THIS INVENTION relates to mineral processing. In particular, the invention relates to the treatment of iron ore.

There is an increasing demand and incentive in the field of mineral processing to treat low grade ores. One of the problems with treating these ores is that the input costs required to obtain an acceptable recovery are often not justified. In particular, one of the problems encountered with the treatment of flaky or banded low grade ores is that significant comminution energy is required to obtain acceptable liberation of the valuable minerals. In addition, ultra fine grinding of such ores (required for efficient liberation) increases the slimes content and reduces downstream efficiencies. It is also desirable to liberate more efficiently impurities from certain low grade ores.

It is an object of this invention to at least alleviate these problems.

According to one aspect of the invention, there is provided a method of assisting the liberation of a first mineral from a particulate low grade ore containing said first mineral in a layered grain structure with at least a second mineral, said method including increasing the aspect ratio of grains of the low grade ore containing the first mineral by exposing the low grade ore to microwave energy.

In this specification the term "low grade ore" is meant to include any iron ore in which the valuable mineral content is at a concentration that does not exceed about 55% (w/w) iron. The first mineral may be an iron-containing mineral. The low grade ore may be banded ironstone. Typically, banded ironstone includes iron-containing layers of haematite and goethite separated by gangue material such as silica.

The microwave energy may be at a frequency of between 300 MHz and 300 GHz, preferably from 300 MHz to 50 GHz, more preferably from 300 MHz to 20 GHz and, most preferably from 300 MHz to 5 GHz.

The low grade ore may be exposed to microwave energy while being held at or below atmospheric pressure.

The microwave energy may have an input power of at least 1 kW.

The low grade ore may be continuously irradiated with microwaves for a predetermined time period. Instead, the low grade ore may be exposed to pulsed microwave energy for a predetermined time period.

The low grade ore may be exposed to microwave energy for less than 5 seconds, preferably less than 1 second and, more preferably, between 0.5 seconds and 0.001 seconds.

The low grade ore may have a particle size of 100 % passing 100 mm and, preferably, larger than 2.5 mm. Preferably, the particle size is smaller than 100 % passing 53 mm.

The method may be carried out as a continuous or batch process.

The low grade ore may be held in or may be passed through a mono-mode microwave cavity during exposure to the microwave energy. The mono-mode cavity may be the same as or similar to that which is described in US 5,824,133, which is incorporated herein in its entirety by reference.

The low grade ore exposed to microwave energy may subsequently be subjected to further thermal shock, e.g. by quenching. According to another aspect of the invention, there is provided a method of assisting the liberation of phosphate-containing minerals from iron-containing mineral grains in a particulate low grade ore having said iron-containing mineral in a layered grain structure with the phosphate-containing minerals, said method including exposing the low grade ore to microwave energy.

The ore may have a particle size of 100 % passing 100 mm and most preferably 100 % passing 53 mm. This ore is subsequently typically milled down to a preferred particle size of 5 mm and, most preferably, down to 2.5 mm to achieve substantial liberation of phosphate minerals from the ore.

The phosphate-containing minerals may be liberated from specularitic haematite present in banded ironstone thereby decreasing the phosphate mineral content.

The bulk phosphate mineral content of the banded ironstone present as P2O5 may be reduced to less than 0.21 % by mass. Typical phosphate containing minerals include apatite, goyazite, gorceixite and florencite.

The microwave energy and its application may be as hereinbefore described.

The method may also include subjecting the low grade ore to thermal shock, e.g. by quenching.

The invention will now be described by way of the following non-limiting example and the accompanying illustrations in which Figure 1 shows a microscope photograph of ore treated in accordance with the invention; Figure 2 shows a microscope photograph of the same ore as in Figure 1 , but milled in conventional fashion; and Figures 3 and 4 show microscope photographs of phosphate mineral associations with haematite. Example

10kg of banded ironstone particles having an iron concentration of about 47% (w/w) and particle size of - 31 + 26mm was exposed to microwave energy for 1 second in a microwave apparatus having a 80mm diameter mono-mode cavity supplied by SAIREM. The sample was quenched after the microwave exposure. The input power was 15kW and the operating frequency was 2.5GHz. The exposed product was subjected to compression using a 100 ton press (supplied by Goldquest) at a pressure of 25 MPa to reduce the particles to a size range of 1 mm to 4.75mm. During microwave exposure localized increases in temperature were experienced. Maximum localized temperatures of about 8000C were experienced with a bulk temperature of less than 4000C.

Figure 1 of the illustrations shows the grain structure of the ore after treatment in accordance with the invention observed under a Wetzlar microscope in reflected light. Blocky specular haematite particles (white) which have broken parallel to the lamina direction are visible. The width of field of view = 1.6 mm. Figure 2 shows a sample of the banded ironstone which was milled in accordance with conventional techniques. A 30 kg sample was milled down to -4.75 mm in a rod mill for 20 minutes and the -1.4 mm size fraction was mineralogically examined. Blocky specular haematite (white) with earthy haematite (black) is visible. The width of field of view = 1.6 mm.

As is evident from Figure 1 the microwave treated sample has mineral containing grains having a generally higher aspect ratio (i.e. the ratio of the length of the grain to its width). Particles in Figure 1 which are generally rectangular were found to be more abundant than the untreated particles shown in Figure 2 of the illustrations due to even breakage.

Surprisingly, it was also observed that particles having layered grain structure were found to give the best response to the treatment. In many cases, the layers split violently when the heated particles emerged from the mono-mode cavity. Particles consisting predominantly of haematite with little or no layered grain structure showed a tendency to heat and cool without significant fracturing. Mineralogical analysis shows that the aspect ratios of the specularitic haematite found in the banded ironstone vary between 2 to 2.5 in liberated ore. Particles having an average length of 350 μm and a width of 235-300 μm were observed. Banded ironstone that has been subjected to the method in accordance with the invention exhibits an increase in aspect ratio of up to about 5. Particles having an average length of 650 μm and a width of 250 μm were observed.

The phosphate content in banded ironstone is about 0.05 % to 0.06 % (about 0.21 to 0.28 % P2O5). Conventional milling as described above reduced the phosphate content to 0.035 %. In the example in accordance with the invention the phosphate content of the treated ore was reduced to below 0.035% and, more particularly, to below 0.03%. Figures 3 and 4 show some phosphate mineral associations, such as apatite and goyazite, respectively, with haematite.

In Figure 3, a particle of apatite (light grey) intergrown with haematite (white) and containing small intergrowths of muscovite (dark grey) is visible. In Figure 4, goyazite (indicated by the arrow), comprising almost 50 % of a particle, is shown.

Without wishing to be bound by theory but in order to promote understanding of the invention, an explanation of the mechanisms is discussed below. Owing to the layered nature of the material (having adjacent minerals of differing microwave energy absorptivity) there is differential expansion between adjacent layers. This expansion creates tensile stress within the ore structure which promotes or induces fracture along the grain boundaries instead of fracture across the grains which would, typically, be associated with conventional crushing techniques. In addition, the quenching of the ore after microwave exposure promotes the stress and induces fracture along the grain boundaries. It is believed that the susceptibility to microwave treatment is a function of a significant difference in the thermal expansion coefficients of adjacent materials or differences in the dielectric properties of the different phases.

The inventor believes that it is an advantage of the invention that by subjecting the layered low grade ores to the method in accordance with the invention, the mineral particles are liberated or the structure is at least weakened (as a result of the intergranular fracture) to be more susceptible to subsequent mechanical liberation steps. The mineral particles being liberated have a marked and surprisingly higher aspect ratio when compared to conventional milling techniques and exhibit an abundance of rectangularly shaped particles. This allows for the reduction in energy consumption in down stream crusher stages. For example, a typical design crusher circuit that would be required for banded ironstone would involve secondary (64 x 5mm), tertiary (53 x 5mm), quaternary (minus 19mm) and fine recirculating (minus 19mm) crushing stages. Material that has been treated in accordance with the invention, with a reduced work index, reduces the number of crushing stages required as well as the size of crushers used in certain crushing stages. It is also believed that the problem of slimes generation present during conventional ultra-fine liberation is alleviated by improving liberation at coarser particle size.

It is a further advantage of the invention that the removal of phosphate impurities from banded ironstone is achieved more efficiently as compared to conventional methods of which the inventor is aware. Conventional methods require milling to a particle size of about 150 μm to achieve substantial liberation as compared to samples treated in accordance with the invention which are substantially liberated at a coarser particle size.