In a preferred embodiment the invention provides a process for removal of chromite from ilmenite. In a general aspect the process of the invention comprises three steps, namely

(i) a reduction step, optionally followed by magnetic separation;

(ii) a leaching or aeration step, and (iii) a magnetic separation step.

In step (i), the iron contents of the metalliferous ores containing chromite are partially converted to metal by gaseous or carbothermic reduction. After such reduction mineral which has not been metallised may be separated by magnetic separation. This mineral may include a proportion of chromite in the feed.

In step (ii), the product of step (i) is subjected to an alteration treatment such as leaching, aeration or oxidation roasting, in order to oxidize that portion of the metallic iron present which is susceptible to such treatment. It will be appreciated that where the chromium content in metal phases is originally relatively high, these metal phases may be chemically resistant and not respond to said treatment. In step (iii), material remaining magnetic after the treatment in step (ii) is separated by magnetic means. The magnetic material chiefly comprises particles retaining metal which was resistant to chemical treatment, e.g. metal of sufficient chromium content to exhibit passivity. Consequently, metallised chromite grains containing ferrochrome may be separated to a magnetic fraction, leaving a low chromium content non magnetics fraction.

Additional steps may be employed as will be described below.

The process of the invention may or may not include the gainful recovery of chromium values from the chromite originally present in the metalliferous ores. The separated chromium bearing fraction may be upgraded for sale or simply discarded.

In the prior art, chromite removal and recovery from iron bearing metalliferous ores is commonly carried out by magnetic separation. The mineral chromite has a spinel structure which frequently exhibits significant contents of magnesium and aluminium. Chromite composition is considered to fall within the ranges defined by the spinel end members FeCr ₂ 0 ₄ (chromite), MgAl ₂ 0 ₄ (spinel), e ₃ 0 ₄ (magnetite) and FeAl-O, (hercynite) and its composition within and between

mineral deposits varies dramatically. Consequently the magnetic susceptibility of chromite is not well defined, even within individual deposits. However, magnetic separations at field strengths of between 500 and 15,000 oersteds have enabled separations of chromite from various mineral sources. The separations are often difficult especially where high recoveries are desired.

An example of a difficult separation of chromite is the removal of chromite from ilmenite. Ilmenite concentrates are only useful for titania pigment production by sulphate solution and hydrolysis where the contained chromium levels are low (e.g. less than 0.1% Cr) since chromium acts to discolour white titania pigment.

Ilmenite from the Australian East Coast typically contains 0.5 - 2% Cr-,0-, and only a fraction of the ilmenite (e.g. 50%) can be recovered by magnetic separation as low chromium ilmenite. The remaining ilmenite has to be sold into low value applications such as a flux in blast furnace iron making or as a sand blasting medium or as abrasive. Alternatively it has to be sold at low prices for use in high cost processes for synthetic rutile production. Frequently ilmenite is stockpiled as an unsaleable by-product because of its chromium contamination.

Broadly the major problems associated with chromite removal and recovery evident in the prior art can be related either to poor recoveries in separation due to indistinct bands of magnetic susceptibility or other separation parameters concomitantly with the increased cost of the greater process sophistication required, which cannot be recovered from added value in the product.

It is the object of the present invention to overcome, or at least alleviate some of these difficulties.

According to a preferred embodiment of this invention a process is provided for removal or upgrading and recovery, if desired, of chromium values present in chromite in metalliferous ores or concentrates, more particularly ilmenite.

In step (i) of the process the iron present may be reduced by coke or coal, gaseous fuels, such as natural and petroleum gas and products thereof, or liquid fuels such as oil or products thereof. The temperature of reduction is generally maintained above 1000°C resulting in a mineral product in which the iron content of the iron bearing material is at least partially metallised. Various additives, e.g. alkali metal salts, chlorides, and magnesium or manganese compounds, may be added in the reduction step to assist separation.

Reduction may be carried out in any suitable device including fluidised beds, or preferably, rotary kilns which are well suited to reduction at high temperatures at high fuel efficiencies. Reduction is normally carried out in a temperature range of 1000 C-1300 and the reductant used, especially in kiln processing, is often coal, coke or char.

The lower temperature limit f"or reduction (usually 1000 C) is determined by both the temperature requirement for reduction of iron bearing minerals to metal and what is an appropriate temperature for reasonable reaction rates. The upper temperature is limited by a tendency for metals and oxides to form carbides and nitrides at high temperature under conditions of carbothermic reduction, and also by a sintering effect which occurs as metal segregates to particle surfaces during reduction, causing troublesome and uncontrollable agglomeration. The practical temperature limit imposed by these effects depends on the nature of the iron bearing mineral and the metallisation process and will vary from approximately 1100°C-1250°C.

During reduction the iron bearing minerals are in general partially metallised. Contained chromite grains may be partially metallised, remain non-metallised or be apportioned between these two behaviours. Overall reaction which occur during carbothermic metallisation of chromite are believed to be:

⁽ g ⁾

In view of said reactions the presence of both reacting species and product species in solid solution with other species in phases, such as spinel and ferrochrome alloy must be taken into account. Accordingly, it has been found that additives which may stabilize the spinel phase as iron is metallised, such as magnesium and manganese compounds or minerals, may assist in metallisation of chromite. It is not considered necessary for conversion of iron and chromium to metallic forms to be complete. Where chromite grains are metallised, degrees of metallisation as low as 10% will suffice to provide the desired separations under optimised conditions. Consequently reduction times of between 15 minutes and several hours can provide useful degrees of metallisation, depending on the desired separation.

After reduction and the attainment of the desired degree of metallisation, the material being heated must be cooled almost to room temperature in an essentially oxygen free environment. Cooling may be conducted in a cooler which forms an integral part of the reduction unit or in a separate cooling unit through which is passed an atmosphere of inert gases or reduction product gases. Separation of any carbonaceous material from the reduced minerals is then performed by a suitable combination of magnetic and size separations with the carbonaceous component recirculated, as appropriate. Mineral which has not been metallised (possibly

including a proportion of the chromite) is normally directed to the non-magnetic fraction for subsequent separation from carbon before carbon recycling.

In step (ii) of the process iron metal may be removed from the mineral particles which were not derived from chromite by suitable means. Chemical methods which act on metallic iron but maintain a sufficiently oxygen rich environment to provide a passivating oxide film on metal containing chromium are effective, e.g. oxidizing or aerated dilute acid attack, aerated aqueous dilute ferric/ferrous salt attack or simple aqueous aeration in the presence of an activating agent. Thermal oxidation roasting has also been found to be effective.

One method for metal removal which has been found to be successful is aeration in an aqueous medium in the presence of low concentrations (e.g. 0.5 - 1.0 wt.%) of ammonium chloride at temperatures near 80 C. Aeration is typically carried out for from 5 to 16 hours under strongly agitated conditions,the exact conditions depending on factors such as degree of metallisation in the reduced mineral and the desired degree of subsequent separation. Activating agents such as sodium and iron chlorides and some organic acids and salts have also been found to be effective.

The products of aqueous aeration are chrome free mineral grains from which the bulk of reduced metal has been removed, chrome bearing grains in which the passivated metal phase is still present, and hydrated iron oxides. The chromium content of the metal phase would typically be between 10 and 50 wt.%, although sufficient chromium for passivation need only be present in the metal. Within this range of chromium contents the metal retains room temperature ferromagnetism, rendering the chromium bearing grains well suited to magnetic separation.

The reactions occurring during aeration are believed to be of the type:

_ _

2Fe + 3/20 ₂ + H ₂ 0 = 2FeO (OH) 3Fe + 20 ₂ = ^Fe 3°4

Any production of magnetite (Fe^0 ₄ ) during aeration suggests that a clean direct separation of magnetic chromium bearing grains will not be possible by magnetic separation. However, the oxidation of iron during aeration results in the formation of finely suspended iron oxides such as goethite and magnetite in the aeration liquid at locations away from the metal surface. Washing of the mineral products in cyclones, or by agitation and decantation, or by wet screening will result in separation of iron oxides from the altered mineral products, leaving a separable magnetic chromium bearing product and a non-magnetic, virtually metal and chromium free one. Enhanced separations may be achieved by use of additives which stabilise formation of hydrated iron oxides rather than magnetite.

In step (iii) low intensity wet magnetic separation or dewatering and drying followed by dry magnetic separation is performed to produce chromium bearing and chromium free upgraded products. Typically magnetic field strengths of 150 - 1000 oersteds will produce the desired separations, although the separating conditions used will be strongly dependent on the degree of metallisation and grade and recovery considerations for each of the final products. Multiple stage separations may also need to be considered.

Other separation techniques such as froth flotation, heavy medium separation, other gravity separation techniques or electrostatic separation may also be applied to the separation of chromium bearing grains from the chemically treated mineral. In particular gravity or specific gravity based separations for chromium removal will be enhanced after metal removal from chromium free grains due to the reduction in mass of these grains as metal is removed.

The efficiency of a final magnetic separation will be dependent on the effectiveness of metallic iron removal from chromium free grains in the aeration or leaching step. This in turn is affected by the degree of segregation or concentration of metal in the reduction step at sites in individual mineral grains which will be available for aeration or leaching access. Such segregation will often be inadequate to allow complete metal removal, leaving up to one percent of iron metal in aerated chrome free grains. The presence of this iron metal may result in inefficient magnetic separations from chromium bearing metallised grains. This condition may be alleviated by additives to acid separation, including sodium or chloride bearing salts in the reduction stage, including but not limited to sodium chloride, sodium sulphate, sodium carbonate, ferric chloride, calcium chloride and magnesium chloride.

An optional processing step which may be applied as part of the present invention is a pre-oxidation of the charge aimed at improving the rate of reduction or altering the degree of metal segregation in the reduction stage.

Pre-oxidation may be necessary to prevent sintering in subsequent reduction, and is carried out by heating to within the temperature range 700 - 1300 C in any suitable arrangement such as a fluidised bed or rotary kiln, using air as the oxidizing agent. Typically pre-oxidized mineral may be fed directly to the reduction stage while hot or it may be cooled beforehand.

Further upgrading of the magnetic chromium bearing fraction may be carried out especially where segregation of metal has been encouraged. Grinding of said fraction to liberate contained ferrochrome followed by magnetic separation, froth flotation or other separation method, including sizing, will enable separation of a saleable ferrochrome product.

In yet another aspect the process of the invention may be applied to the treatment of a mineral mixture which contains chromite and other mineral contaminants which are less readily metallised in reduction than the valuable mineralisation to be recovered. Such mineral contaminants may include, e.g. rutile, leucoxene, quartz, zircon, monazite, xenotime, and many silicates as well as chromite. The reduction conditions may in this case be set such that chromite does not metallise and will then report to a non-magnetic product, separate from the magnetic metallised mineral to be recovered. In the case in which metallisation conditions are such that a proportion of chromium bearing mineral grains is metallised and a proportion is not, it is therefore possible to direct some chromite after reduction to a non-magnetic product and the remaining chromite after chemical treatment to a magnetic product, leaving behind an altered mineral product which is relatively free of chromite grains.

Example 1 In western Victoria there are a number of deposits of mineral sands containing the mineral ilmenite. The ilmenite concentrate produced as a magnetic fraction upon separation of these mineral sands contains overall approximately 0.93% Cr-0^ . Attempted magnetic separations on this ilmenite have resulted in three fractions at 0.65%, 1.16% and 1.20% Cr ₂ 0, accounting for 50.5%, 26.1% and 23.4% of the mineral weight respectively. Separate chromite grains have been shown to be the source of over 90% of the chromium contamination in the ilmenite. Until the advent of this invention it has not been possible using established mineral separation techniques to produce a mineral fraction of this material which is free of chromite grains, even at very low recoveries. Table 1 presents a chemical analysis of an ilmenite fraction which was used in the present examples. Table 2 provides the results of electron microprobe analyses

SUBSTITUTE SHΞF

performed on a number of chromite grains in the "as received" ilmenite concentrate. The obvious variability of the chromite composition is a major contribution to the difficulty of magnetic separation of ilmenite from contained chromite. Approximately 80% of the chromium in the ilmenite concentrate was shown to be present as separate chromite grains.

A mixture of 350g of the "as received" ilmenite and 300g of -4mm + 1.4mm Victorian brown coal char was prepared and placed above and below char layers in a high temperature alloy pot. The pot was placed in a muffle furnace and allowed to reach a charge reaction temperature of 1180 C over approximately one hour and held constant at this temperature for a further two hours. The pot and contents were then removed from the furnace and allowed to cool in air to room temperature. Screening and magnetic separation were used to separate char and reduced mineral.

The reduced mineral was subjected to aeration at 500cc air/minute in an agitated 10% suspension with aqueous 1% NH.C1 originally at 0.01% H ₂ S0.. Aeration was continued for 12 hours at 80°C. The products of aeration were wet screened at 38 pm aperature for separation of fine iron oxides. The iron oxide formed was predominantly magnetite

The aerated seperated and dried +38 micron product was subjected to magnetic separation via a Carpco induced roll lift type magnetic separator, operated at 0.1A current. In this separation 87.1% of the contained titania in the feed was recovered in a non magnetic fraction containing only 32.7% of the original chromium. The analyses of reduced, aerated and separated mineral products are provided in Table 3.

TABLE 1

Composition of Ilmenite Used in Examples

Components wt %

Ti0 ₂ 53.5

Fe (total) 30.4

^Si0 2 0.96

^A1 2°3 0.73 Zr0 ₂ 0.17

^Cr 2°3 0.65

^P 2°5 0.46

^V 2°5 0.23 CaO 0.05

MgO 1.58

MnO 1.60

TABLE 2

Grains Cr O, A1 ₂ 0 ₃ FeO MgO TiO. ^V 2°5 SiO. Na ₂ 0 Total

SUBSTITUTE SHEET

TABLE 3

Analyses of Various Fractions in Example 1

wt % Ti0 ₂ Fe (total) Fe(metal) MgO MnO Si ^Q ₂ ^A1 ₂ ° ₃ ^Cr ₂ ° ₃

Reduced

Ilmenite 59.86 34.01 31.60 1.73 1.66 1.29 0-79 0.84

Aerated -Ilmenite 82.40 6.34 2.39 2.42 2.09 1.72 1.38 1.07

Aerated

0.1A magne _t ics 63.54 12.30 0.28 2.55 2.34 2.58 1.94 4.24

Aerated 0.1A non- magnetics 86.32 4.47 0.22 2.36 2.13 1.51 0.98 0.45

Example 2

The above test was repeated with the exception that a 5% magnesite powder addition was made to the ilmenite prior to carbonaceous reduction. In this case a magnetic fraction comprising 23.2% of the titania in feed was found to contain 83% of the original chromium, producing a non-magnetic product with only 0.24% Cr ₂ 0 ₃ at 77.7% Ti0 ₂ -

Example 3

In a further test conducted in a similar way to Example 1, a magnetic separation rejecting 6% of the reduced material to a non-magnetic fraction (conducted at 0.04A on a lift type Carpco induced roll magnetic separator) was performed prior to aeration. Otherwise identical aeration was

4656 , .,

- 13 -

performed with the exception of the addition of 0.12% of an organic reagent which has the effect of stabilizing hydrated ferric oxides in place of magnetite as the iron oxide product of aeration. In the magnetic separation after reduction 4.1% of the titania present was rejected with 40.2% of the contained chromium. Following aeration a further 35.1% of the contained chromium was rejected to magnetics with a further 7.8% of the titania in the original feed. The final -63 micron, non- magnetic product had the composition 83.9% Ti0 ₂ , 0.23% Cr ₂ 0 ₃ . Table 4 provides analyses of the various fractions obtained.

TABLE 4

Analyses of Various Fractions in Example 2

wt %

Tio. Fe (total) Fe (metal) Cr ₂ 0 ₃

Reduced Ilmenite 58.10 32.30 32.20 0.75

0.04A magnetics (reduced) 60.41 35.00 31.8 0.50

0.0 A non-magnetics

(reduced) 38.50 19.0 13.8 5.08

Aerated 0.10A magnetics 70.70 12.40 2.05 4.13

Aerated 0.10A non-magnetics 83.90 4.01 0.14 0.23 (-63 micron)

SUBSTITUTESHEE _T