TECHNICAL FIELD

A method of dry, physical separation of a valuable iron component from iron- bearing material is disclosed. The method is applicable, although not exclusively, to tailings, waste from metal processing and non-magnetic iron-bearing materials (e.g. low grade ore, such as hard cap, goethitic ore and pisolitic ore). Particularly, the method relates to preparing non-magnetic iron-bearing material for magnetic separation of the valuable iron components from non- valuable components.

BACKGROUND ART

Magnetic separation of the valuable iron component from iron-bearing material requires the valuable iron component to be in a magnetically susceptible state. Efforts, therefore, have focused on recovering additional quantities of magnetic iron-bearing materials, such as magnetite (Fe ₃ 0 ₄ ) and hematite (Fe ₂ 0 ), from ore stockpiles considered to be too low in iron content to be economical to process.

An example of one such process has been developed by Magnetation, Inc. and involves wet processing of iron ore that contains magnetite, hematite and other weakly magnetic minerals. More specifically, the ore is refined to a small size (typically less than 0.6 mm) and is carried in a water-based slurry past a series of magnetic stations so that magnetically susceptible particles are retained at the stations. The particles are then collected as an iron-bearing concentrate.

To ensure that the process is economical in capturing as much magnetic and weakly magnetic materials as possible, the magnetic stations are operated with a magnetic field of about 920 gauss. Furthermore, the collected material will have a relatively high water content due to absorption of water during the separation process. Removing this water from the collected material adds a further cost to the process of extracting iron from the collected material.

Considerable volumes of iron ore have an iron content that is considered to be too low to process economically. These ores have an iron content that falls below a threshold of 60wt% Fe content which is typically required for ore to be suitable as blast furnace feedstock. To take advantage of the iron ore having an iron content of less than 60wt%, it is blended with ore having an iron content of greater than 60wt% to provide a metallurgical process feedstock that has an overall iron content of about 60% 5 or higher.

While this enables some goethitic ore (FeO(OH)) having close to 60wt% Fe to be utilised, the vast majority of goethitic ore has a much lower iron content.

Specifically, the greatest volumes of goethitic ore comprise oolites that comprise between 20wt% to 50wt% Fe. Another form of goethite comprises low grade pisolites o which have between 45wt% to 55wt% Fe, but are less abundant that oolites. Less

abundant still are high grade pisolites which have an even higher Fe content (between about 55wt% to 60wt%).

In particular, the lower iron content of the goethitic ore is accompanied by higher silica and alumina content. It is preferable to reduce silica and alumina content in 5 feedstock because they are costly to heat and process through a blast furnace or other iron or steel making process when they make no valuable output contribution.

In order to make goethitic ore, including oolites, a valuable resource, it is necessary to upgrade the ore by separating the iron-bearing component from non-iron- bearing component. Goethite is not magnetically susceptible. Therefore, the valuable o iron-bearing component in the goethite cannot be recovered through magnetic

separation techniques such as proposed by Magnetation, Inc.

There is a need, therefore, for an economically viable process that enables the valuable iron component to be recovered from low grade iron ore, including but not limited to goethite, and from other iron-bearing materials, such as tailings, and metal 5 processing wastes.

SUMMARY OF THE DISCLOSURE

There is provided in accordance with one aspect of the invention, a method of 0 treating non-magnetic iron-bearing material to form an iron-rich component that is separable from a gangue-rich component, the method comprising treating the nonmagnetic iron-bearing material by roasting the material under reducing conditions which cause (a) the formation of separate iron-rich and gangue-rich components and (b) the iron-rich component to become magnetic.

The reducing conditions may comprise exposing the iron-bearing material to reducing conditions to increase metallisation of the iron-containing components to at least 70%.

The method may further comprise a step of reducing the particle size of the iron- bearing material. This may be achieved by crushing, grinding or pulverizing and may be conducted before and/or after the treatment. Following the treatment by roasting under reducing conditions, the material may be allowed to cool before reducing the particle size.

Following the treatment and reducing the particle size, the method may further comprise dry, magnetic separation of the iron-rich component from the gangue-rich component using a low strength magnetic field. The term "low strength magnetic field" is taken to mean a magnetic field of less than 1000 gauss.

Through extensive laboratory test work carried out on goethitic iron ore, the applicant has observed that roasting the ore under reducing conditions to achieve a high degree of metallisation (at least 60%) converts the non-magnetic ore to a magnetic forai. This is significant because it enables non-magnetic iron-bearing materials to be recovered by magnetic separation techniques. More importantly, however, the applicant has observed also that the valuable iron component and the non-valuable component in the ore undergo a phase separation which results in discrete iron-rich phases in a gangue-rich phase matrix.

The applicant subsequently found through the test work that the roasted ore preferentially breaks along grain boundaries between the iron-rich phases and the gangue-rich matrix. Accordingly, crushing of the roasted ore produces magnetic, partially metallised, iron-rich particles that can be dry-separated from the gangue-rich particles under relatively low magnetic fields, i.e. less than 1000 gauss.

It is anticipated that the roasting and magnetic separation process can be applied to the significant volumes of non-magnetic iron-bearing materials that are available, such as stockpiles of non-magnetic iron ore that have an iron content of less than 60wt% Fe (i.e. low grade ore), tailings and wastes from metal processes. In terms of iron ore, however, it is anticipated that the roasting and magnetic separation process can be used to upgrade low grade ores into a valuable resource.

The conditions for treating the iron-bearing material may comprise roasting the iron-bearing material to a temperature in the range of 800°C to 1200°C. Optionally, the temperature may be in the range of 850°C to 950°C

The roasting period may be varied provided it is sufficient to cause the formation of separate iron-rich and gangue-rich components and to cause the iron-rich component to become magnetic. The iron-bearing material may be subjected to the treatment for a period in the range of 1 minute to 30 minutes. The treatment period may be in the range of 5 to 30 minutes. During testing, longer roasting periods of up to around 60 minutes were also found to be effective.

The reducing conditions may be strongly reducing. For example, the conditions may comprise an oxygen-deficient environment comprising 50 to 100 % H ₂ gas by volume and 0 to 50% N ₂ gas by volume. Alternatively, various forms of fuel gas including CO, CH ₄ (natural gas) and other gaseous hydrocarbons may also be used to control the oxygen partial pressure to provide reducing conditions required.

The iron-bearing material may be goethite and the iron-rich component may be hematite.

The step of reducing the particle size may involve reducing the particle size of the iron-bearing material to a size that is suitable for treatment according to the method. This may, alternatively, involve reducing the particle size of the iron-bearing material after the treatment to make the iron-rich component available for separation from the gangue-rich component. In a further alternative, this may involve reducing the particle size before and then again after the treatment either by recycling the treated material to the initial size-reduction step or by passing the treated material to a separate size- reduction step. Optionally the treated material is allowed to cool before reducing the particle size. The particle size reduction step or steps may produce particles of iron- bearing material having a powder-like form, such as less than 4mm and preferably less than 2mm.

The size-reduction step or steps may comprise reducing the size of the iron- bearing material to a size that causes the iron-bearing material to break along grain boundaries between the iron-rich component and the gangue-rich component. This size- reducing step may form particles of the iron-rich component and the gangue-rich component having a size less than 2 mm.

The method may involve treating the iron-bearing minerals to the reducing conditions in a fluidised bed.

The low strength magnetic field is arranged to separate the iron-rich component from the gangue-rich component. The low strength magnetic field may have a field strength of less than 1000 gauss and optionally less than 500 gauss and may be in the range of 100 to 250 gauss. Additionally, the separation step may be followed by one or more further dry, magnetic separation steps. For example, the non-valuable material from the dry, magnetic separation step may be recycled directly to the magnetic separation step and/or may be passed to the size-reduction step or a separate size- reduction step and then passed to the dry, magnetic separation step or to a separate dry, magnetic separation step. The conditions (e.g. magnetic field strength) of each dry, magnetic separation step may differ.

The method may further comprise controlling the reducing conditions and the separation step to recover at least 80% of iron contained in the iron-bearing material.

Another aspect of the invention provides a method of preparing iron-containing feedstock for a metallurgical process, the method comprising:

(a) treating iron bearing material according to the aspect described above to produce an iron- rich component and a gangue rich component;

(b) reducing the size of the treated iron-bearing material to a particle size that

enables magnetic separation of the iron-rich component from the gangue-rich component; and

(c) applying a magnetic field to the iron-bearing material produced by step (b) to separate the iron-rich component from the gangue-rich component.

The method may further comprise consolidating the iron-rich component into a form suitable for metallurgical processing in a metallurgical vessel. The consolidating step may involve agglomerating, briquetting or pelletizing the iron-rich component.

The metallurgical processing may comprise a process that increases

metallisation of the iron-rich component. The metallurgical processing may comprise a process that produces iron metal from the iron-rich component. BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the method as set forth in the Summary, a specific embodiment will now be described, by way of example only, with reference to the accompanying drawing in which:

Figure 1 shows a flow chart of a method as described above for treating iron ore.

Figure 2 is a hot-stage SEM micrograph of goethite prior to treatment according to the method described above.

Figure 3 is another a hot-stage SEM micrograph of goethite subsequent to treatment according to the method described above and showing discrete phases of the iron-rich component (light) in a matrix phase of the gangue-rich component (dark).

DESCRIPTION OF AN EMBODIMENT

The following description of an embodiment of the method described above is in the context of processing goethite iron ore. It will be appreciated, however, that the method can be applied to alternative ore types and to other forms of iron-bearing materials, with suitable adjustments to processing conditions, to achieve the same result. Accordingly, the following description should not be construed as limiting the scope of application of the method to goethite.

Having regard to Figure 1 , goethite 2 is provided as an as-mined ore to a cmsher 10 to reduce the size of the goethite 2 particles to a size less than 4mm such that it is suitable for processing in downstream stages.

The downstream processing stages involve treating the goethite 2 under reducing conditions which cause iron-containing components to separate into an iron- rich component and a gangue-rich component and which cause the iron-rich component to become magnetic.

Specifically, the goethite 2, having passed through the crusher 10, is supplied to a reactor 20, which may be, but not limited to, a fluidised bed reactor, which is supplied with reducing gas from a gas source 30. The conditions in the reactor 20 are selected to cause reduction of the goethite to a metallisation degree of at least 60%. Those conditions include exposing the goethite to low oxygen partial pressure environment using strongly reducing conditions, provided by an atmosphere comprising 50 to 100% ¾ gas by volume at a temperature greater than 800°C and up to 1200°C. Alternatively, various forms of fuel gas, including CO, CH ₄ (natural gas) and other gaseous hydrocarbons, may also be used to control the oxygen partial pressure to provide the reducing conditions required. The residence time of the goethite particles in the reactor 20 is controlled depending upon the size of the particles. Specifically, the goethite 2 particles are retained in the reactor 20 for a period of time sufficient to cause the iron-containing materials to reduce to a metallisation degree of at least 60% and for phase separation into the iron-rich component and the gangue-rich component to occur. This time may be in the range of 1 minute to 30 minutes. The treatment time may be in the range of 5 to 30 minutes, although longer treatment times of up to 60 minutes have also been found effective. "Before" and "after" treatment electron microscopy images of the goethite are shown in Figures 2 and 3. Prior to treatment, the goethite appears as a single phase of nano- size crystals. However, after treatment, a relatively pure iron-rich phase (the light coloured phase in Figure 3) forms as discrete particles in a gangue-rich matrix (shown in Figure 3 as a darker phase). This phase separation phenomenon is understood by the applicant to be produced by the specific conditions selected for the treatment.

Off- gas 32 from the reactor 20 is passed to a gas (G)-solids (S) separator, such as a cyclone separator 34, to remove dust and fine particles from the off-gas 32. A solids-free gas stream 38 is released from the cyclone separator 34. This may be processed and released to the atmosphere.

The treated goethite particles from the reactor 20 are sent to a crushing or grinding stage 40 that reduces the size of the particles further. The applicant has found that the treated particles have a tendency to break along grain boundaries between the iron-rich phase and the gangue-rich phase when cmshed or ground. The cmshing stage 40, therefore, acts to make the iron-rich phase available for separation from the gangue- rich phase.

The treated ore leaving the cmshing stage 40 is passed to a magnetic separation stage 50. However, solid particles and dust removed from the off-gas 32 in the cyclone separator 34 are sent via line 36 to combine with the treated and crushed goethite so that it too passes through the magnetic separation station 50.

The magnetic separation stage 50 is configured to expose the treated and

5 crushed particles to a magnetic field that separates the iron-rich phase from the gangue- rich phase. The iron-rich phase is magnetic and it reacts to the magnetic field by, for example, being attracted to the surface of a magnet. The iron-rich particles are then collected from the magnet. Test work carried out by the applicant revealed that exposing the treated and crushed particles to a magnetic field less than 1000 gauss o sufficient to separate the iron-rich phase from the gangue-rich phase when a magnet is placed in appropriate proximity to the treated and crushed goethite. However, the magnetic iron-rich particles may be separated under a magnetic field in the range of 100 to 250 gauss. It has been found that a drum magnet is particularly suitable for use in separating the iron-rich phase from the gangue-rich phase. As the iron-rich phase is 5 attracted to the drum it may be considered that the drum magnet acts by separating out the gangue-rich phase from the iron-rich phase.

Being able to separate the iron-rich phase from the gangue-rich phase with a low strength magnetic field is an important improvement over previous magnetic separation processes which require considerably greater magnetic fields. Accordingly, the

o treatment process described above contributes to a lowering of the overall economic input into recovering iron from goethite, including reducing costs associated with the magnetic separation stage.

The iron-rich phase 54 is recovered from the magnetic separation stage 50 as a reduced ore product comprising 90 to 95% of the iron contained in the mined goethite. 5 Laboratoiy test work carried out by the applicant involved subjecting low grade

(pisolite) goethite to the treatment process described above. Specifically, the treatment conditions comprise crushing ore to a size less than 2mm, exposing the ore to a reducing atmosphere of predominantly ¾ gas or other reducing gas and the balance of N ₂ gas at temperatures of greater than 800°C in fluidised bed reactors. The ore was 0 retained in the reactor for a period of time to achieve greater than 60% metallisation of the iron-bearing minerals. The treated ore was then subjected to magnetic separation by exposure to a magnetic field of less than 1000 gauss and as low as 100 gauss. The following table shows an example of some of the results of the above test work carried out on iron ore waste from the Mesa A mine in the Pilbara region of Western Australia. Specifically, the table shows the content of iron, silica and alumina of the as-mined ore, the treated ore, the reduced ore product obtained from the magnetic separation step and the non-magnetic waste product.

The reduced ore product obtained from the method has an iron content of almost 79%. This is a significant upgrading of the as-mined ore which contained an iron content of slightly more than 50%, i.e. well below the 60% threshold for use in metallurgical processes. The method described above, therefore, is capable of upgrading ore to a reduced ore having considerably higher iron content. This means that low grade ores can be upgraded to form economically valuable resources. It is anticipated that the method may be used to upgrade tailings, hard cap and ore waste streams, such as low grade ore, including pisolites and goethite. The test work suggests that ore having an iron content as low as 45% may be upgraded to form a product comprising greater than 60% iron on an ore equivalent basis.

The product obtained from the magnetic separation is used as feedstock in metallurgical processes to obtain iron metal (i.e. by increasing metallisation to 100%). While the product may be used as a feedstock to molten bath-based metallurgical process, the relatively fine particle size of the product means that it cannot be conventionally added directly to a metallurgical process that relies on exposing iron- bearing materials to a reducing gas, such as a blast furnace or rotary hearth furnace, because the product will choke flow paths of reducing gas passing through a burden. Accordingly, the product may be formed into suitably sized lumps by agglomerating, briquetting or pelletizing processes so that it can be used in a blast furnace or rotary hearth furnace. A range of processes are known for forming lumps feedstock of iron- containing materials. Any of those processes may be used to form the lump feedstock. Alternatively, the product maybe injected into the blast furnace via the tuyeres such as with pulverised coal.

Whilst one method embodiment has been described, it should be appreciated that the method may be embodied in many other forms.

It is to be understood that, if any prior art publication or existing or typical process is referred to herein, such reference does not constitute an admission that the publication or process forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.