HIGH TEMPERATURE LEACHING OF SULFIDE ORES

PC1 I« i0 /CAN.

The present invention relates to the oxidative leaching of aqueous slurries of sulfide ores to recover metal values therefrom.

The oxidative leaching of a sulfide ore is a well known process whereín sui{ur present as sulfide in the ore is oxidized to its elemental form. It is highly desirable for kinetic reasons to be able to perform such leaching at as high a temperature as practicable. Apart from the improved reaction kinetics, operation at higher temperatures results in slurries which exhibit good solid/ liquid separation characteristics, the latter being an important economic factor in any hydrometallurgical process.

However, a problem which is encountered when leaching is performed at temperatures higher than the melting point of sulfur (i.e., I12°C) is that the liberated sulfur tends to coat the surface of the sulfide ore particles and passivate them against further reaction with the leach medium. For this reason, inspite of the obvious commercial attractions of high temperatures, the generally advocated procedure has been to leach such sulfide ores at temperatures lower than I12°C. After oxidation of the sulfide to produce elemental sulfur, a "pelletizing" step has been advocated, wherein the slurry is treated at a temperature higher than ll2°C to transform the liberated sulfur into globules of a convenient size. Additives can be used in the pelletizing step to aid formation of the sulfur globules.

More recently, in Canadian Application Serial No. 141,591, a process was disclosed wherein a surfactant is present during the leaching operation to ínhibit the passivation of the sulfide particles by liberated sulfur i0 10Z4 and thus enable leaching temperatures higher than I12°C to be used. The surfactants recommended in the abovementioned application are water-miscible organic compounds such as mixtures of water-soluble aliphatic amines, polyalklene gycol esters, polyoxyethylene lauryl ethers and sulfonated aromatics. A serious disadvantage of such reagents is their high costs in relation to the ores treated.

It has now been found that sulfide ores can be leached at temperatures higher than the melting point of sulfur by the use of relatively inexpensive additives, and in particular by the use of additives previously considered as industrial waste products.

Accordingly, the present invention provides a process for oxidizing at least part of the sulfur present as sulfide in an ore to elemental sulfur, comprising treating an aqueous slurry of %te ore at a temperature higher than the melting point of sulfur, wherein the aqueous slurry includes an additive comprising a rosin acid and a fatty acid to minimíze coating and wetting of the sulfide ore by molten elemental sulfur during the oxidation.

The additive used in the process can consist of crude or refined "tall oil", but as explained below it is preferred that the additive consist of "tall oil pitch".

Tall oil is a mixture of rosin acids and fatty acids together with unsaponifiable matter. It is obtained in crude form as a by-product in the manufacture of paper, by acidifying the black liquor which results from digesting wood pulp with sulfite or alkali. The exact composition of crude tall oil varies in accordance with the origin of the wood pulp employed in the digesting process, typically the z - 10Z4355 crude oil can contain from about 18 to about 62% fatty acids and from about 30 to about 65% rosin acids.

While the crude tall oil may be used as the additive in the present process, it is a distinct advantage that the additive can in fact comprise the far cheaper material known as tall oil pitch. The term tall oil pitch is a well known term of the art and is defined for example, in the Encyclopaedia of Chemical Technology, Kirk-Othmer, Vol. 19 page 619. The term is used to describe the residue which remains after distillation of tall oil. This residue contains typically about 12-30% rosin acids and esters, about 35-50% fatty acids and esters, the balance being made up of neutral materials such as alcohol, sterols, hydrocarbons and sulfur lignin. Thus, while crude tall oil is itself an industrial by-product, the tall oil pitch is generally considered a waste product and provides a very low cost additive for the present process.

Reference in this specification to ores is intended to include such metallurgical intermediates as ore concentrates and leach residues which contain solid sulfide material.

An advantageous embodiment of the invention is the treatment of an aqueous slurry containing the sulfide of at least one of the following metals: cobalt, copper, iron, lead, nickel and zinc. For economic reasons it is advantageousthat the ore contain at least one non-ferrous meta! value. Examples of sulfide minerals include, although the invention is not limited thereto,chaloopyríte, pentlandite, chalcocite,sphalerite, cubanite, galena, pyrrhotite, violarite, covellite, digenite, millerite, bornite, cobaltite, polydymite and iron monosulfide produced by heating iron pyrites in non-oxidizing atmospheres to drive off the labile sulfur.

ÆOZ4355 In addition to the improved kinetics resulting from the high temperature operation, other advantages of the process include the fact that a separate pelletization step is obviated in that sulfur liberated during the leach forms pellets which can be separated, for example, by screening. Moreover, the high temperature oxidation results in a residue having good settling properties, thus minimizing the need for expensive flocculants in subsequent settling operations.

The ore does not have to be treated in any manner other than preliminary crushing or grinding to increase the surface area of the material for facilitating the leaching reactions. In most instances, grinding to about 90% minus i00 mesh Tyler Screen Size (TSS) is advantageous in providing commercially attractive leaching rates. For best results in terms of non-ferrous metal value dissolution, sulfide sulfur conversion, reaction rates and apparatus utilization, the ore is ground to a particle size of about 100% minus 325 mesh TSS.

After crushing or grinding, if necessary, the ore is pulped with water or an acidic aqueous leaching solution.

A wide range of pulp densities, e.g., from about 2% solids by weight to about 50% solids by weight, can be employed.

However, it is preferred to employ pulp densities between about 5% solids, by weight, and 30% solids, by weight, to minimize materials handling problems, to provide efficient utilization of process apparatus, and to produce pregnant solutions more concentrated in the non-ferrous metal values.

Any leaching process that oxidizes the sulfide sulfur associated with a metal value to be extracted to elemental sulfur and dissolves the metal value can be i0 10Z4355 employed. For example, solutions of at least one ferric salt selected from the group consisting of sulfate, chloride and nitrate can be employed to oxidize sulfide sulfur of non-ferrous metal sulfide minerals to elemental sulfur while dissolving the non-ferrous metal value.

Alternatively, the oxidation can be accomplished by passing gaseous chlorine through an aqueous slurry of the sulfide mineral. Most advantageously, leaching is conducted by oxidizing an aqueous slurry of the sulfide mineral with a free-oxygen-containing gas as described below.

An aqueous slurry of the finely divided sulfide ore is established in an autoclave made of a suitable acidresisting material and equipped with means for agitating the slurry. The slurry is heated to a temperature above about I12°C., preferably between 120° and 170°C, under an oxygen partial pressure of between 0.35 and 3.5 MPa, preferably between 1.4 and 2.1 MPa, to oxidize the suifide su1fur to elemental sulfur and to dissolve the metãl value.

A preferred operating sequence consists of heating the slurry to about 150°C for a short time, decreasing the temperature to about 130°C and holding at that temperature for a period sufficient substantially to complete the desired oxidation, and thereafter raising the temperature to 150°C for a short time. Very high leaching temperatures are to be avoided because above about 190°C the sulfide sulfur is oxidized to sulfate instead of being left as elemental sulfur, thus necessitating subsequent neutralization thereof.

As stated above tall oil pitch is preferred, due to its cheapness, for use as additive in the process of the invention. It is available for example, under the i0 ' 1024355 trade name PAMAK CTP and is solid ah room temperature.

The tall oil pitch may be added to the slurry in such solid form, or alternatively it may be warmed and introduced as a liquid or possibly as a water emulsion. The amount of additive used should be between 0.25 and kg/tonne and preferably between 0.5 and 2.5 kg/tonne.

While the additive may be introduced at the start of the leach, it is preferable especially when higher temperatures are used to minimize degradation of the additive by adding all or part of it during the course of the leach.

During the oxidation reaction the slurry should be vigorously agitated, and at the end of the reaction sulfur pellets formed on cooling can be separated from the slurry by screening with a 65 mesh (TSS) screen.

In a preferred application of the process of the invention to an ore consisting of pyrrhotite, pentlandite and chalcopyrite, the oxidation is allowed to proceed until substantially all of the sulfide sulfur of the pyrrhotite and pentlandite has been converted to elemental sulfur; the oxidation is then terminated before substantial amounts of the sulfide in the chalcopyrite have been oxidized. In this way, a slurry as obtained which contains elemental sulfur, iron oxide, gangue and uncreated chalcopyrite in a pregnant nickel-containing sulfate solution. The chalcopyrite, after separation from the elemental sulfur, iron oxide and pregnant solution, can be treated for copper recovery, and nickel can be recovered from the pregnant solution.

In order to give those skilled in the art a better appreciation of the invention, the following illustrative examples are given:

i0 10243 EXAMPLE 1 A series of oxidative leach tests were performed on a pyrrhotite concentrate, ground to a particle size of about 98% minus i00 mesh (TSS) and containing, by weight: 0.09% copper, 0.91% nickel, 56.5% iron and 34.7% sulfur. The concentrate was mixed with water to give a 20% solids slurry. Various amounts of tall oil pitch were added to samples of the slurry, which were then leached at different temperatures using an oxygen partial pressure of 1.7 MPa, while stirring at 800 rpm. The additive used in these tests was a commercially available tall oil pitch sold under the trade name PAMAK CTP, which has a fatty acid content of about 8% and a rosin acid content of about 25%.

In each case after the oxidation, the resulting residue was screened with a 65 mesh (TSS) screen. The amount and sulfur content of the screened pellets, as well as the metal values extracted in the pregnant liquor are shown in Table 1 below. This Table includes results of three tests (1-3) using different temperatures and amounts of additives, as well as a test (4) wherein the temperature was varied during the leach. Also included in the Table are results of a comparative test (A) wherein no additive was employed.

10; 4355 TABLE 1 Test Ho.

LEACHING Temp Duration (°C) (min) 130 120 140 120 150 120 150 I 135 142 15 I Z50 130 90 Additive (kg/ tonne) 1.5 2.5 2.5 2.5 Screened Pellets Wt of Pellets (as of Concentrate) Sulfur Content Metal Recovery (%) Cu Ni Fs 91 4 94 3 89 3 91 4 51 3 Contrasting the results of Tests 1-4 with those of Test A, it will be seen that the use of the additive led, under the various tests conditions to good metal recovery as well as easily separated high purity sulfur pellets. In the case of Test A passivation of the concentrate with molten sulfur resulted in inferior metal recoveries and in sulfur pellets of only 46% purity.

EXAMPLE 2 To test the efficacy of a tall oil pitch of different composition, the following Test (5) was conducted. A sample of pyrrhotite concentrate having a particle size of 0.8% plus 100 mesh (TSS) and containing by weight: 0.1% copper, 1.0% nickel, 56.8% iron and 35.5% sulfur, was leached under the same conditions (i.e. temperature, time and amount of additive) as Test No. 1 of Example l,except that the additive used was commercially available tall oil pitch known as PAMAK CTPD containing about 30% fatty acids and 46% rosin acids.

10Z4355 The results obtained are shown in Table 2, from which it will be seen that there is very little difference between the efficacy of this additive and that of the additive used in Example l.

TABLE 2 Test N, : Wt of Pellets : s % of : Concentrate Screened Pellets : Sulfur : Content (%) : 98 : tal Recovery (%) : Cu : 41 : Ni : Fe : 88 • 4 EXAMPLE 3 To demonstrate the advantages of high temperature leaching, two Tests (6 and 7) in accordance with the invention were compared with a Test (B) performed at a temperature below the melting point of sulfur• In each case a slurry was used of a concentrate having a particle size of 98.2% mìnus 100 mesh (TSS) and containing by weight: 0.09% copper 0.92% nickel, 58.3% iron and 34.7% sulfur. The slurry was leached at an oxygen partial pressure of 1.7 MPa. The Tests 6 and 7 involved use of the additives of Examples 1 and 2 respectively, whereas in Test B no additive was used. After the leach settling tests were carried out at 80°C in a measuring cylinder, and filtration rates were determined by passing 500 cc's of hot water through a 1 cm deep cake on a Buchner funnel, underflow solids from the settling tests, and the filter cake moisture were also determined. The results are shown in Table 3, and illustrate clearly the vastly superior solid/ liquid separation characteristics of the slurries obtained from the processes in accordance with the invention.

p o Test N° Se -: % Solids : in slurry TABLE 3 : Additive TOUCHING : ,Amt. of : additive : (kg/tonne) :T np : (°C) Dura- : tion : (mins) : (%) : Res:dUe Properties : Nickel : Settlihq Results : Filtration Results : Extrac- :In:- :Under--= Set- :

: tion :tial :flew :fling :

: % : % : rate :Pate : Cake % :as Example 1 :as Example 2 1,5 1.5 : 150 : 135 : 140 : 150 : 150 : 135 : 110 i0 5o i0.

:I : 82 91 : 23 - 16 : 23 :Solids :Solids : 64 : n<h : 0.3 : 0.5 : 0.02 : i/m2h.

: 300 : ,300 : Fbisture : 32 : 38 4055 Although the present invention has been specifically described with reference to preferred embodiments, it will be understood that various modifications may be made, for example, to the ore treated or to the oxidation process aRd conditions, without departing from the scope of the invention which is defined by the following claims.

-llPC-II21/CAN.

IOZ4355 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

i. A process for oxidizing at least part of the sulfur present as sulfide in an ore to elemental sulfur, comprising treating an aqueous slurry of the ore at a temperature higher than the melting point of sulfur, wherein the aqueous slurry includes, in an amount of between 0.25 and 5 kg/tonne of ore, an additive comprising a rosin acid and a fatty acid to minimize coating and wetting of the sulfide ore by molten elemental sulfur during the oxidation.

2. A process as claimed in claim 1 wherein the additive comprises tall cil.

3. A process as claimed in claim 1 wherein the additive comprises tall cil pitch.

4. A process as claimed in claim 1 wherein the amount of additive used comprises between 0.5 and 2.5 kg/tonne of ore.

5. A process as claimed in claim 1 wherein the aqueous slurry is treated at a temperature between 130°C and 170°C.