AU667437B2 - Primary beneficiation of ilmenite - Google Patents

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT PRIMARY BENEFICIATION OF ILMENITE THE FOLLOWING STATEMENT IS A FULL DESCRIPTION OF THIS INVENTION, INCLUDING THE BEST METHOD OF PERFORMING IT KNOWN TO ME:- 5 TECHNICAL FIELD This invention relates to a process which enhances the extraction of ilmenite from deposits of mineral sands, or mineral concentrates thereof and is a patent of addition to, being an improvement in or a modification 0 of, the subject matter described in our co-pending application 76298/91.

a O, BACKGROUND ART Mineral sands may contain many valuable minerals, among which are principally ilmenite, rutile, zircon, °o a leucoxene, monazite and gold. These minerals are extracted by using differences in density and differences in the magnetic and electrical properties of the individual mineral species to separate them from the less valuable mineral components of the sands, and from each other.

Several prior art techniques are available for the separation of mineral sands into their valuable components. The most common method is generalized in Figure 1 in block diagram form. The mineral sands are delivered as a wet raw sand to a gravity circuit (WET PLANT) to produce a coarse heavy mineral concentrate (HMC). This HMC may then be fed to a second stage

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raaLLII..- -2where the magnetic properties of some of the component minerals are used to effect a further separation and concentration.

Ilmenite is a composite of iron and titanium oxides and is weakly magnetic. Highly magnetic minerals, such as magnetite, are removed from the HMC by a low intensity magnetic separator. The residual material may then be subjected to a wet high intensity magnetic separation (WHIMS) stage to concentrat2 the ilmenite.

The WHIMS product may then be processed through an electrostatic stage in a DRY MILL.

The compound of particular interest for which ilmenite is the principal source is titanium dioxide, and the typical titanium dioxide concentration when the above prior art process is applied to ilmenite from the West Coast of The South Island of New Zealand ranges between 45%-47% TiO 2 with typical assays of silicon dioxide (silica) in the range of 4% to 6% and dialuminium trioxide (alumina) of 2% to By contrast, concentrates of West Australian ilmenites 0 commonly contain TiO, in excess of Due to the presence of iron oxides in ilmenite, the magnetic susceptibility of ilmenite can be increased by roasting under a variety of conditions. This increase in magnetic susceptibility is a well-known phenomenon and occurs through alteration of the chemical composition and crystalline structure, for example as discussed in the articles referred to below and allows the ilmenite to be readily separated from other minerals for example chromite, quartz, garnet and rutile, etc. by magnetic separation techniques.

A discussion of various prior art processes was included in our earlier application 76298/91 and will -3not be repeated here.

Ilmenite deposits are found in many countries for example South Africa, United States of America, Australia, India, New Zealand and other areas of the world. The ilmenite deposits in various countries and locations can differ in their-compositions.

In particular the ilmenite found in the South Island of New Zealand contains abundant inclusions and selvedges of silicate minerals. Metallurgically these inclusions have the effect of lowering the magnetic o° susceptibility and conductivity of grains of ilmenite containing inclusions, while enhancing the content of silica and alumina and other deleterious compounds in an ilmenite concentrate with a consequent relative o depletion of the titanium dioxide content. Such o composite grains can be difficult to separate magnetically or electrostatically, and can result in 20 lower than average yields and higher than average S0° capital anrid direct operating costs than are usual in the mineral sands industry.

The South Island of New Zealand ilmenites also occur in common association with abundant garnet. The garnet has a specific gravity and size range close to that of the ilmenite and this also creates problems in the first stage of gravity separation in the known processes. The magnetic susceptibility and conductivity of this garnet are also close to those of the ilmenite such that the employment of the known separation stages is costly while the loss of ilmenite from the process is also high.

Because the silicate inclusions give significant "inbuilt" levels of silica and alumina in a slag or synthetic rutile feedstock, it is important to remove -4discrete crystals such as garnet, quartz or other deleterious silicate minerals in the mineral dressing process. The conventional mineral dressing process as shown in Figure 1 can remove nearly all the unwanted discrete minerals from a West Coast South Island of New Zealand mineral sand but at the cost of an overall recovery ranging from 65% to 75% of the ilmenite. The best ilmenite concentrate that can be achieved may contain from about 1% to 2% of discrete silicate minerals and will assay approximately 46.5% to 47% titanium dioxide. When this concentrate is processed in an electric arc smelting furnace it can provide, according to Figure 3, an equivalent of approximately 73%-83% titanium dioxide in slag, depending on the level of iron (FeO) in the slag acceptable in the slag-making process and to the consumer.

DISCLOSURE OF INVENTION The present invention seeks to overcome disadvantages in the prior art and to provide a modified process for the sepa'ration of ilmenite ores from raw sands including those with high garn.t content or minerals such as chromite and some radioactive materials.

Another object of the invention includes enhancing the Ti0 2 content by removing silicate selvedges and inclusions, where such are present.

According to one broad aspect of the invention there is provided a process for the separation of ilmenite from raw sand, or mineral concentrates thereof, which includes the steps of, a single stage magnetising roast in a roaster, other than a fluidised bed roaster, at a te;,perature in the range 650 0 C 900 0 C using ~RA4/ 1 i i -L IPY~it~i an excess of carbon as hereinafter defined to provie an atmosphere in which the oxygen potential is controlled; followed by a i a low tj medium intensity magnetic separation stage.

According to another aspect of the invention there is provided a process for the separation of ilmenite from raw sand, or mineral concentrates .hereof, which includes the steps of, in sequence: a specific gravity separation stage; a low intensity rmagnetic separation stage; a single stage magnetising roast in a roaster, other than a fluidised bed roaster, at a temperature in the range 650 0 C 900 0 C using an excess of carbon as hereinafter defined to provide an atmosphere in which the oxygen S potential is controlled; and a low to medium intensity magnetic separation 20 stage.

ii SAccording to a further aspect of the invention there is i provided a process for the separation of ilmenite from i raw sand,or mineral concentrates thereof, which includes the steps of, in sequence: I a specific gravity separation stage; a low intensity magnetic separation stage; a single stage magnetising roast in a roaster, other than a fluidised bed roaster, at a temperature in the range 650C 900 0 C using an excess of carbon as hereinafter defined to provide an atmosphere in which the oxygen potential is controlled; a cooling stage comprising cooling of the roasted ore under controlled conditions; and a low to medium intensity magnetic separation S-stage.

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1 -6- The cooling stage may be performed gradually, for example over a period of one and a half hours to cool the roasted ore to ambient temperature, or may be performed rapi while preventing oxidation, for example by forced cooling within typically 15 minutes either directly or indirectly with water or a neutral gas so ac to prevent contact with oxygen or air.

According to a further aspect of the invention there is provided a process for the separation of ilmenite from raw sand, or mineral concentrates thereof, of the type having a high relative concentration of i deleterious silicates (including garnet) including the I steps of, in sequence: a specific gravity separation stage; a low intensity magnetic separation stage; a single stage magnetising roast in a roaster, other than a fluidised bed roascer, at a temperature in the range 650 0 C 900 0 C using an excess of carbon as hereinafter defined to provide an atmosphere in which the oxygen potential is controlled; a low to medium intensity magnetic separation stage; a grinding stage; and a low to medium intensity wet magnetic separation stage.

An attritioning stage may be introduced between the magnetising roasting and the low to medium intensity magnetic separation stages with or without a cooling stage.

9 -V rdr r, i- i. i -i -6a- A high intensity magnetic separation stage may be introduced prior to the roasting stage. This stage may employ a WHIMS or a rare earth drum magnet.

The magnetising roast may be performed in a rotary ji kiln, or any type of roaster within the knowledge of a person skilled in the art may be employed, for example any other type of kiln such as a brick kiln but ii excluding, for the purpose of this patent of addition,

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i R i E -Y^mtn~ -7a fluidised bed roaster. A magnetising roast employing a fluid bed roaster is described in Australian Patent Application 76,298/91 (649,441), to which this application is a Patent of Addition.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a conventional separation process; i0 Figure 2 is a block diagram of a first embodiment of the process according to the present invention; Figure 3 is a diagram relating titanium dioxide in ilmenite to titanium dioxide content in slag; Figure 4 is a block diagram of a second embodiment of the process according to the present invention; Figure 5 is a Molar Ternary Diagram of the TiO 2 -FeO-Fe 2 3 system; Figures compare the stability of the inventive process to that of the prior art at various roasting temperatures; and Figure 7 is a block diagram of a third embodiment of the process according to the present invention.

i PREFERRED MODES FOR CARRYING OUT THE INV I4TION As shown in Figure 2 the process accordigjg to one aspect of the invention relates to the processing of ilmenite in deposits with high relative concentration of silicate and garnet materials and comprises the conventional step of first passing the raw sand through a wet gravity concentration stage (step 1), followed by screening (step and the removal of the highly susceptible minerals such as magnetite by low L

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-8intensity magnetic separation (step The resulting product may then be passed through a high intensity magnetic separation stage (step 3a) before being introduced into a roaster, (step in which the temperature, oxygen potential, residence time and cooling are carefully controlled. The roaster product may then be attritioned (step and then passed to a low to medium intensity magnetic separation stage, (step Though a fluid bed roaster or rotary kiln is shown in Figure 2 any type of roaster within the knowledge of a person skilled in the art may be employed, for example a brick kiln or other type of kiln.

Depending on the characteristics of the ore being treated, it may not be necessary to screen (step 2) or attrition (step or grind (step 7) the ore.

Concentrates from step 6 show a significant improvement in the recoveries of ilmenite, as compared to Levels achievable by conventional methods.

In the roasting operation, (step the magnetic susceptibility of the ilmenite fraction can be enhanced by a factor of up to 50, depending on the atmosphere and other factors selected, whilst the magnetic susceptibility of the silicate and other deleterious minerals, including garnet, remains virtually unchanged.

Following the roasting operation, (step and attritioning, (step the enhanced magnetic susceptibility enables a clean separation of the ilmenite fraction from the other mineral components, using a low to medium intensity magnetic separation (step 6).

i- -9- The flowsheet outlined above in effect does away with the primary DRY PLANT beneLiciation concentration procedures in common use in the mineral sands industry worldwide and replaces them with a roasting/low to medium intensity magnetic separation.

The process also pretreats ilmenite for the manufacture of synthetic rutile, or for the manufacture of titania slag.

.i 4' With respect to New Zealand South Island ilmenite, reduction in the garnet and silica components of the resulting concentrate optimises the smelter feed in the slag-making process, and the quality of the final ilmenite product may be enhanced by introducing a grinding stage, (step as shown in Figure 2. After 04. grinding, a high quality concentrate is then achievable with only about a 3% by weight loss. This loss is understood to be mostly accounted for by the removal of deleterious silicate material still persisting in the concentrate prior to the grinding stage, (step and of some of the silicate inclusions and some of the silicate selvedges attached to the edges of the ilmenite grains. The output from the grinding, (step is then passed through a low to medium intensity wet magnetic separation (step 8).

The resultant ilmenite product shows an enhanced concentration of the titanium dioxide as shown in Table i.

The inventive process results in an assay of the resulting ilmenite product of approximately 49% titanium dioxide compared with the assay employing the conventional process of approximately 46.5%. In addition, the silica and alumina concentrations are significantly reduced, and these differences provide substantial commercial advantages over the conventional heavy mineral sand processing methods.

The inventive process allows a lower grade HMC to be accepted from the Wet Plant or gravity-processed stage, (step than would normally be desirable.

For example, a 25% (approx.) ilmenite concentrate can be acceptable compared with a-35% (approx.) ilmenite concentrate in the prior art techniques. In such a circumstance recoveries can be increased by S° 10 approximately 4% overall, while reducing capital and o o operating costs.

o TABLE 1 COMPARATIVE ASSAYS I o o Product of Flowsheet Product of Conventional of Figure 2 Flowsheet of Figure 1 S 0: TiO 2 48.9% 46.6% SiO 2 3.8% 4.78% Al 2 0 3 1.17% 1.95% The known techniques for the separation of ilmenite o from mineral sands with high concentration of garnet o may result in low recoveries of ilmenite and may require a large and costly DRY MILL to remove the volume of garnet waste.

The inventive process does not require a DRY MILL process. Overall recoveries of ilmenite are significantly enhanced and consequently the overall direct operating costs are lower than for conventional processes, and the mineable reserves of deposits are extended.

Depending on the type of ore being treated, the roasting temperature, (step can range between 650 0 C to 900 0 C (but preferably is in the range Fee $227.00 File: C94 049 -11- 7 50 0 -850 0 C),and residence time can range between minutes and 90 minutes or more depending on the ore.

The wide temperature range and long residence time has the advantage of simplifying operating conditions and thereby allowing ease of control.

The invention stabilizes the roasting reaction in the zone of maximum magnetic enhancement (Figure 5) by 10 controlling the oxygen potential so that for an o ilmenite with a high Fe 2 0 3 :FeO mole ratio the reaction oo condition may be reducing and for an ilmenite with a .i low Fe 2 0 3 :FeO mole ratio the reaction condition may be s oxidizing. Others (Bozorth et al, Ishikawa, or Curnow Parry) have established that maximum magnetic enhancement is achieved when the mole ratio Fe 2 0 3 :FeO is within the range 1:1 and 1.57:1 (shaded region 24 i in Figure For most ilmenites the reaction condition is mildly oxidizing.

The reaction stability is achieved by using excess carbon fuel mixed with the ilmenite feed stock and S" combusted with air in amounts so that the amount of oxygen in the exit gas is readily maintained at the level most suited to the particular ore type being processed In most cases this will be within the range 0.1% to 1.0%.02 by volume of the exit gases.

Thus the invention is applicable to ilmenites of i 30 different composition such as, but not restricted to, the examples shown in Table 2 below.

TABLE 2 EXAMPLE %BY WEIGHT FeO Fe 2 0 3 1. Westport (New Zealand) 37.6 3.2 I 1 -12- 2. Richards Bay Africa) 22.5 25.0 3. West Australia 24.0 18.0 4. West. Australia 33.9 13.2 Queensland 18.8 21.9 6. New South Wales 16.2 22.6 I 7. Tellnes (rock ilmenite) 34.6 11.6 Figure 6 illustrates the difference in results i achieved from a reaction that is not buffered by 1 0 excess carbon and one that is. The unbuffered i i '1 reaction results in a sharp curve 30 as compared to i the smoother curve 32 for the buffered reaction according to the invention thus allowing better control in plant practice.

Figures plot the magnetic susceptibility versus roasting time at roasting temperatures respectively of 750 0 C, 800 0 C and 850 0 C. Each curve shown in broken line, demonstrates that the resultant susceptibility as a function of time using i high percentage oxygen atmosphere roast employed in the prior art peaks and then falls within a narrow time window. The prior art is thus more susceptible i to an inconstant result or requires more rigid control. The process according to the invention is graphed in curves 32,shown in unbroken line, from Swhich it is clear that maximum susceptibility is achieved more gradually tending to a plateau with time. This result provides a more efficent and more S 30 easily controlled process compared to the prior art.

Throughout the specification the term "carbon" while including carbon per se charcoal) includes "carbon containing" or carbonaceous con ounds, for example CO, CO steam, or hydrocarbon fuels in addition to or in place of the char used in the examples described herein. The excess of carbon used 1; -13may thus be in part supplied by the fluidising gas and/or the bed of the roaster.

In one series of experimental tests, the following parameters were used: Feed to roaster 5000 g Heavy mineral concentrate 1000 g Recycled char 600 g Bituminous coal Temperature of oO roaster bed 800 0

C

Residence time in roaster 60 minutes Fluidising gas Air Roaster atmosphere (exhaust) 0.3% to 0.5 02 Do a After roasting under these conditions, it was possible S. 20 to separate "magnetically enhanced ilmenite" from the gangue minerals at better than 98% recovery of the ilmenite component, using a low intensity magnetic separator.

The mass magnetic susceptibility (10-6m 3 /kg) at a field strength x field gradient of 1,0T 2 /m of the roaster feed and product were as follows: TABLE 3 Ilmenite Garnet Roaster Feed 0.9 0.9 Roaster Product 50.0 0.9 The heavy mineral concentrate used for the example cited above was specifically Westport (New Zealand) concentrate but similar results were obtained in other experimental tests using other ilmenites which did not

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i I-P *~sllUP l~ -14contain silicate inclusions and hence did not require a grinding stage (step and subsequent magnetic separation stage, (step That is, only a low to medium intensity magnetic separation stage was necessary after roasting. In some cases the mass magnetic susceptibilities were measured at between and 100.

Thus a second embodiment of the invention, as shown in 1 0 Figure 4 includes conventional stages of gravity separation screening and attritioning, (12), 0 followed by a low intensity magnetic separation stage, to remove highly magnetic materials such as magnetite. Subsequent roasting, followed by a low to medium intensity magnetic separation stage, results in a high recovery of ilmenite, In addition to providing a mechanism whereby ilmenite can be readily and economically recovered from mineral sands in general and recovered and upgraded and separated from garnet in the particular case of West Coast, South Island, New Zealand ores, or separated from deleterious chromite and/or zhrome spinels as is the particular case of Eastern Australian ilmenite, or from magnesia-rich minerals as in the case of Norwegian rock ilmenite (Tellnes), the invention provides a single stage roasting reaction which has the additional effect of pretreating the ilmenite so that the reactivity of ilmenite is enhanced and the mineral thereby made amenable to synthetic rutile production by selective leaching of its iron content by hydrochloric acid. Other known processes in the prior art require multiple stage roasting to achieve the same effect.

Yet a further improvement in the magnetic susceptibility has been found to result from L -1 I- El controlling the rate of cooling of the roasted product.

For example, in one series of tests, four identical samples of ilmenite were roasted for 90 minutes in separate runs using an excess of coal/char as fuel as previously described. Two runs were conducted at 800'C and two at 850 0

C.

At the completion of the roasting, one of each of the separate temperature runs was rapidly quenched in i S, water bath, while each of the remaining samples was ,cooled gradually to ambient temperature over periods ranging up to 90 minutes.

S When cooled, each of the four samples was cleaned of residual char, magnetically separated from gangue minerals and tested for magnetic susceptibility, with the results shown in Table 4.

i r 4 44 TABLE 4 Mass magnetic susceptibility 6 m 3 /kg) at a field strength S 5Rs x field gradient of 1,OT 2 /m 25 Roasting Temperature 800 0 C 850 0

C

Quenched 38 Cooled 58 76 Therefore, a third embodiment of the invention comprises the steps set out in Figure 7 where in between the steps of roasting 16 and magnetic separation 18 a cooling step 17 is performed as described above. Controlled cooling of the roasted product, compared to quenching, enables an improved recovery of the roasted ilmenite in the magnetic separation stage due to the further improvement in c i -16magnetic susceptibility.

Though the invention has been described above with respect to preferred embodiments thereof it is to be understood that variations in the above-described method are contemplated within the knowledge of a person skilled in the art. For example, the roasting temperature, atmosphere and residence time of step 4 of Figure 2, or step 16 of Figure 4 can be varied within parameters determined by suitable 0 experimentation. In addition, the grinding stage of 0 step 7, when required, can be varied within parameters 0 determined by suitable experimentation. In addition, o the grinding stage of step 7 of Figure 2 when carried out seeks to produce grains in the range from minus 125 microns to plus 75 microns together with the grading of the resultant product. It is contemplated that these ranges are not absolute but relative to the o feed stock and are determinable by experiment within the knowledge of a person skilled in the art.

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Claims (10)

1. A process for the separation of ilmenite from raw sand or mineral concentrates thereof including the steps of, in sequence: a specific gravity separation stage; a low intensity magnetic separation stage; a single stage magnetising roast in a roaster other than a fluidised bed roaster at a 0 temperature in the range 650 0 C 900 0 °C using an excess of carbon as hereinbefore defined to o 10 provide an atmosphere in which the oxygen S0. 0 potential is controlled; and 0 ,0 a low to medium intensity magnetic separation stage.

2. A process as claimed in claim 1 further including the step of coolinr, the product of said roasting stage under controlled conditions prior to said low to medium intensity magnetic separation S' stage.

3. A process as claimed in claim 1 or claim 2 Sfurther including, subsequent to said low to I' medium intensity magnetic separation stage, the Ssteps of: t i a grinding stage; and 'I 25 a low to medium intensity wet magnetic ]separation stage.

4. A process as claimed in any one of claims 1-3 further including, prior to said magnetising roasting stage, the step of: a high intensity magnetic separation stage. A process as claimed in any one of claims 1-4 wherein said magnetising roast is psrforcmed in a 4rotary kiln using bituminous coal and recycled u' char. i i -18-

6. A process as claimed in claim 5 wherein said atmosphere is controlled to have an oxygen concentration in the exit gases of 0.1% to 1.0% by volume.

7. A process as claimed in claim 6 wherein said roasting atmosphere is air.

8. A process as claimed in claim 7 wherein said roasting temperature is in the range 750C So 850 0 C. o 10 9. A process as claimed in claim 8 wherein said roasting stage is performed for a residence time of 30-90 minutes or longer depending on the ore. 000 10. A process as claimed in claim 8 when appended to claim 2, wherein said cooling stage lasts for a time of between 15 and 90 minutes. o o11. A process as claimed in any one of claims 2-9 0:0 when appended to claim 2, wherein said cooling SOo stage is performed rapidly within substantially o 2O minutes while preventing oxidation.

12. A process as claimed in claim 11 wherein said rapid cooling is performed with water or a neutral gas while preventing contact with oxygen or air.

13. A process as hereinbefore described with respect to any one of Figure 2, 4 or 7 wherein roasting is performed in a roaster other than a fluidised bed roaster.

14. Ilmenite ore when treated according to a process as claimed in any one of claims 1-13. r -i c ;-19- ABSTRACT A process for enhancing ilmenite from deposits of mineral sands or mineral concentrates comprises a single stage fluidised bed or rotary kiln magnetising roast 16. A temperature of 650°C-900 0 °C in an excess of a carbonaceous fuel (such as coal/char, CO or hydrocarbon) is used to provide an atmosphere in which the oxygen potential is controlled resulting in a consistently high magnetic susceptibility product. Rc, asting has been used before but the current process O requires a lower intensity magnetic separation stage 18 and provides improved recovery (even of tailings produced by prior art roasting process). Controlled 15 cooling 17 of the roasted product improves the resultant magnetic susceptibility. A high intensity magnetic separation stage may be introduced prior to the roasting stage. Ilmenites having inclusions or selvedges of silicate 0. °ominerals may be further improved by employing a grinding step after magnetic separation prior to slagging or use as synthetic rutile feedstock. S. 25 The roasting stage potentiates the ilmenite for leaching in the production of synthetic rutile. sh. i I M