AU726261B2 - A process to improve mineral flotation separation by deoxygenating slurries and mineral surfaces - Google Patents

Description

-1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

(I

Name of Applicant: Actual Inventors: Address of Service: Invention Title: BOC GASES AUSTRALIA LIMITED, A.C.N. 000 029 729 David William CLARK and Andrew James NEWELL SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "A PROCESS TO IMPROVE MINERAL FLOTATION SEPARATION BY DEOXYGENATING SLURRIES AND MINERAL SURFACES" Details of Associated Provisional Application No. PO 5909 dated 26th March 1997 The following statement is a full description of this invention, including the best method of performing it known to us:- -2- FIELD OF THE INVENTION This invention relates to the physical separation of minerals and, in particular, to the separation of minerals of different mineralogical character.

BACKGROUND OF THE INVENTION Many ore bodies comprise a mixture of valuable sulphide minerals with a number of non-sulphide minerals, including carbonaceous minerals (eg graphite, carbon based residues as exist in Mt Isa, Australia ore bodies), talcose minerals (eg talc, brucite etc which are associated with Western Australian nickel deposits and the Woodlawn, New South Wales, Australia base metal deposit) as well as amphiboles.

The non-sulphide minerals have naturally hydrophobic characteristics. The degree of hydrophobocity varies according to mineral and ore type from weakly hydrophobic to strongly hydrophobic. As a result, these "gangue" minerals have a tendency to float and are very difficult to separate from other valuable minerals, notably the sulphide minerals (eg chalcopyrite (CuFeS 2 pentlandite ((Ni,Fe) 9 Ss) and sphalerite When present in 15 mineral concentrates, these "gangue" minerals often attract penalty charges at the smelter and, indeed, may be the cause of rejection of the concentrate by the smelter.

S•In practice, two approaches to this problem exist, namely to minimise the flotation of the non-sulphide "gangue" minerals using specific reagents or, alternatively, to encourage flotation of the "gangue" minerals in a pre-flotation step prior to the flotation of the desired minerals.

In the first approach, reagents such as depressants (guar gum, CMC, etc) or dispersants (eg sodium silicate, etc) are employed to minimise the flotation rate of the nonsulphidic minerals. In some cases for example with copper-nickel-iron bearing ores nitrogen is used as a flotation gas in combination with organic depressants. This tends to -3strengthen pyrrhotite depression and increase nickel recovery. While successful to some extent, the use of these organic depressants is non-specific and adversely affects the flotation behaviour of the sulphide minerals in terms of metallurgy as well as froth structure. In addition, the use of such reagents is costly and, if it were possible, should be avoided.

Furthermore, the use of such reagents not only adversely affects flotation behaviour, it affects downstream operations such as dewatering and settling of the minerals.

Additionally, and particularly with depressants, there is a requirement to add more reagent at each stage of the separation process.

In the second approach, a separate flotation system is dedicated to the recovery of the naturally floating mineral. Reagents are added to prevent the flotation of the valuable sulphide minerals, however with varying degrees of success. Inevitably, there will be at least some loss of the valuable mineral with the gangue recovered from the pre-flotation system. Such losses represent an economic disincentive and should ideally be avoided.

The applicant has previously attempted to address this problem by providing a preflotation stage in which the major proportion of the non-sulphidic or naturally floating materials are separated from the valuable sulphidic mineral prior to the primary flotation step. In this process, which is subject of Australian patent application no 28746/95, a :mineral slurry is subjected to a sequence of mineral dressing operations in which an inert gas and/or reducing agent are added to the slurry to maintain an electrochemical potential S. conducive to the separation of the minerals by flotation.

However, apart from the requirement of an additional pre-float stage, such preflotation may adversely affect the recovery of the valuable sulphidic mineral in the subsequent primary flotation step.

It has been previously reported that nitrogen, with and without organic depressants, may have an effect in the recovery of nickel. These previous disclosures, however, generally use nitrogen as a flotation agent to maximise sulphide flotation eg pyrrhotite, pentlandite or pyrite which has nickel, cobalt or some precious metals associated therewith.

Increasing quantities of depressers are required to provide effective separation of the nickel and pyrrhotite for example.

In an effort to ameliorate at least some of the disadvantages of the prior art it is proposed to provide a method for conditioning a slurry or flotation concentrate which improves the separation of valuable sulphidic minerals from non-sulphidic "gangue" material.

SUMMARY OF THE INVENTION In a first aspect, the present invention provides a method of treating a slurry or flotation concentrate having a mixture of valuable sulphidic mineral and non-sulphidic "gangue" material wherein the milled slurry or flotation concentrate is conditioned with an inert, non-oxidising gas and/or a reducing/deoxifying agent to achieve a controlled Sdissolved oxygen content or electrochemical reduction potential conducive to the flotation of the valuable sulphidic material from the non-sulphidic "gangue" material by reducing the floatability of the "gangue" material, followed by flotation of the valuable sulphidic mineral from the non-sulphidic "gangue" material using an inert/non-oxidising gas as the .20 flotation gas, the conditioning step being conducted simultaneously with or prior to the flotation step.

S: In a preferred embodiment, the amount of inert/non-oxidising gas and/or reducing/deoxifying agent added to the milled slurry or flotation concentrate is sufficient to increase rejection of the non-sulphidic "gangue" minerals in a subsequent flotation step.

25 Alternatively, the inert/non-oxidising gas and or reducing/deoxifying agent may be added.

in sufficient quantity to improve selectivity between the valuable sulphide minerals and non-sulphide "gangue" minerals.

The applicant has found that non-sulphidic "gangue" minerals have an affinity for oxygen. Oxidation or attachment of oxygen to talc, for example, renders the material even more hydrophobic ie floatable, than in its natural state. The inventive method for conditioning a slurry or flotation concentrate overcomes at least some of the difficulties associated with the naturally floatable non-sulphide "gangue" minerals. Not wishing to be bound by any particular theory, the applicant believes such a conditioning step with nitrogen or other inert/non-oxidising gas, and optionally a reducing agent, creates an environment which physically and chemically removes oxygen from non-sulphide "gangue" minerals. This subsequently improves their rejection in the flotation process while not adversely affecting the recovery of the valuable sulphide minerals.

The conditioning step can be conducted simultaneously with or prior to the flotation step. To explain, as will be clear to persons skilled in the art, flotation may be carried out 15 in a mechanical flotation vessel or a pneumatic column. Such vessels and columns can have substantial residence times. While a slurry or flotation concentrate is resident in the S. flotation vessel or column, conditioning may be effected. Indeed, some flotation machines lend themselves to being used for conditioning prior to or simultaneously with the flotation step.

It will be understood that the term "inert/non-oxidising gas" used throughout this specification refers to commercial grades of such gases.

In a preferred embodiment, the inert/non-oxidising gas and/or the reducing/deoxifying agent are added to provide a dissolved oxygen content of less than 1 ppm.

-6- In another preferred embodiment, the inert/non-oxidising gas and/or reducing/deoxifying agent are added to provide an electrochemical potential of between 0 to -700 mV more preferably between -100 mV and -500 mV which is conducive to depression of the non-sulphidic "gangue" minerals.

MODE(S) FOR CARRYING OUT THE INVENTION The present invention will now be described by way of example only with reference to the accompanying figure 1 which is a flow diagram of a typical flotation circuit in accordance with an embodiment of the present invention.

The inventive process is suitable for ores related to mafic and ultramafic intrusions typically containing metal sulphides and precious metals and non-sulphide "gangue" minerals. Suitable ores for application of the process are shown in Table 1.

Specifically, the inventive process is particularly suitable for recovery of nickel eg millerite, valerite, pentlandite; copper eg chalcopyrite, chalcocite; precious metals such as gold, silver, platinum group metals (pgms) and commonly associated sulphides including 15 pyrite, marcasite, pyrrhotite, cobalt etc.

Suitable non-sulphide "gangue" materials which may be subjected to the present invention include magnesium bearing minerals, talc, lizardite, brucite etc and others such as antigorite, chlorite, certain micas, amphiboles etc and generally other so-called naturally floating minerals.

TABLE 1 TYPE MAJOR METALS EXAMPLES MINERALS* EXTRACTED ORES RELATED TO MAFIC AND ULTRAMAFIC INTRUSIONS Sudbury nickel-copper po, pn, py, cpy, viol Ni, Cu, Co, PGM Sudbury, Ontario Merensky reef platinum po, pn, cpy Ni, Cu, PGM Merensky Reef, South Africa JM Reef Montana ORES RELATED TO FELSIC INTRUSIVE ROCKS Tin and tungsten skarns py, cass, sph, cpy, Sn, W Pine Creek, California wolf Zinc-lead skarns py, sph, gn Zn, Pb Ban Ban, Australia Copper skarns py, cpy Cu, Au Carr Fork, Utah Porphyry py, cpy, bn, mbd Cu, Mo, Au Bingham Canyon, Utah copper/molybdenum Climax, Colorado Polymetallic veins py, cpy, gn, sph, ttd Camsell River, NWT ORES RELATED TO MARINE MAFIC EXTRUSIVE ROCKS Cyprus-type massive py, cpy Cu Cyprus sulphides Besshi-type massive py, cpy, sph, gn Cu, Pb, Zn Japan sulphides ORES RELATED TO SUBAERIAL FELSIC TO MAFIC EXTRUSIVE ROCKS Creede-type epithermal py, sph, gn, cpy, Cu, Pb, Zn, Ag, Au Creede, Colorado veins ttd, asp Almaden mercury type py, cinn Hg Almaden, Spain ORES RELATED TO MARINE FELSIC TO MAFIC EXTRUSIVE ROCKS Kuroko type py, cpy, gn, sph, Cu, Pb, Zn, Ag, Au Japan asp, ttd ORES IN CLASSIC SEDIMENTARY ROCKS Quartz pebble py, uran, Au Au, U Witwatersrand, South conglomerate gold- Africa uranium Sandstone-hosted lead- py, sph, gn Zn, Pb, Cd Laisvall, Sweden zinc Sedimentary exhalative py, sph, gn, cpy, Cu, Pb, Zn, Au, Ag Sullivan, BC lead-zinc (Sedex) asp, ttd, po Tynagh, Ireland ORES IN CARBONATE ROCKS Mississippi Valley type py, gn, sph Zn, Pb, Cd, Ga SE Missouri *ABBREVIATIONS used as follows: po pyrrhotite, pn= pentlandite, py pyrite, cpy chalcopyrite, viol violarite, cass cassiterite, sph sphalerite, wolf wolframite, gn galena, bn bornite, mbd molybdenite, ttd tetrahedrite, asp arsenopyrite, cinn cinnabar, uran uraninite 9 a.

9@ a a -8- Any inert or non-oxidising gas may be used with the present inventive process but nitrogen, argon, CO 2

SO

2 or admixtures thereof are particularly suitable. Suitable reducing/deoxifying agents include sulphoxy agents, SMS, MBS, sulphite agents, K, Ca,

NH

4 salts thereof, NaSH, Na 2 S etc and organic depressants for naturally floating minerals such as CMC, dextran, guar gum etc and modifications or derivatives thereof.

The applicants have found that the present inventive process provides improved oxygen removal from surfaces of non-sulphide "gangue" minerals thereby increasing "gangue" mineral rejection and improving valuable sulphide (particularly nickel) flotation metallurgy eg better concentrate grade in the flotation circuit. It has also been found that the present inventive process increases non-sulphide "gangue" mineral rejection and MgO rejection, if present, while maintaining existing valuable sulphide mineral (specifically nickel) recovery.

The present inventive process may be used for conditioning a milled slurry or flotation concentrate that has been exposed to reagents including collectors, frothers, 15 activators and organic depressants etc. According to the present invention, such a slurry or concentrate is conditioned with nitrogen and/or a reducing agent eg NaSH group, for a specific conditioning period prior to flotation to provide a controlled dissolved oxygen content or electrochemical reduction potential suitable for floating the valuable sulphidic minerals and sinking the non-sulphidic "gangue" minerals. Preferably the conditioning period is between one and six minutes.

Subsequent flotation is then carried out preferably using nitrogen as the carrier gas.

This process improves the selectivity between valuable sulphides and non-sulphide "gangue" minerals thereby improving the concentrate grade of the valuable sulphide at the same recovery levels and improving rejection of the non-sulphide "gangue" mineral.

-9- Figure 1 is a typical flow diagram of a flotation circuit. As shown in this drawing, the present invention is particularly suitable for, but not limited to, the final cleaning/scavenger circuits in which the valuable concentrate from the previous flotation circuit is dosed with a suitable reducing/deoxifying agent such as NaSH or Na 2 S and subjected to final flotation with nitrogen gas. The nitrogen gas and NaSH type reducing agent effectively suppress flotation of the non-sulphidic "gangue" minerals thereby increasing the recovery of the valuable sulphidic mineral.

Example 1 N 2 /NaSH conditioning with nitrogen flotation.

By way of example, two tests were conducted in which 1 kg charges of crushed ore containing disseminated nickel sulphide were slurried in salt water to obtain a pulp density of 60 wt% solids and milled in a stainless steel rod mill employing stainless steel rods to achieve P80 of approximately 160 microns. An appropriate quantity of collector eg sodium ethyl xanthate, was added to the mill.

The milled slurry was then repulped and deslimed in the 25 mm diameter Mosley 15 cyclone. The cyclone underflow stream was collected for flotation testing.

a.° The deslimed milled slurry was transferred to a 2.5 litre Denver flotation cell.

Frother and additional collector was added and the slurry was conditioned for a period of time prior to flotation.

S Flotation with air was commenced and a rougher concentrate and scavenger concentrate were produced from 3 and 27 minutes respectively of flotation. Additional collector and frother was added during flotation. The scavenger concentrate was then reflotated in 0.5 Denver cell at 700 rpm according to the following two methods: Test A Control Tests Using Air As The Flotation Gas Scavenger Concentrate Stage Reflotation Performance Product Assay Distribution Ni MgO Wt Ni MgO Conc 1 5.63 28.9 1.9 4.7 1.6 Cone 1+2 6.53 27.5 7.7 22.2 6.1 Cone 1+2+3 6.20 27.5 20.4 56.1 16.3 Feed 2.25 34.3 Test B Test Using N 2 /NaSH Conditioning Followed By Flotation With N 2 Gas In accordance with the present invention, in this test the scavenger concentrate was conditioned in a 0.5 L Denver cell at 700 rpm for 2.5 minutes with 1 L/min of nitrogen gas and NaSH additions as the reducing/de-oxifying agent. The NaSH addition was controlled by measuring and maintaining the sulphide potential (Es) at approximately -500 mV.

Flotation with nitrogen was commenced after conditioning.

Scavenger Concentrate Stage Reflotation Performance Product Assay Distribution Ni MgO Wt Ni MgO Conc 1 9.63 23.2 3.2 11.6 2.2 Cone 1+2 9.78 22.7 10.1 37.7 6.8 Cone 1+2+3 8.02 25.2 21.8 67.1 16.3 Feed 2.61 33.8

S.

.5 S S 11 Conc 1 is the first concentrate floated in the flotation test. Cone 1+2 is the combination of the first and second concentrates floated in the flotation test etc.

It is clear from the above results that Test B, using the inventive conditioning step provides a higher concentrate nickel grade and higher flotation recovery of nickel with a lower concentrate of MgO grade.

Example 2 Nitrogen Conditioning With Nitrogen Flotation In the second example, two tests were conducted where 1 kg charges of crushed ore containing disseminated nickel sulphides were slurried in salt water and ground in similar equipment as example 1 to achieve P80 of 75 microns.

The milled slurry was then transferred to 2.5 L Denver flotation cell and floated in a manner similar to example 1 to produce a rougher concentrate and scavenger concentrate.

The scavenger concentrate was then refloated in a 0.5 L Denver flotation cell as discussed in example 1.

Test C Control Test Using Air As The Flotation Gas Scavenger Concentrate Stage Reflotation Performance Product Assay Distribution Ni MgO Wt Ni MgO Cone 1 2.47 34.8 3.1 4.0 Conc 1+2 3.29 33.5 11.1 19.0 10.5 Cone 1+2+3 4.50 31.7 20.1 47.2 18.1 Feed 1.92 35.3 a a.

q. a a ar a a. a -12- Test D Test Using N 2 Conditioning Followed By Flotation With N 2 Gas In this test, the scavenger concentrate was conditioned in a 0.5 L Denver flotation cell with 1 L/min nitrogen gas addition. Flotation with nitrogen was commenced after conditioning.

Scavenger Concentrate Stage Reflotation Performance Product Assay Distribution Ni MgO Wt Ni MgO Cone 1 2.94 33.7 3.0 4.2 2.9 Cone 1+2 4.06 32.3 10.8 21.0 10.0 Cone 1+2+3 5.09 30.7 23.2 56.5 20.4 Feed 2.10 35.0 *o 0* °The test data indicate a slightly higher concentrate nickel grade, higher flotation orecovery of nickel and a slightly lower concentrate MgO grade in test E using the nitrogen conditioning step followed by nitrogen gas flotation.

Example 3 Nitrogen Flotation In this example, two tests were conducted on fresh samples of reagentised flotation

S*

plant feed slurry from an ore containing a mixture of massive and disseminated nickel **sulphide. This slurry assayed 1.7% nickel and 24% Mg. sulphide. This slurry assayed 1.7% nickel and 24% MgO.

°,oo* -13- The slurry was transferred to a 2.5 L laboratory flotation cell and flotated according to the following operations and reagent additions.

Operation Time Minutes Guar Addition, gpt SEX Addition gpt Conditioning 2 Flotation Concentrate 1 4 Conditioning 2 2 Flotation Concentrate 2 4 Conditioning 2 Conditioning 2 2 Flotation Concentrate 3 4 Conditioning 2 2 Flotation Concentrate 4 4 SEX Sodium Ethyl Xanthate 5 Each test produced four flotation concentrates and one flotation tail.

Test E Control Test Using Air As The Flotation Gas Flotation Feed Stage Flotation Performance Product Assay Distribution Ni MgO Wt Ni MgO Conc 1 8.30 12.2 15.6 77.6 Cone 1+2 6.36 15.5 22.7 86.5 14.8 Cone 1+2+3 5.70 16.4 26.3 89.7 18.2 Conc 1+2+3+4 5.34 17.1 28.5 91.0 20.4

C

C

a a.

a.

a. a a.

a.

a a a. -14- Test F Test Using N 2 For Flotation Gas Flotation Feed Stage Flotation Performance Product Assay Distribution Ni MgO Wt Ni MgO Cone 1 11.00 8.40 11.3 72.7 3.9 Cone 1+2 8.61 11.9 16.8 84.6 8.3 Cone 1+2+3 7.33 13.5 20.8 89.0 11.6 Cone 1+2+3+4 6.65 14.6 23.3 90.6 14.1 The above test data clearly indicates higher concentrate nickel grade and lower concentrate MgO grade in Test F than Test E.

It will be understood by persons skilled in the art that the present invention may be embodied in forms other than that shown in the present invention without departing from the spirit or scope of the present invention.

4 a a. a

Claims (12)

1. A method of treating a slurry or flotation concentrate having a mixture of valuable sulphidic mineral and non-sulphidic "gangue" material wherein the milled slurry or flotation concentrate is conditioned with an inert, non-oxidising gas and/or a reducing, deoxifying agent to achieve a controlled dissolved oxygen content or electrochemical reduction potential conducive to the flotation of the valuable sulphidic material from the non-sulphidic "gangue" material by reducing the floatability of the "gangue" material, followed by flotation of the valuable sulphidic mineral from the non-sulphidic "gangue" material using an inert/non-oxidising gas as the flotation gas, the conditioning step being conducted simultaneously with or prior to the flotation step.

2. A method as claimed in claim 1 wherein the amount of said gas and/or said agent *g added to the milled slurry or flotation concentrate is sufficient to increase rejection of the S•non-sulphidic "gangue" minerals in a subsequent flotation step.

3. A method as claimed in claim 1 wherein said gas and/or said agent is added in a sufficient quantity to improve selectivity between the valuable sulphidic minerals and non- sulphidic "gangue" minerals.

4. A method as claimed in any one of the preceding claims wherein said gas and/or said 0* "agent are added to provide a dissolved oxygen content in the slurry of less than 1 ppm.

5. A method as claimed in any one of the preceding claims wherein the said gas and/or said agent are added to provide an electrochemical potential of the slurry of between 0 to 700 mV.

6. A method as claimed in claim 5 wherein said gas and/or said agent are added to Sprovide an electrochemical potential of the slurry of -100 mV to -500 mV. -16-

7. A method as claimed in any one of the preceding claims wherein the valuable sulphidic mineral includes nickel, copper, precious metals such as gold, silver or platinum group metals (PGMs) and their commonly associated sulphides including pyrite, marcasite, pyrrhotite, cobalt.

8. A method as claimed in any one of the proceeding claims wherein said non-sulphidic gangue materials include magnesium bearing minerals, talc, lizardite, brucite, antigorite, chlorite, micas, amphiboles and other naturally floating minerals.

9. A method as claimed in any one of the preceding claims wherein said gas is selected from the group consisting of nitrogen, argon, neon, carbon dioxide, sulphur dioxide or admixtures thereof. A method as claimed in any one of the preceding claims wherein said agent is selected from the group consisting of sulphoxy agents, SMS, MBS, sulphide agents, K, Ca, NH4 salts thereof, NaSH, Na 2 S or organic depressants for naturally floating materials such as CMC, dextran, guar gum modification, derivatives or mixtures thereof.

I5

11. A method as claimed in any one of the preceding claims wherein the conditioning 9* with said gas and/or said agent is carried out for a time period up to 6 minutes.

12. A method for conditioning a slurry or flotation concentrate substantially as herein before described with reference to figure 1 and any one of the accompanying examples but excluding comparative examples. A DATED this 9th Day of March, 1998 BOC GASES AUSTRALIA LIMITED Attorney: PAUL G. HARRISON Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS