrap metal te.g. Ni, Co, Cu, etc.) may be optionally added together with make-up sulfur.
A typical granulated matte is one having the follo~-ing composition: 50.5% ~i, 28.1% Cu, 0.6% Co, 2% Fe and 18.8%
S.
Putting it succinctly, a typical unit operation would comprise subjecting the gxanulated matte in the commi-nuted state to acid leaching at a temperature of about 150F
to 185F (65C to 85C) while the leaching solution is strongly aerated. Generally speaking, the pulp density may range from about 10% to 40% by weight solids. The acid leaching solution is preferably spent electrolyte recycled from copper electroly-sis process carried out later on in the process. An example of a spent electrolyte is one containing 55 gpl ~i, 20 gpl Cu, 0.85 gpl Co, 122 gpl of S04 ions and 43 gpl H2S04. Broadly speaking, the spent electrolyte may comprise up to about~75 gpl nickel combined as sulfate, 5 to-30 gpl copper combined as sul-fate and ~ree sulfuric acid in an amount ranging ~rom about 20 to 100 gpl.
The copper is re~ected from solution by hydrolysis _ and/or cementation during atmospheric leaching and a nickel-~; rich solution obtained which is treated to remove iron and then sent to nickel and cobalt recovery. The methods for recovering nickel and cobalt are well known in the art and need not be described here.
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.Following atmospheric leaching, the residue is fil-tered, washed and slurried in an acid solution to remove most of the iron therefrom. The residue may contain at this stage about 28% Ni, 48% Cu, 0.8% Co, 2.4% Fe and 22~/o S~ The residue after washing is pulped (e.g. 20% to 50% solids) and high pres-sure leached in an acid solution at a pressure of about 600 to 700 psig at a temperature of about 350F to 400F (177C to 204~C). This results in a pregnant liquor which, after filter~
ing, becomes the feed solution for copper electrowinning after acidifying and dilution to contain about 40 to 50 gpl Ni, ~0 to 50 gpl Cu and about 100 gpl free sulfuric acid.
The foregoing solution is sent to copper electroly~
sis for copper recovery, the spent solution for recycle to atmospheric containing about 55 gpl ~i, 20 gpl Cu, 0.85 gpl Co, 122 gpl S04 ions and 43 gpl H2SO4.
An important advantage of producing the desired feed matte by blending together mattes of different grades and hls-tories, with or without the addition of scrap metal (~i, Co, Cu, alloys thereof, etc.), is that a feed product is obtained following granulation which is independent of the pr1or hLstory of the blended materials. Examples of blended compositlons for ; ` use in producing the granulated feed material of the invention are as follows:
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Table 7 - r Blend Material Wt. ~/~ 7i Co nposltion I % Fe % Otherc _ . _ _ lA 45 4039.6 16.4 0.23 3 8 . ._.
lB 45 683.4 21.0 3.40 3~? _ Cu Scrap _ _ 10 _ 100 0 - _ ~ _ _ . .
Blend Com-p~sition 100 48.629.5 16 9 1 1.6 3.4 Table 8 !

Blend Com ~osition _ % Other Material Wt. % Ni % Cu % S % Fe (bal.) lB 30 68.0 3.4 21.0 3.4 4.2 lC 40 51.6 38.4 5.0 ~1.8 3.2 Monel Scra~ 13 67.0 30.0 ~ 1.5 1.5 Scrap 10 _ 100.0 _ _ _ Sulfur 7 _ _ 100.0 _ _ ~ _ Blend Com-Position100~49~8 30.3 -15.3 1.9 2.7 . ~
~ As wlll be apparent, any desired blend can be produced so long as the final composition is melted and granulated in accordance with the invention. While cobalt is not listed~in _ Tables 7 and 8, it is understood to be present in~small~amounts in most nickeliferous sulfide mattes. Of course, cobalt scrap `; : :~ :
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in the form of the metal or alloys thereof may be employed in producing blended compositions containing cobalt ranging up to about 10% or higher.
Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may bè 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.

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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method for the atmospheric acid leaching of comminuted nickeliferous sulfide mattes containing about 30% to 75% nickel, up to about 50% copper, up to about 10% cobalt, about 4% to 25% sulfur and up to about 5% iron, the nickel (cobalt), copper and sulfur content making up at least about 90% by weight of the matte, wherein the amount of sulfur con-tained in said matte in the foregoing range is less than that amount required stoichiometrically to combine with the metal values therein and does not substantially exceed that amount required to combine with nickel as Ni3S2, and wherein said com-minuted ore is leached at atmospheric pressure in hot acid, the improvement for enhancing the dissolution rate of said matte in the acid solution which comprises, establishing a bath of molten matte of predetermined composition having the aforementioned characteristics, the temperature of said molten matte being at least about 100°C higher than the liquidus-solidus temperature of said predetermined matte composition being treated, lowering the temperature of said molten matte to a granulating temperature relative to said liquidus-solidus temperature, wherein said granulating temperature falls within the range of about 50°C
above to about 100°C below said liquidus-solidus temperature, said molten matte within said tem-perature being capable of being granulated and quenched with a quenching fluid, granulating said matte and quenching it in the presence of said quenching fluid from said granu-lating temperature within said temperature range, and then comminuting said granulated matte, whereby a feed product is obtained charac-terized by improved dissolution character-istics with respect to the nickel therein when subjected to atmospheric leaching in a hot acid solution containing free sulfuric acid.

2. The method of claim 1, wherein the nickeliferous sulfide matte contains about 40% to 70% Ni, 2% to 40% Cu and about 5% to 20% S.

3. The method of claim 1, wherein said matte is granulated at a temperature below said liquidus-solidus tem-perature of said matte in the range of 15°C to 75°C below said temperature.

4. The method of claim 1, wherein the comminuted granulated matte is thereafter leached in hot sulfuric acid at a starting pH of less than about 0.5 to dissolve selective-ly substantial amounts of nickel therefrom.

5. In a method for the atmospheric acid leaching of comminuted nickeliferous sulfide mattes containing about 40% to 70% nickel, about 2% to 40% copper, up to about 10%
cobalt, 5% to 20% sulfur and up to about 5% iron, the nickel (cobalt), copper and sulfur content making up at least about 90% by weight of the matte, wherein the amount of sulfur con-tained in said matte in the foregoing range is less than that required stoichiometrically to combine with the metal values therein and does not substantially exceed that amount required to combine with nickel as Ni3S2, and wherein said comminuted ore is leached at atmospheric pressure at temperatures ranging up to about 100°C, the improvement for accelerating the disso-lution of said matte in the acid solution which comprises, establishing a bath of molten matte of predeter-mined composition having the aforementioned characteristics, the temperature of said molten matte being at least about 100°C higher than the liquidus-solidus temperature of said predetermined matte composition being treated, lowering the temperature of said molten matte rela-tive to said liquidus-solidus temperature to a temperature ranging from about 15°C to 75°C below said liquidus-solidus temperature, said molten matte within said temperature range being capable of being granulated and quenched with a quenching fluid, granulating said matte and quenching it in the presence of said quenching fluid from said tem-perature within said temperature range, and then comminuting said granulated matte so that at least 50% passes through 270 mesh, whereby a feed product is obtained charac-terized by improved dissolution character-istics relative to the nickel therein when subjected to atmospheric leaching in a hot acid solution containing free sulfuric acid.

6. The method of claim 5, wherein the comminuted granulated matte is thereafter leached in an aerated hot sul-furic acid at a starting pH of less than about 0.5 to dissolve selectively substantial amounts of nickel therefrom.

7. The method of claim 6, wherein the temperature of leaching ranges from about 65°C to 85°C.