FIELD OF THE INVENTION The present invention relates to a process for treating red mud or for treating a concentrate containing desilication product from red mud.

BACKGROUND TO THE INVENTION The Bayer process is widely used in the manufacture of alumina. The Bayer process involves contacting bauxite with a strongly alkaline digestion solution at elevated temperatures and pressures. The most commonly used alkaline digestion solution is caustic soda. The alkaline digestion solution dissolves aluminium compounds in the bauxite to form a pregnant digestion liquor. After appropriate residence time in the digestion stage, the pregnant liquor is separated from undissolved solids in a clarification or settling stage. Alumina is subsequently recovered from the pregnant liquor by passing the pregnant liquor through a precipitation stage to recover aluminium hydrate precipitates. The aluminium hydrate precipitates are subsequently calcined in a calcining stage to form particulate alumina.

One of the by-products of the Bayer process is the slurry containing undissolved solids that is separated from the pregnant liquor. This slurry is commonly known as red mud. The red mud contains iron oxide compounds (which give it the characteristic red colour), other metal oxide compounds and silica compounds. The red mud also contains quantities of entrained alkaline digestion solution.

Once the red mud has been separated from the pregnant liquor, it is washed to remove some of the entrained alkaline digestion solution. The red mud is typically then discharged to tailings dams or to red mud pans. Bauxite ore typically contains aluminium compounds, iron oxide compounds, clay, silt and silica compounds. During the digestion process, the alkaline digestion solution dissolves the aluminium compounds. However, silica compounds are also readily soluble in alkaline digestion solutions and therefore the free silica compounds and clay materials in the bauxite are also dissolved during digestion. This causes increased consumption of alkaline material.

In some Bayer process plants, the bauxite is subjected to a desilication step prior to the digestion step. In the desilication step, the bauxite is contacted with alkaline solution at atmospheric temperature and elevated pressure in a desilication leaching vessel. This results in dissolution of silica compounds and some aluminium compounds. The residence time of the liquor in the desilication vessels is controlled such that a desilication product precipitates out in the desilication vessel. This reduces scale buildup in the digestion vessels.

In the Bayer process plants that do not utilise a separate desilication step prior to digestion, a desilication product will precipitate during the digestion step. The desilication product forms part of the solids that report to the red mud. The desilication product (which is also referred to as sodalite) is a product that contains sodium, silica and alumina. The desilication product is normally discharged with the red mud and it represents a significant loss of both soda and alumina from the Bayer process.

Various methods have been proposed for recovering soda and alumina from the desilication product. Many of these proposals involve reacting the desilication product with lime (calcium oxide) at high temperature. However, these proposals have not found wide commercial use, most likely due to their high lime consumption.

International patent application number PCT/AU97/00073 describes a method for recovering soda and/or alumina values from desilication product by mechanically activating the desilication product to induce a mechanochemical reaction whereby soda and/or alumina values are solubilised. In this process, red mud may be treated directly to mechanically activate the desilication product. Alternatively, the red mud could be subjected to an initial treatment to produce a desilication product-containing concentrate, with that concentrate subsequently being subjected to mechanical activation. This patent application also states that the particle size of the red mud/desilication product may be reduced by grinding or the like prior to mechanical activation.

In this International patent application, it was stated that the recovery of soda and/or alumina values can be enhanced by mechanically activating the desilication product in the present of reagents. Suitable reagents include oxides and hydroxides such as CaO, NaOH and Ca(OH) ₂ .

International patent application number PCT/AU97/00073 states that, preferably, the red mud/desilication product concentrate is mechanically activated in a mechanical mill. The expression mechanical mill as used in the patent application is to be understood to include ball mills, nutating mills, tower mills, planetary mills, vibratory mills, attritor mils, gravity dependent type ball mills, jet mills, rod mills, high pressure roller mills and the like. In a preferred embodiment, a thermally insulated high energy mill, such as an attritor is used: According to International patent application number PCT/AU97/00073 , with such high-intensity mills power inputs of the order of 100kW/m ³ can be achieved.

In the examples given in International patent application number PCT/AU97/00073, a red mud slurry was loaded together with 3 kg of 6 mm grinding balls into a 1 liter capacity horizontal attritor mill operated with a rotor speed of 600 rpm. Milling time varied from 15 minutes to 60 minutes. Soda and alumina recovery decreased with decreasing milling time. For example, test number 16 used a milling time of 15 minutes and obtained soda recovery of 32% and alumina recovery of 5.5%. Test number 14 used a milling time of 60 minutes and obtained a soda recovery of 48:1% and an alumina recovery of 1 1.9%. To the best of the applicant's knowledge, the process described in International patent jpplication number PCT/AU97/00073 has not been used commercially. Throughout this specification, the term "comprising" and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicates otherwise. The applicant does not concede that the prior art described in this specification forms part of the common general knowledge in Australia or elsewhere.

BRIEF DESCRIPTION OF THE INVENTION In a first aspect, the present invention provides a method for treating red mud containing a desilication product or for treating a desilication product-containing concentrate obtained from red mud, the method comprising grinding the desilication product for a period of less than 15 minutes. In one embodiment of the present invention, the desilication product is subject to grinding in a high intensity mill having a power input of at least 50 kW/m ³ , more preferably between 50 to 600kW/m ³ , even more preferably between 80 to 500 kW/m ³ , even more preferably around 100 to 500k W/m ³ , even more preferably 150 - 500 kW/m ³ , even more preferably 200 to 400 kW/m ³ , most preferably about 250 to 350 kW/m ³ .

In some embodiments of the present invention, the desilication product is subject to grinding for a period of from 0.5 minute to 15 minutes, preferably from 0.5 minute to 10, even more preferably from 0.5 minute to 5 minutes. In one embodiment, the method of the present invention utilises a horizontal shaft grinding mill. Examples of a suitable horizontal shaft grinding mill is a horizontal shaft grinding mill as described in some embodiments of United States patent 5,797,550, or such as a horizontal shaft grinding mill as manufactured and sold by Xstrata Technology under the trade name IsaMill™ . Other horizontal shaft grinding mills or modified IsaMills, or vertical stirred mills may also be used. The IsaMill utilizes circular grinding discs that agitate the media and/or particles in a slurry. A classification and product separator keeps the grinding media inside the mill, allowing only the product to exit.

The mill used in the present invention may have a power of at least 500kW. More suitably, the mill has a power of at least 750kW. Even more suitably, the mill has a power of 1MW or greater. Preferably, the mill has a power from 1MW to 20 MW. In this regard, the power of the mill is determined by the power draw of the motor or motors powering the mill. In preferred embodiments of the present invention, the grinding mill comprises an IsaMill (as described above). In the IsaMill, a series of stirrers are positioned inside the grinding chamber and these stirrers are rotated by an appropriate driven shaft. The high power intensity is achieved through a combination of high stirrer speed and compression of the media arising from back pressure applied in the grinding mill. Suitably, the tip speed of the rotating stirrers falls within the range of 5 to 35 meters per second, more preferably 10 to 30 metres per second, even more preferably 15 to 25 metres per second.

The stirrers used in an IsaMill are typically discs. However, it will be appreciated that an IsaMill may be modified to use different stirrers and the present invention encompasses use of such modified mills. It will also be appreciated that other horizontal or vertical stirred mills may also be used in accordance with the present invention where those other stirred mills incorporate appropriate rotating structures, for example, peg mills, mills that are stirred by a rotating auger flight, tower mills etc. The tip speed of those rotating apparatus preferably falls within the ranges given above.

Other high power intensity mills may also be used, provided that the mills have the minimum power input as specified above. Examples of other mills that may be used in the present invention include vertical or horizontal stirred mills such as those produced by Deswik, Metso Minerals (Verti Mill and Stirred Mill Detritor (SMD)), Nippon Eirich Co. as well as Union Process Inc The mill used in the present invention suitably utilises a grinding media. Preferably, the grinding media is a man-made grinding media. Examples of man-made grinding media that may be used in the present invention include ceramic grinding media, steel or iron grinding media or grinding media based upon metallurgical slags. By "man-made grinding media", it is meant that the grinding media has been manufactured by a process that includes a chemical transformation of a material or materials into another material. The term "man-made grinding media" is not meant to encompass materials that have been treated solely by physical means, such as tumbling or screening of natural sands. The grinding media may have a specific gravity that falls within the range of 2.2 to 8.5 tonnes per cubic metre.

In some embodiments, the method of the present invention utilises a ceramic grinding media. The specific gravity of the ceramic grinding media preferably falls within the range of 2.4 to 6.0 tonnes per cubic meter. More preferably, the specific gravity of the grinding media is greater than 3.0 tonnes per cubic meter, even more preferably about 3.2 to 4.0 tonnes per cubic meter, yet even more preferably about 3.5 to 3.7 tonnes per cubic metre.

The ceramic grinding media may comprise an oxide material. The oxide material may include one or more of alumina, silica, iron oxide, zirconia, magnesia, calcium oxide, magnesia stabilized zirconia, yttrium oxide, silicon nitrides, zircon, yttria stabilized zirconia, cerium stabilized zirconia oxide or other similar hard wearing materials.

The ceramic grinding media is preferably generally spherical in shape although other shapes may also be used. Even irregular shapes may be used.

In other embodiments, the present invention utilises iron or steel grinding media. In these embodiments, the grinding media is suitably in the form of spheres or balls, although other shapes may also be used. The specific gravity of steel or iron grinding media normally is greater than 6.0 tonnes/m ³ , more preferably about 6.5 to 8.5 tonnes/m ³ . Other embodiments of the present invention utilise metallurgical slags or other byproducts available to the operator as the grinding media. The metallurgical slag or other byproducts may be used in the form of irregular shaped particles or, more preferably, as regular shaped particles . If regular shaped particles are used, those particles are suitably of generally spherical shape. However, it will be understood that the present invention also extends to using other shapes.

It will be. appreciated that other grinding media may also be used in the present invention. The grinding media may be added to the grinding chamber such that it occupies from 60% to 90% by volume of the space within the grinding chamber, or even from 70 to 80% by volume of the space within the grinding chamber. However, it will be appreciated that the present invention also encompasses a grinding method in which the grinding mill has a volumetric filling of less than 60% of grinding media. The grinding media may be present in an amount such that the weight ratio of grinding media to red mud or the ratio of grinding media to desilication product-containing concentrate obtained from red mud in the mill (calculated on a dry basis) falls within the range of 1 to 15, more preferably within the range of 5 to 15, even more preferably within the range of from 5 to 10. These ranges are significantly lower than used in international patent application number PCT/AU97/00073, which used a weight ratio of grinding balls to red mud of 20.

In some embodiments, the present invention includes further step of removing a slurry from the mill and separating a liquid component from the slurry. The recovered soda and/or alumina will report to the liquid fraction. The liquid fraction may be further treated to recover soda and/or alumina therefrom. Alternatively, the liquid fraction may be returned to the Bayer process circuit.

In some embodiments of the present invention, the desilication product is treated in the milling step in the presence of one or more reagents. Suitable reagents include those that are capable of reacting with the desilication product to solubilise soda and/or alumina.. Some examples include oxides and hydroxides, such as CaO, NaOH and Ca(OH) ₂ . Suitably, the reagents are present with the desilication product in the milling step. The reagents may be added to the mill as a separate stream to the stream containing the desilication product. More preferably, the reagent(s) is mixed with the stream containing the desilication product prior to feeding the desilication product stream to the mill. This is desirable in that separate feeding arrangements to the mill for the reagent stream are not required.

The present inventors believe that, somewhat surprisingly, utilisation of very short grinding times, optionally in conjunction with high specific power inputs during grinding, can result in the recovery of significant amounts of soda and/or alumina ^" contained in the desilication product. As the grinding time of the desilication product in the present invention is minimised, large throughputs can be achieved and the large quantities of desilication product generated in the Bayer process can be efficiently treated to recover soda and/or alumina from the desilication product. In some embodiments, a lower ratio of grinding media to red mud is used than in previous attempts to treat red mud.

EXAMPLES: Environmental:

In one embodiment, the present invention provides a method for treating red mud to enable it to be used for remediation purposes by reducing or eliminating the desilication product present in the red mud. By remediation, it is meant that the red mud is treated to enable vegetation of some type to establish itself on the treated red mud and stabilise it. The treated red mud to be used for remediation will be discharged to a tailings dam or red mud pans, and may be used by itself, or with materials such as mulch, fertilizers, chemicals, gypsum, soil modifiers or the like, either added to the red mud during processing or post processing, to enable the red mud to be used for remediation. In these applications either all, or part of the red mud is treated to enable the remediation of the red mud to take place. The red mud may also be treated by itself, or with reagents that are capable of reacting with the desilication product to solubilise soda and/or alumina, as described previously, to ensure the treated red mud can be used for remediation.

When the red mud is treated to reduce or eliminate the desilication product present in the red mud, the priority of the process is to enable the red mud to be used for remediation purposes, and the soda and/or alumina that is removed from the mud may be retreated in the Bayer process, or transformed into alumina and/or soda and/or other useful products, or in some cases be treated as a waste product.

In another embodiment, the present invention provides a method for partial treatment of red mud as it is produced, prior to discharging to dams or ponds. In this embodiment the red mud may only be partially neutralised of the desilication product, and, while not being fully remediated to allow vegetation to be established on it, the treated red mud will contain less desilication product than red-mud that has not been treated by the invention.

Those skilled in the art will appreciate that the present invention may be subject to variations and modifications other than those specifically described. It will be understood that he present invention encompasses all such variations and modifications that fall within its spirit and scope.