AU633608B2 - Leaching of graphite ores - Google Patents

Description

aaarar 633608 Form COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: PJ 5721 Lodged: 11 August, 1989

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Complete Specification Lodged: Accepted: Published: Priority Related Art Name of Applicant o o o Address of Applicant INDUSTRIAL MINERALS RESEARCH AND DEVELOPMENT PTY. LTD.

CURTIN UNIVERSITY OF TECHNOLOGY SCHOOL OF APPLIED CHEMISTRY HAYMAN ROAD, BENTLEY, W.A. 6102 M.E. KEENEY Suite 16, 32 Jubilee Street South Perth, W.A. 6152 WATERMARK PATENT TRADEMARK ATTORNEYS.

LOCKED BAG NO. 5, HAWTHORN, VICTORIA 3122, AUSTRALIA Actual Inventor Address for Service Complete Specification for the invention entitled: "LEACHING OF GRAPHITE ORES" The following statement is a full description of this invention, including the best method of performing it known to I L I -2- LEACHING OF GRAPHITE ORES This invention relates to the refining of graphite ore by chemical to remove associated minerals from the ore after it- as been subjected to a preliminary physical separation treatment.

Graphite is found in nature as a black lustrous soft mineral and occurs mostly in metamorphic rocks. Due to its softness it marks other substances readily and is greasy to touch. It is virtually infusible, resistant to chemical attack and a good conductor of heat and electricity. These properties make it widely applicable for a number of industrial uses.

Graphite is graded according to its flake size, carbon content and type and amount of impurities. Generally 15 the larger the flake size and higher the carbon content the more valuable the graphite. Normal or crucible grade 0 graphite must have a carbon content of 85% and be coarser than 150 mesh. Graphite used for pencils, lubricants and o 20 batteries requires a grade of about 94% and for electric motor brushes a grade of 98% carbon is required.

Some refined graphite products commercially available have unacceptable levels of natural minerals particularly silica in the form of quartz but also 0000 kaolinite, magnesite and iron substituted magnesite.

0.0 5 It is an object of this invention to produce a refined graphite of more than 97% graphite.

"o To this end the present invention provides a method of treating graphite ore comprising subjecting the beneficiated ore to an acid leaching treatment followed by a 30 caustic leaching treatment.

It is thought that the two stage process eliminates in the correct sequence the acid soluble and the alkaline soluble gangue material.

The sequential leach process eliminates most gangue minerals present in the flotation concentrate but it does not reduce the amount of quartz. Surprisingly, it has been found that the process liberates quartz to the extent that K ^s "W 7 C I i o 9, o os *99 0 q 99 04 4 o 0 0 o P4 a a 3 it may be separated by other physical methods such as flotation, gravity concentration or heavy media separation.

Using the sequential leach coupled with such a physical separation technique, a flake concentrate assaying 98%+ carbon can be obtained.

ACID LEACH It is preferred that the ore subjected to the two stage leaching method of this invention is an attritioned ore. Preferably the attritioned ore has been obtained following the steps outlined in the copending Patent application 62& U. ted-n PJ 72?.

Preferably the conditions of the acid leaching step are to use acid at a concentration of at least 7.5% v/v to form a slurry of about 40% solids content at a temperature 15 above 25 0 C for a time sufficient to achieve the required degree of leaching. The most preferred acid concentrations were in the range of 15 to 30% v/v.

A preferred temperature is about 50 0 C and generally higher temperatures have only a slight effect on the leach rate and little effect on the final grade of the refined product. A leach period of 6-8 hours is generally sufficient and at lower acid concentrations and 50 CA12 hours is quite sufficient. The preferred leach acid is HCl but HNO 3 or H2So are also suitable. Agitation during leaching is also desirable.

CAUSTIC LEACH The second stage leach is a caustic -leach tag preferably using NaOH. The caustic concentration _+hfA~ae above 25% w/w and 50% w/w is most preferred. Again P rpir e- r%\0 rv> temperature conditions shouldAbe at least 25 C preferably above 50 0 C and most preferably about 80 0 C. The solids content is generally about 30% due to higher viscosities and agitation during leaching is desirable.

At 80 0 C and 50% w/w caustic a leach time of 3 to 4 hours is sufficient.

"3YSICAL SEPARATION In order to remove the remaining gangue, flotation e j ~an~ o* 00 a a a 9 a Ga a a~ a aa a *a a -4or a heavy media separation treatment is applied to the caustic leach concentrates.

The following is a description of experimental results for the preferred 3 stage process.

HC1 Leach Study The results indicate that elevated temperatures 500C) result in the highest carbon grade for each acid strength. Higher temperatures have no effect on the final grade and have only a slight effect on the rate of the leach.

Acid strengths of 30% v/v and 15% v/v produce nearly identical leaching rates over all temperature ranges and are superior to 7.5% v/v. Presumably the 7.5% v/v solution has insufficient HCl, at 40% solids, to leach the Sgangue. A leach of 6-8 hours gives the highest achievable grade of carbon. This period is somewhat variable depending upon the initial grade and content of the flotation concentrate.

A strong orange-colour imparted to the HCl solution indicates the iron level in the graphite is strongly effected by both the acid concentration and the temperature of the leach solution. Figure 8 shows a marked difference in the iron content with acid concentration. Under these conditions, elevated temperatures 75 0 C) compensate 25 for the slower leaching rate of the iron. However, even at 0 C, the lowest iron-contaminant level in the graphite is achieved within 12 hours.

It is thought that the acid leach is effective in removing the alumina and also the iron and magnesium oxides 30 leaving a silica matrix which can not be removed until the second alkali leach stage.

The magnesite content quickly diminishes at 50 0

C,

with acid strength not playing a significant role in the rate of dissolution.

At the end of the HCI leaching, differences in elemental analysis can be seen, as shown in Table 1. X-ray diffraction analyses on HCl leached samples show quartz and I I kaolinite as the major contaminants.

Table 1 Sample Fe203 Si02 CaO Mg AI203 Na 0 LOI 1).

Attritioned ore 8.2% 12.0% 0.65% 5.7% 5.9% 0.13% 66.1% After HC1 0.89% 14.0% 0.01% 0.10% 8.7% 0.007% 75.8% leach 2).

Attritioned ore 5.4% 18% 1.3% 2.4% 4.7% 0.13% 67.2% After HC1 <0.01% 19% 0.01% <0.01% 0.6% 0.06% 82.6% leach 50 0 C 15% v/v HC1 S" 15 From the HC1 leach studies, it appears that a 500C leach o with an acid strength of 15% v/v would be effective in the first stage of the upgrading of the graphite flotation concentrate. The time over which the leach is conducted is |o determined primarily by the rate of iron dissolution. About 12 hours at 500C is required to minimise iron levels in the graphite concentrates used for these tests.

To reduce the leach time, a slightly higher acid S o. strength should be used. However, HC1 concentrations should IHo be maintained below 25% v/v. Above this, strong fuming by 25 the acid creates handling problems.

Improved leaching rates and increased removeal of acid soluble gangue is also attained if surfactants such as Teric are added to the leaching solution. Any acid stable surface active agent is suitable in improving the process.

30 NaOH Leach Study ooAcid leached flotation concentrates were subjected to a series of leaching experiments with sodium hydroxide under different temperatures and concentrations. Leach temperatures ranged from 25 0 C to 135 0 C with NaOH concentrations of 50% w/w to 12.5% w/w.

The HCl-leach base material used for the caustic leach work was not the highest grade achievable. The HC1 I_ g 6 leach was conducted in 30% v/v HC1 at 25 0

C.

Based upon the colour of leach solutions showing iron dissolution, it was assumed all gangue materials were also completely leached.

Although the assumption was incorrect, trends were still seen in the caustic leach indicating the optimum caustic leach conditions.

The efect of NaOH concentration at room temperature on leaching is minimal.

At 50 0 C, the 50% w/w caustic has the largest effect on carbon grade. Lower caustic concentrations (25% w/w and 12.5% w/w) have only a small effect.

At 80 0 C using 50% w/w caustic solution, the rate of 1 the leach is substantially increased. The highest grade is 15 achieved within 3-4 hours. Increasing temperatures to reflux Sdoes little to effect the leach rate.

t The NaOH leach leaves quartz behind as the major ,a gangue mineral. Only small amounts of other gangue minerals Sare observed by X-ray analyses and optical microscopy.

1, 20 Furthermore, qua'rtz liberation from the graphite matrix is achieved following the caustic leach, without significant attritioning of the graphite flake, suggesting additional upgrading is possible.

Leaching rates are increased and silica removal is 25 increased if surface active agents are added to the caustic i leach solution S °Table 2 shows elemental analysis following the two-stage leach.

00 t S° 30 Table 2: Elemental Analysis after NaOH Leach Sample Fe 03 Si2 CaO Mg0 Al 2 0 Na 2 0 LOI 1).

Attritioned ore 8.2% 12.0% 0.65% 5.7% 5.9% 0.13% 66.1% HCl leach 0.89% 14.0% 0.01% 0.10% 8.7% 0.007% 75.8% NaOH leach *0.12% 12.0% 0.01% 0.06% 0.6% 0.006% 86.3% 7 2).

Commercial 2.20% 7.2% 0.94% 0.23% 2.9% 0.040% 86.3% graphite*** Attritioned ore After HC1 and NaoH 5.4% 18% 1.3% 2.4% 4.7% 0.13% 67.2% I Ir 4 r leach 0.97% <0.01% <0.01% 0.01% 98.9% 50°C 15% v/v 80 0 C 50% w/w Sample from China The low Al 03 analysis indicates that all of the SiO 2 is present as quartz. This has been confirmed by XRD analyses. The conditions which produce the best results in the caustic leach are a concentrated caustic leach (50% w/w) at 80 0

C.

The final grade of the graphite will vary depending upon the amount of quartz present in the flotation concentrate. The quartz levels in the graphite samples used in these tests were high, samples running at 11-14%. Removal of this gangue by refloat or by gravity concentration techniques will result in a exceptionally high grade flake graphite.

Recycling %it 25 44 4 *9

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In the preferred two-stage HCl/NaOH leach process developed for upgrading graphite, SiO0 and Al 2 03 levels in the caustic solution increase due to the dissolution of clays in the ore. Close-loop recycling of the NaOH leach solution, necessary for the economic viability of the process can result in supersaturation levels sufficient for the precipitation of sodium silicates and sodium aluminosilicates onto the surfaces of the graphite flake.

This produces unacceptably high levels of SiO2 and Al 0 in the graphite product. However, an acid wash of the graphite, following the caustic leach, is extremely effective for dissolving the precipitated aluminosilicate species. The -8wash could be conducted in the HCl solution used for acid leach, thereby avoiding the necessity for a separate crystallisation circuit to remove silica and alumina contaminants from the caustic leach liquor.

In any leaching operation, recycling of leach liquors can result in the excessive contamination of the solution with species detrimental to the leach process unless techniques are employed to limit impurity levels.

Purging of a side-stream is the most common industrial method for controlling impurity levels in a process liquor stream. Unfortunately, liquor purging can be extremely expensive, especially if the process stream contains valuable chemical species.

1 The two-stage HCI-NaOH leach process for upgrading Munglinup graphite ore requires both the HCl and NaOH to be recycled for the process to be economically viable.

During the development work on the caustic leach process, it was found that the use of recycled caustic leach liquors resulted in a flake graphite with high levels of Al 2 03 and Si0 2 From a preliminary evaluation of the leach liquor and graphite chemical analyses from the caustic recycle tests, it was suspected that the problem was not due to ineffective leaching of the clays by the recycled caustic solution but to the precipitation of sodium aluminosilicate S 25 species onto the graphite surfaces. Sodium aluminosilicates are extremely soluble in acid solutions and any material precipitated onto the graphite from recycled caustic solutions should be easily dissolved using an acid-wash step S 3 following the caustic leach.

To confirm the presence of precipitated sodium aluminosilicates on the caustic-leached graphite and the ability of an HCl wash to dissolve aluminosilicate species, graphite leached in recycled caustic was examined by elemental analysis and X-ray diffraction before and after an HCl wash. No attempt was made to optimise the acid wash step, only to confirm the ability of an acid wash to fully dissolve the precipitated aluminosilicates.

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z i' i' i I 9 Experimental Graphite samples from the third and fourth caustic recycle were first treated by heavy media separation (TBE/CC1 sg 2.45) to remove the quartz gangue liberated in the caustic leach.

Acid treatment of the caustic-leached, HMS-treated graphite was conducted using 10 gram samples of graphite in 150mL of a 20 weight HCl solution. The slurry was gently agitated at ambient temperature for six hours. The acid-washed graphite was filtered, washed repeatedly in deionised water and dried overnight at 100C.

Samples of the graphite were analysed by X-ray diffraction and ICP techniques before and after acid washing.

15 Results 4* II 1 I 1.

#4 11 I I ti *r I bitr lull I I 61 1

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.41800 X-ray diffraction results confirmed the presence of sodium aluminosilicate as a contaminant in the HMS-treated graphite following the fourth caustic recycle leach. The species has been identified as a hydrated form of NaAlSi 206 The flake graphite appeared to be a light grey colour, with an absence of the 'shine' found in graphite samples leached in fresh caustic. No sodium aluminosilicate species were found after the HC1 wash step and the graphite was a shiny, black flake.

25 Analytical results for the graphite samples are shown below. All values are in percentages.

Sample Si02 Al 2 0 Fe 2 0 MgO CaO Na20 LOI Initial 19.0 0.6 0.1 0.0 0.0 0.1 83 4th cycle 10.0 7.7 0.2 0.0 0.1 1.3 73 4th cycle HCl wash 4.8 0.5 0.0 0.0 0.3 0.0 The results show a complete removal of both soda and alumina contamination in both samples confirming the X-ray analyses that the sodium aluminosilicates have been removed. The reduction in the silica content of the 4th-cycle sample agrees with this result.

If the quartz content of the samples is discounted,

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10 the final graphite has LOI values between 95 and 99%. This result shows that even after four recycles of the caustic liquor and the precipitation of sodium aluminosilicate, the caustic solution is capable of efficient leaching.

If the acid wash step is conducted such that the spent wash-acid is sent to the initial HCl leach circuit, a separate crystallisation circuit to remove the alumina and silica contaminants from the caustic leach liquor may not be necessary.

An acid wash following the caustic leach is capable of completely removing any sodium aluminosilicates which may precipitate on the graphite surfaces as a result of crystallisation from solution. The alumina and silica o contaminants in the caustic leach liquor do not appear to 15 affect the efficiency of the caustic leach.

Therefore, with proper circuit design, a crystallisation circuit to remove the silica and alumina a a. contaminants from the caustic leach liquor may not be 0 o necessary.

020 A further variation to the invention is to use a vigorous acid leaching step to avoid contamination of the caustic liquor circuit with aluminium species.

Under vigorous conditions, a concentrated HCl leach removes most of the alumina from the graphite.

Complete removal of iron and magnesium oxides is as expected. Neither the moderate nor the vigorous HCl leach 0 *removed any silica from the graphite.

83% graphite was recovered from the vigorous HCl leach. This agrees with the losses of alumina, iron and magnesium oxides as determined by the elemental analysis.

When a moderate HCl leach of float concentate is used in the two-stage HCl/NaOH process for upgrading Munglinip graphite, SiO and Al 03 levels in the caustic solution increase. Closed-loop recycling of the NaOH leach solution results in supersaturation levels sufficient for the precipitation of sodium silicates and sodium aluminosilicates onto the surfaces of the graphite flake.

C III~P-Lf t ie 11 This produces high levels of SiO and Al 20 in the graphite product. An acid wash following the caustic leach is effective in completely removing these surface deposited species and an acceptable flake of 90-95% total carbon is obtained after five recycles of the caustic leach solution.

Hence, precipitation of sodium aluminosilicates onto the graphite surface does not appear to affect the leaching efficiency of the recirculating caustic solution.

When a more vigorous HCl leach is used, the graphite entering the caustic cycle is almost fully depleted in alumina. Alumina levels in the recirculating caustic solution are kept to a minimum and the solution can be repeatedly recycled without precipitation of aluminosilicates onto the graphite surface. Discounting quartz contribution, a graphite flake of 95%+ total carbon can be obtained.

Flake Attrition/Size Analysis Two fractions (+250 micron and -250 micron +106 micron) from the flotation concentrate were leached by the sequential acid/caustic leach process to determine the extent of flake attrition during leaching.

Conditions chosen for the leach were those that gave the most favourable results from testwork previously discussed. Samples were taken prior to and at the completion of each leach stage and sized on the Endecotts wet sieve shaker. Results from these tests can be seen in the following attached sheets. The original material was rescreened to simulate the attritioning effect of the stainless steel screens.

30 The +250 micron fraction can be seen to show the largest degree of attritioning with about 15% loss of the coarse material to the finer fraction. It should be noted that a higher degree of attritioning would be expected on a laboratory scale in such a small vessel and this may not be encountered on the industrial scale. Chemical attack does not appear to cause significant flake attrition.

The finer fraction (-250 micron to +106 micron) 4t. i I *1 or 4 -4t* 0r 1~_1 12 appears to experience negligible attritioning to due the leaching testwork. It can be concluded that careful handling of the coarse material would be required during the leaching stage to maintain the level of coarse mineral in the concentrate.

Physical Separation Following the two stage leaching process it is preferred to subject the leached ore to a physical separation process to remove the remaining gangue from the graphite. Heavy media separation, gravity concentration or froth flotation are possible.

Heavy Media Separation HMS testing on final leach concentrates using a :1 65% tetrabromoethane/remove quartz (sg 2.7) from flake graphite (sg 2.2) and 35% chloroform mixture (sg 2.45) confirmed the ability to further upgrade the grahpite by physical methods. Carbon analysis conducted on HMS concentrates resulted in a grade of Conventional heavy media techniques are used wherein the heavest quartz is 20 allowed to settle and is then removed. The process is represented several times.

Flotation of Graphite is..

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j 1iI r; i i Froth flotation of the leached graphite concentrate was carried out to obtain the final upgraded graphite 25 product. The success of the final flotation depends on the effectiveness of the leaching step to liberate the quartz gangue from the graphite surface.

The flotation was carried out at a much slower agitator rate than the flotatiouns of the original core 30 samples, to reduce the possibility of silicate material being projected into the froth from the action of the agitator. A large amount of sodium silicate solution was added to depress flotation of the quartz by the maximum possible amount. This would depress quartz material that 35 still may not have been completely liberated from the graphite. The elemental analyses of the flotation concentrates and tails are shown in table 9. The first,

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I I 0 C 13 second, third and fourth floats are taken at longer time intervals over the separation process.

Table 9 Elemental composition of final flotation concentrates and tails.

1st 2nd 3rd 4th Float Float Float Float Tails LOI 100.0 100.0 100.0 99.8 17.2 Al 2 0 3 0.2 <0.1 0.1 0.3 0.8 Fe 2 03 <0.1 <0.1 <0.1 <0.1 0.3 SiO 2 0.37 0.20 0.02 0.56 84.0 CaO <0.01 <0.01 <0.01 <0.01 0.18 MgO <0.01 <0.01 <0.01 0.02 0.13

NA

2 0 0.01 <0.01 <0.01 0.02 0.21 The elemental analysis shows the effectiveness of the final flotation step. The quartz gangue component is effectively removed with an 84% yield in the tails. The gangue material in the graphite concentrate has been effectively reduced to zero. The variation in the silicate 20 values from the elemental analysis for the first three floats does not contribute significantly to the overall graphite concentration. However the fourth float has a noticeable quartz component present and subsequent floats would produce concentrates with greater quartz :25 concentrations.

Accordingly it can be seen from the above that this invention provides a method of improving graphite quality.

Claims (7)

1. A method of refining graphite ore which comprises subjecting the ore to an acid leaching treatment followed by a caustic leaching treatment.

2. A method as defined in claim 1 in which the two leaching treatments are followed by physical separation of gangue from graphite.

3. A method as defined in claim 2 wherein the physical separation step is preceded by an acid wash of the leached ore.

4. A method as claimed in any one of claims 1 to 3 in which the acid leaching step is carried out using 7.5% v/v of mineral acid to form a slurry of the ore and heating the slurry at a temperature above 25 0 C for a time sufficient to liberate the acid soluble gangue material. A method as claimed in any one of claims 1 to 4 in which the caustic leaching stage is carried out above 25% of caustic solution at a temperature above 25 0 C for a time sufficient to liberate the caustic soluble gangue.

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6. A method as claimed in claim 4 or 5 in which the acid leaching step is carried out with 15% v/v mineral acid i rabove 50°C with agitation and part of the acid leaching liquor is used to acid wash the ore following the caustic o leach step.

7. A method as claimed in any preceding claim in which a surface active agent is added to the leaching solution in either or both leaching stages. r8. A method as claimed in any one of claims 2 to 6 in which the physical separation step is froth flotation. DATED the 9th day of August, 1990. KINDUSTRIAL MINERALS RESEARCH AND DEVELOPMENT PTY. LTD. WATERMARK, PATENT TRADEMARK ATTORNEYS, 290 BURWOOD ROAD, HAWTHORN, VIC. 3122. AUSTRALIA. DBM:jl(7L0.3) 0. o