WO2024228165A1

WO2024228165A1 - Pelletisation of a mineral product

Info

Publication number: WO2024228165A1
Application number: PCT/IB2024/054313
Authority: WO
Inventors: Alex Richard FISHER; Garry LONSDALE; Rudolph Johannes FOUCHEE; Wesley Robin GOUNDER
Original assignee: Anglo American Woodsmith Ltd
Current assignee: Anglo American Woodsmith Ltd
Priority date: 2023-05-04
Filing date: 2024-05-03
Publication date: 2024-11-07
Anticipated expiration: 2025-11-04
Also published as: GB202306624D0; GB2629628A

Abstract

The invention provides a method for forming a pelletised evaporite mineral product, the method comprising pulverising an evaporite mineral feedstock to form a powder, mixing the powder with a binder in the presence of a liquid to form a mixture, wet screening the mixture to separate a granule fraction and a fines fraction and processing the fines fraction using a pelletiser to form a quantity of pellets principally composed of the evaporite mineral. The evaporite product is preferably polyhalite.

Description

PELLETISATION OF A MINERAL PRODUCT

INTRODUCTION

This invention relates to forming pelletised mineral products, for example, for use as fertiliser. In particular this invention relates to a method of pelletising evaporite mineral powder mixed with a binder and water utilising screening to maximise efficiency.

BACKGROUND OF THE INVENTION

Polyhalite is an evaporite mineral. It is a complex hydrated sulphate of potassium, calcium and magnesium. Deposits of polyhalite occur in, amongst other countries, Austria, China, Germany, India, Iran, Turkey, Ukraine, the UK and the USA.

Polyhalite has the capacity to be valuable as a source of agricultural fertiliser. In some prior art processes it has been proposed to decompose natural polyhalite to extract specific nutrients. See, for example, WO 2013/074328, US 1 ,946,068 and US 4,246,019. However, intact polyhalite is also usable as a fertiliser, being able to supply sulphur, potassium, calcium and magnesium to the soil.

Mineral polyhalite can be spread in raw, crushed or chipped form. That involves low material processing costs, but it has a number of agronomic disadvantages. Once applied to the soil the raw mineral takes some time to break down, delaying the bioavailability of its constituents. If applied in chipped form, the polyhalite tends to be of irregular shape and size, meaning that there can be issues in applying it uniformly, and that it can be difficult to apply with some types of agricultural spreading machinery. Untreated powdered polyhalite might in some circumstances be capable of being uniformly spread. However, it can have a tendency to flocculate or clump in some storage conditions, making it difficult to spread evenly with some types of machinery.

It is known to form urea into fertiliser pellets and to form limestone into pellets for dressing to increase soil pH. This can be done by mixing powdered urea or limestone with a binder and then processing it in a pan pelletiser. GB 2 533 490 and GB 2 530 757 disclose forming powdered polyhalite into pellets, the powder being bound by starch.

GB2 577 866 teaches a method for forming a pelletised evaporite mineral product, the method comprising: pulverising an evaporite mineral feedstock to form a powder; mixing the powder with a binder in the presence of a liquid to form a blend; processing the blend using a pelletiser to form a quantity of pellets principally composed of the evaporite mineral; separating at least one of an oversized fraction and an undersized fraction from the quantity of pellets; disaggregating the pellets of the oversized and/or undersized fraction to form disaggregated material; and re-pelletising the disaggregated material.

The methods taught in the prior art suffer from the disadvantage that screening the wet pellets before drying often results in clogging of the screens and high rejection fractions. Screening after drying is an alternative but it comes at a much greater energy cost due to non-product material reporting to the dryer alongside the on-spec granules.

There is a need for an improved process for pelletising evaporite minerals.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method for forming a pelletised evaporite mineral product, the method comprising: pulverising an evaporite mineral feedstock to form a powder; mixing the powder with a binder in the presence of a liquid to form a mixture; wet screening the mixture to separate a granule fraction and a fines fraction; and processing the fines fraction using a pelletiser to form a quantity of pellets principally composed of the evaporite mineral.

The method may further comprise separating at least one of an oversized fraction and an undersized fraction from the quantity of pellets; disaggregating the pellets of the oversized and/or undersized fraction to form disaggregated material; and re-pelletising the disaggregated material.

Preferably the wet screening involves the application of a rotary drum screen which may be a self cleaning trommel screen. The granules from the wet screening may be directed through an oversize screen to separate product granules from non-product granules. The product granules may be directed to a drier to form pellets.

The non-product granules may be directed to a secondary mixer which in turn is fed back to the wet screening.

The method according to the present invention may comprise disaggregating the pellets of the oversized and/or undersized fraction in an operating chamber of a mixer/granulator and re-pelletising the disaggregated material in that same operating chamber. That may be a different operating chamber from an operating chamber in which the blend or mixture is processed to form the quantity of pellets.

The method may further comprise mixing the powder with the binder in an operating chamber of a mixer/granulator and processing the mixture to form pellets in the operating chamber.

The method may comprises separating only an oversized fraction. The method may comprise separating only an undersized fraction.

The said disaggregating step may involve the application of a higher shear than the mixing step.

The method may comprise disaggregating the pellets without substantially drying them subsequent to the re-pelletising step. The water content of the disaggregated material input to the re-pelletising step may comprise substantially the same proportion of water by mass (e.g. plus or minus 5 or 10 percent) as the oversized and/or undersized fraction at the time when it is separated. This can assist in re-pelletising the material efficiently and without the need to add additional water.

The oversized fraction may comprise those of the formed pellets that exceed a predetermined size, e.g. diameter. The undersized fraction may comprise those of the formed pellets that are below a predetermined size, e.g. diameter. The mixing step may comprise holding the powder and the binder in a vessel and agitating the powder and the binder by the operative region of a mixing tool, at least a part of the operative region having an instantaneous linear speed of at least 14 m/s relative to the vessel during the mixing step.

The said disaggregating step may comprise holding the oversized and/or undersized pellets in a vessel and agitating those pellets by the operative region of an agitating tool, at least a part of the operative region having an instantaneous linear speed of at least 2 m/s relative to the vessel during the disaggregating step. The highest instantaneous linear speed of any part of the operative region of the tool during the disaggregating step may be less than 10 m/s or less than 5 m/s.

During the mixing step the highest instantaneous velocity of at least a part of the operative region of the mixing tool relative to the mixing vessel may be in the range from 5 to 10 times the highest instantaneous velocity of at least a part of the operative region of the agitating tool relative to the agitating vessel.

The binder is preferably a farinaceous material, i.e. comprising starch, for example in the form of flour.

The powder may have a mass average grain size in the range from 50 to 400pm.

The liquid may comprise or may be water.

The pellets of the quantity of pellets may contain greater than 80%, preferably greater than 90% by mass of the evaporite mineral. The evaporite mineral may be polyhalite.

DESCRIPTION OF PREFERRED EMBODIMENTS

In one example of a process, an evaporite mineral feedstock (e.g. a polyhalite feedstock) can be mechanically processed to form a dry powder. That powder can then be mixed with a binder to form an intermediate mixture. The intermediate mixture can be processed using a pelletiser to form pellets that are principally composed of the evaporite mineral. The pellets can be processed (e.g. by drying) to stabilise them structurally. As indicated above, polyhalite is a complex hydrated sulphate of potassium, calcium and magnesium. Polyhalite may have the general formula K2Ca2Mg(SO4)4.2H20, or substantially such. Polyhalite has a Moh's hardness of around 2.5 to 3.5. As-mined polyhalite may be intimately combined with gangue. The gangue may include other evaporite minerals such as halite (NaCI) and anhydrite (CaSO4). The gangue is preferably in low proportions (e.g. less than 10% or less than 5% in good quality ore).

Once mined, polyhalite may be broken into blocks or chips of suitable size for transport and processing. For example, the as-mined rock may be fed to crushers such as jaw crushers and/or cone crushers in order to yield a chipped material of generally uniform size. It may be desirable to limit the moisture uptake of the chipped polyhalite in order to reduce any variation in the subsequent processing steps as a result of, for example, variations in the weather.

It may be desired to form the polyhalite into pellets, e.g. to act as a spreadable fertiliser product. One way in which this can be done will now be described.

The raw or chipped polyhalite is processed to form a powder essentially of polyhalite. This may suitably be done using high pressure grinding roller (HPGR) equipment, or in a ball mill (e.g. a continuous "Hardinge" ball mill) or an attritor mill. The average grain size of the powder is dependent on various process parameters including the dwell time of the feedstock in the powdering equipment and the configuration of the powdering equipment. Oversized particles exiting the powdering equipment may be returned to the equipment for further processing. The desired powder size will depend on the nature of the subsequent processing steps, but it has been found that air classifying the output of the powdering process with a 500pm air classifier (AC) and accepting the material passing the AC for further processing yields good results. Oversized particles exiting the powdering equipment and not passing the screen may be returned to the powdering equipment for further processing. A convenient profile of the powder passed to the next step of the process is: 100% passing a 500pm screen and 80% (by mass) passing a 200pm screen. Conveniently at least 50% or more preferably at least 70% of the mass of the powder is composed of grains having a grain size, or a largest or average diameter, in the range from 50 to 400pm. The grain size may be as measured by means of a Malvern Mastersizer 2000 or as measured by means of a sieve shaker. Gangue may be separated before the mined rock is powdered. Alternatively, if the gangue is in reasonably low proportion to the desired mineral then it may be retained and powdered. Thus the powdered polyhalite may comprise other minerals too. The powder preferably comprises greater than 80% by mass polyhalite, more preferably greater than 90% by mass polyhalite. The powder preferably comprises greater than 90% by mass of evaporite minerals.

Water and a binder are added to the powdered polyhalite. In the description below, the additions of water and binder are specified by mass with reference to the mass of the powder to which they are added. The amount of water to be added will depend on the inherent water content of the powdered polyhalite and the nature of the subsequent processing steps. However, it has been found that when the binder is a starch-based binder such as starch itself or flour, acceptable results can be achieved by adding water in the range of 5% to 10% by mass, more preferably between 7% and 8% by mass. At a subsequent stage in the process excess water is removed from the formed pellets by drying. That can consume energy, so it is preferred to minimise the amount of water added, provided that is consistent with the production of an acceptably bound pellet product. The preferred amount of water can readily be determined by testing. The amount of binder to be added will depend on the qualities of the binder. For typical binders, e.g. starch or flour, the amount added may be in the range of 0.5% to 1 .5% by mass. The binder may be a farinaceous material including a starch-based binder such as a purified starch or a flour, or an adhesive such as PVA. The binder may be added directly to the powder, or it may first be added to the water and then the water and binder combination may be added to the powder.

One option as a binder is purified starch. This can be added in the range 0.5% to 3.0% by mass.

Another option as a binder is flour. The flour may be formed by grinding a starchy vegetable base such as one or more types of vegetable root or seed. The flour may, for example be formed from the seeds of a cereal such as wheat, corn or rye or of a pulse such as pea. The flour may be a raw flour: that is a flour formed by the grinding of the base biological material with no substantial bleaching or refinement. The flour may be a wholegrain flour. The flour may be formed from material from which the germ and/or the bran has not been separated. The flour may comprise starch together with fat (e.g. oil) or protein or both. The germ is a significant source of fat. The flour may comprise greater than 1 .0% or greater than 2.0% germ by mass. The flour may comprise greater than 0.05% or greater than 0.1 % or greater than 0.2% fat by mass or greater than 0.4% fat by mass. The bran is a significant source of protein. The flour may comprise greater than 8% or greater than 12% bran by mass. The flour may comprise greater than 0.5% or greater than 1 .0% or greater than 4.0% or greater than 8.0% or greater than 10% of protein by mass. The flour may comprise greater than 50% or greater than 55% or greater than 60% starch by mass. The flour may comprise less than 90% or less than 80% or less than 70% starch by mass. The flour may comprise greater than 1% or greater than 2% lipids by mass. The flour may comprise greater than 0.2% or 0.4% fatty acids by mass. A suitable example composition of the flour is: lipids 3%, protein 7.5%, moisture 13.5%, fibre 3.6%, fatty acids 0.5%, ash 2.5%, starch 69.4%, The flour may comprise gluten. The flour may comprise greater than 5.0% or greater than 8.0% or greater than 10% gluten by mass.

Flour has been found to be advantageous as a binder for the present process for a number of reasons:

1. When a pelletised fertiliser bound with flour is introduced to a growing medium (e.g. soil) and breaks down, the protein and oil in flour can be released to the growing medium. The protein and potentially also the oil can attract and support the growth of mycorrhizal organisms. Those organisms can promote the growth of the target plants and supplement the nutrient-giving effects of the fertiliser composition itself.

2. Flour is typically less flammable than refined starch, which can even be explosive. This improves safety and makes flour less expensive to handle.

3. Flour is significantly cheaper than some alternative binders, such as refined starch, and has a lower carbon footprint.

4. The type of flour selected for use can depend on the availability of suitable starchy crops convenient to the pelletising plant, and may vary seasonally without significantly affecting pellet yield.

When the binder is starch or flour, it is convenient to mix it with water, and then subsequently to add the mixture of binder and water to the polyhalite powder. This can improve the intermixing of the binder and the mineral powder. Starch contained in the binder may be gelatinised prior to being mixed with the mineral powder. This may, for example be done by atmospheric cooking or by jet cooking. To this end the binder may be added to water at a temperature of greater than 55°C, more preferably greater than 80°C; or the binder may be added to water and the binder/water mixture heated to a temperature in that range. The binder may be added to the water in equal proportion to the intended additions of the same to the powder, or alternatively additional water may be added after the water/binder mix is added to the mineral powder. As part of the process of cooking the starch, water may be added to the binder in liquid form or in the form of steam. The process of cooking the starch may be performed at atmospheric pressure and/or at greater than atmospheric pressure. The cooking process may be completed before the binder mixture is added to the polyhalite powder.

The powder/blender mixture is mixed until it is homogeneous, and pelletised. In one approach, the powder/binder mixture is mixed in a suitable mixer (e.g. a ribbon blender) and then pelletised in a suitable pelletiser (e.g. a pan pelletiser). In an alternative approach, which has been found to be efficient, the powder/binder mixture is passed to equipment that can both mix and pelletise. An example of such equipment is an intensive mixer/granulator, e.g. as available from Maschinenfabrik Gustav Eirich GmbH & Co KG. A pelletiser may be configured to expel processed material as it operates, allowing it to run continuously. Alternatively the pelletiser may operate on a batch basis, with material being processed according to a defined programme and then expelled en masse.

According to the present invention a wet screening step is introduced between the mixing and pelletisation step, using a rotary drum screen which may be a trommel screen and which may be self cleaning.

A trommel screen, also known as a rotary screen, is a mechanical screening machine used to separate materials, mainly in the mineral and solid-waste processing industries. It consists of a perforated cylindrical drum that is normally elevated at an angle at the feed end. Physical size separation is achieved as the feed material spirals down the rotating drum, where the undersized material smaller than the screen apertures passes through the screen, while the oversized material exits at the other end of the drum.

Trommel screens are typically used in a variety of applications such as classification of solid waste and recovery of valuable minerals from raw materials. Trommel screens come in many designs such as concentric screens, series or parallel arrangement and each component has a few configurations. Considerations for a trommel screen include the screening rate, screening efficiency and residence time of particles in the screen. When designing a trommel screen, the main factors affecting the screening efficiency and production rate are the rotational velocity of the drum, mass flow rate of feed particles, size of the drum, inclination of trommel screen, aperture size and type of screening media, for example polyurethane panels vs woven mesh. Depending on desired application of trommel screen, a balance has to be made between the screening efficiency and production rate.

The inventors have found that pelletisation efficiency is greatly improved through pre-screening of the wet mineral / binder / water mixture using a rotary drum screen as described. In particular smaller recycle streams are created which results in lower capital and operating costs, lower maintenance, particularly of the screens, all of which results in maximised plant run time. In particular, it is found that the pelletisation process according to the prior art resulted in fouling of the screens as taught because the material being screened was still wet.

Against this technical prejudice, the inventors now find that a further initial wet screening step using a rotary drum screen significantly reduces fouling of the downstream screens resulting in smaller recycling streams. During mixing of the evaporite powder, binder and water, some granulation takes place. The rotary drum screen removes these granules and other oversize which frees up capacity in the subsequent pelletisation step. The granules and other oversize are then preferably separated through a subsequent oversize screen with the granules being sent to drying to form pellets and oversize being recycled back to the deagglomeration step.

The fines from the wet screening are directed to a pelletiser (granulator) for a pelletisation step.

Before or after being expelled from the pelletiser, the formed pellets may be coated with dry powder before subsequently being screened. This can help to resist them sticking or breaking up during the screening or related processing. Further polyhalite powder may be added to the pelletiser towards the end of the pelletising process. The polyhalite powder may coat the pellets with a dry coating which can assist with subsequent processing. The amount of polyhalite powder added at this stage may be between 5 and 15% by mass of the content of the pelletiser, more preferably between 8 and 12% by mass. The additional polyhalite powder may be added between 10 and 15 seconds prior to completion of the pelletising process. At completion of the pelletising process may be when the pellets are expelled from the pelletiser. The material expelled from the pelletiser can be screened to separate undersized or oversized pellets from pellets of a desired size range. The desired size range may, for example, be that which passes a 4mm screen but does not pass a 2mm screen. Alternatively, other sizes may be chosen as appropriate to the desired application. The outsized pellets may be recirculated. Any pellets that are oversize can be ground and then returned to the pelletiser. Undersize pellets can be returned directly to the pelletiser.

The output of the pelletiser is wet, substantially spherical pellets. The pellets that meet the desired size are conveniently dried before packaging. To achieve this the pellets that have been output from the pelletiser can be passed to a drier.

The granules separated by the rotary drum screen and possibly also the oversize screen can also be passed to the drier.

It has been found that a retention time of around 20 minutes in a drier capable of heating the pellets to a temperature of around 150°C is sufficient to adequately dry the pellets. This can harden them. Pellets manufactured using polyhalite powder and with flour as a binder can have a crush strength in excess of 4.0kgf and/or in excess of 5.0kgf. This compares favourably with a generally accepted lower limit of 2.2kgf for acceptable agricultural pellets. Moisture can be extracted from the dryer using a reverse jet air filter. The operating temperature and retention time in the dryer can be selected to provide pellets of the desired strength for subsequent handling.

It has been found efficient to mix the constituents and pelletise them by the following method.

1 . The mineral powder, binder and water are subject to a first mixing step. The first mixing step is performed so as to provide a homogeneous mixture of the constituents.

2. The wet mixture is subject to a wet screening step to separate granules and overside from fines. The fines are directed to a pelletising step.

3. The output of the pelletising step may be tested for size. Pellets within the desired size range are accepted for drying. Pellets above or below the desired size are rejected. The sizing may be performed using suitably sized screens. 4. The accepted pellets and the granules from the wet screening step are dried to harden them, e.g., by heating.

5. The oversized rejected pellets (an possibly non-product granules from the wet screening step) are, without having been subjected to a heating step, agglomerated to a second mixing step. The second mixing process is performed at sufficiently high shear to break the pellets down. This can be done in a mixer rather than a grinder or crusher because the oversized rejected pellets have not been dried after the first pelletising step. The undersized rejected pellets may be passed to the second mixing process or returned to the first pelletising step.

6. The output of the second mixing process is directed back to the wet screening step.

SPECIFIC DESCRIPTION

The present invention will now be described by way of example with reference to the accompanying drawings in which.

Figure 1 shows a generalised overview of a pelletising process according to the prior art.

Figure 2 shows an alternative overview of a pelletising process according to the prior art.

Figure 3 shows a first embodiment of the present invention.

Figure 4 shows a second embodiment of the present invention.

Figure 1 shows as-mined raw polyhalite 12 is primary crushed in one or more comminution devices such as a jaw crusher 1 and secondary crushed in a cone crusher 2. This produces a crushed polyhalite product. Roll sizers, hammer mills or vertical shaft impact crushers may also be used. The crushed polyhalite may be stored, e.g., in a warehouse 3, until shortly before it is to be processed by the subsequent steps. Preferably the steps illustrated at 4, 6 and 7 follow quickly one after the other, reducing the scope for the polyhalite powder to absorb ambient moisture. When required, the crushed polyhalite can be withdrawn from the store 3 and passed to an HPGR mill 4 where it is rendered to a powder. The HPGR mill may preferably operate in a closed circuit with an air classifier. The binder (flour) 13 is combined with water 14 in a first mixer 5. The polyhalite powder is combined with the binder/water mixture in a second mixer 6. Once the polyhalite and binder are mixed homogeneously the mixture can be passed to a pelletiser 7. Alternatively, the mixer 6 and the pelletiser 7 may be implemented by a single mixer/pelletiser. Additional water may, if needed, be added to mixer 6 and/or pelletiser 7 to achieve proper operation of the process. The pelletiser 7 causes the mixture to aggregate into substantially spherical pellets and dry polyhalite powder may be added to the pelletiser to coat the pellets. The pellets exit the pelletiser gradually or in batches. The exiting pellets are sized by a set of screens 8. Undersize pellets are returned to the pelletiser as indicated at 9, or to the second mixer as indicated at 10. Oversize pellets may be returned to the second mixer indicated at 10. The output of the sizing step is pellets of substantially spherical form and within the size limits defined by the screens 8. Those pellets are dried at 1 1 . The resulting hardened pellets 12 can then be packaged and supplied for agricultural use. Finally, they can be spread on a field or other agricultural or horticultural substrate to act as a fertiliser.

Conveyor belts, auger conveyors or other handling apparatus can be used to move the components between processing stations.

Other additives may be included in the pellets. Such additives may be one or more of the following, in any combination: - a component having the effect of chemically and/or mechanically stabilising and/or preserving the pellets: for example to increase their shelf life, reduce their susceptibility to environmental contaminants or to reduce the likelihood of them being broken up during spreading; - a component having the effect of enhancing the fertilising effect of the polyhalite: for example by accelerating or retarding the breakdown of the polyhalite in the field; - a component having the effect of protecting or enhancing the growth of crops by means other than fertilising: for example a herbicide, fungicide, insecticide, rodenticide, hormone, plant stimulant or mycorrhizal fungus or spore; - a seed: which may be a seed of an angiosperm and/or of a crop species (e.g. a cereal such as wheat, maize, rice, millet, barley, oats or rye); - a further fertiliser composition in addition to the polyhalite: for example a source of nitrogen and/or phosphorus; - a pigment; - a component having the effect of altering soil pH: for example lime, sulphur or a sulphate.

Such a component may be added at various stages in the process, for example it could be combined with the polyhalite powder prior to or during the first mixing stage as described above, or with the binder prior to the first mixing stage as described above, or with the polyhalite/binder mix between the first and second mixing steps as described above, or during the second mixing step as described above, or it could be added to the pan pelletiser, or it could be sprayed or otherwise coated on to the pellets before or after drying.

The polyhalite content of the resulting pellets is preferably greater than 75% by weight, more preferably greater than 80% and most preferably greater than 90%. In the case of pellets that contain seeds this may optionally be varied such that the polyhalite content of the pellets excluding the weight of the seeds may be greater than 75% by weight, more preferably 80%, most preferably greater than 90%.

The pellets are preferably substantially spherical, and of substantially uniform volume and mass. The pellets may have a mean Wadell sphericity of greater than 0.85, 0.90 or 0.95. The pellets are preferably substantially free from voids, for example having not more than 1 %, 2% or 5% by volume of air.

In Figure 2 the polyhalite powder, binder and water are provided as indicated at 20. The polyhalite may be a powder as described above. The binder may be any suitable binder, for example starch, flour or a synthetic adhesive. If the binder comprises starch then it may already have been gelatinised. The proportions of the constituents may be as indicated above. The constituents may be combined before being introduced to a mixer 21 or on introduction into the mixer.

The mixer 21 is a combined mixer/granulator, for example an intensive mixer/granulator, e.g., as available from Maschinenfabrik Gustav Eirich GmbH & Co KG. The mixer/granulator may comprise a rotatable drum defining the operating chamber of the device. The floor of the drum may be tilted to horizontal. The drum may be configured to be driven to rotate about an axis. That axis may be inclined to vertical. A mixing paddle may be located in the drum. The mixing paddle may be configured to be driven relative to the drum, for example by rotating about the axis of the drum but in the opposite direction to the drum. The operative part of the paddle contacts the constituents to be mixed during the mixing step. The rotation of the drum and the mixing paddle may be driven by respective motors. The motors may be controlled by a controller which is pre-programmed to cause the drum and the paddle to operate according to a predetermined programme. That programme can be such as to (i) in a first stage agitate the constituents so as to mix them and (ii) in a second, subsequent stage induce the constituents to bind together into pellets. To that end, the device may operate to apply a higher shear to the contents of the drum in the first step than in the second step. The first stage may operate at a tool speed with a lower limit of 14 m/s and the second stage at a tool speed with a lower limit of 2 m/s. The tool speed for the first stage may be from 5 to 10 times the tool speed in the second stage. In the case of a rotating tool the tool speed may be taken to be the greatest instantaneous linear velocity relative to the vessel holding the contents to be mixed, of the part of the tool furthest from the axis of rotation. For the example of an Eirich RV01 unit, in mixing mode a tool speed of 15 m/s may be applied for 60 seconds after which, in granulation mode, a tool speed of 3 m/s may then be employed for 120 seconds.

The output of the mixer 21 is passed to a screening station 22. At the screening station in-size pellets are extracted and passed to a drier 23 for drying so as to form a dried pellet product 24 at the output of the drier. Undersized material may be returned to the first mixer 21 , as indicated at 25. Alternatively, undersized material may be passed to a second mixer 26. This may be preferable to avoid disruption of the primary pelletising process. Oversized pellets are passed to the second mixer 26.

The second mixer 26 is also a combined mixer/granulator, for example an intensive mixer/granulator, e.g. as available from Maschinenfabrik Gustav Eirich GmbH & Co KG. The mixer/granulator may comprise a rotatable drum defining the operating chamber of the device. The floor of the drum may be tilted to horizontal. The drum may be configured to be driven to rotate about an axis. That axis may be inclined to vertical. A mixing paddle may be located in the drum. The mixing paddle may be configured to be driven relative to the drum, for example by rotating about the axis of the drum but in the opposite direction to the drum. The rotation of the drum and the mixing paddle may be driven by respective motors. The motors may be controlled by a controller which is pre-programmed to cause the drum and the paddle to operate according to a predetermined programme. That programme can be such as to (i) in a first stage agitate the oversized pellets so as to disaggregate them and (ii) in a second, subsequent stage induce the contents of the device to bind together into pellets. To that end, the device may operate to apply a higher shear to the contents of the drum in the first step than in the second step. The first stage may operate at a tool speed with a lower limit of 14 m/s and the second stage at a tool speed with a lower limit of 2 m/s. Thus, the second mixer may comprise a mixing chamber in which the oversize pellets are disaggregated and then subsequently re-pelletised.

The first and second mixers may operate on cycles. In each case, the operating chamber of the respective mixer may be charged with material to be processed. It may then implement its first and second operational stages, and then the processed material may be discharged from the operating chamber for sizing.

The first step of the second mixer may operate at a higher shear than the first step of the first mixer. The first step of the second mixer may impart greater energy than the first step of the first mixer, the energy in each case being per unit time and per unit mass of the contents of the respective mixer. This may facilitate breakdown of the previously formed pellets in the first step of the second mixer. These operational characteristics may be brought about by suitable programming of the mixer controllers. The geometry and form of the mixing tools contained within the first and second mixers may be so optimised to promote the application of shear forces and the subsequent balance between mixing, pelletising and disaggregating. The first and second mixers may also be configured to contain multiple tools which may be of the same or differing design. The duration of the first/mixing step of the first mixer may be greater than the second/disaggregation step of the second mixer.

The undersized particles from the screening station 22 may be passed to the second mixer 26.

It has been found that this is an efficient way to perform the mixing and pelletising process. Screening the pellets before they are dried saves energy because outsized pellets are not dried. Also, it makes the process of recycling the outsize pellets easier because they are not hardened and they contain a close proportion of water to the material already in the process chain to which they are being returned. By breaking down the oversized pellets in the second mixer, the need for a separate grinder or crusher can be avoided. Also, the components of the oversized pellets have already been mixed, and the process described above avoids unnecessarily performing a second mixing operation on the components of the oversized pellets. This process can reduce the need for further binder addition and can save energy.

Optionally a coating agent may be applied to the pellets. The coating agent may, for example, be a wax or wax-like coating. The coating agent may act as a moisture barrier and/or an anti-caking agent and/or a dust suppressant.

Finally the pellets can be cooled and packaged, for example in 600kg bags or 25kg sacks, or shipped loose for use or further processing elsewhere The product can then be shipped for use as a fertiliser. The product may be used as a fertiliser by spreading the pellets on the surface of a growing medium such as soil, or by intermixing the pellets in the growing medium.

Figure 3 shows a preferred embodiment 30 according to the present invention. In Figure 3 the mineral powder, liquid and binder 31 are mixed in a primary mixer 32 to form a wet mixture which is passed to a trommel screen 33. During mixing of the mineral powder, binder and water 31 , some granulation takes place. The trommel screen removes these granules and other oversize 34 which are then preferably separated again through a subsequent oversize screen 35 with the granules 36 being sent to drying 37 to form pellets and oversize 371 being recycled back to the deagglomeration step 38.

The fines 39 from the trommel (wet) screening step 33 are then directed to a pan granulator 331 (pelletisation process). On-specification pellets 333 are fed to the dryer 37 and oversize 332 are fed to the oversize screen 35.

Having a reduced feed to the pelletisation process frees up capacity in the step resulting in improved efficiencies. The wet trommel screen classification 33 prevents energy waste on off-specification material. The high efficiency pelletisation minimises a recycle stream resulting in significant cost savings.

Figure 4 shows a process 40 whereby the granules and oversize 47 screened through the trommel screen 43 are passed directly to the drier 48, thereby foregoing the oversize screening. This circuit therefor goes without oversize screening or secondary agglomeration/granulation/pelletisation resulting in significant costs savings.

The process according to the invention as described above may be used for pelletising minerals other than polyhalite, and in particular for pelletising feedstocks composed principally of one or more evaporite minerals, especially other chloride minerals. These may include any one or more of Anyhdrite, Carnalite, Gypsum, Halite, Kainite, Kieserite, Langbeinite and/or Sylvite. The process is especially suitable for pelletising feedstocks composed principally of minerals that are substantially hygroscopic in recently powdered form and/or that have a Moh's hardness in the range from 2 to 4. The resulting pellets may be used for purposes other than fertilisation. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1 . A method for forming a pelletised evaporite mineral product, the method comprising: pulverising an evaporite mineral feedstock to form a powder; mixing the powder with a binder in the presence of a liquid to form a mixture; wet screening the mixture to separate a granule fraction and a fines fraction; and processing the fines fraction using a pelletiser to form a quantity of pellets principally composed of the evaporite mineral.

2. The method according to claim 1 , the method further comprising: separating at least one of an oversized fraction and an undersized fraction from the quantity of pellets; disaggregating the pellets of the oversized and/or undersized fraction to form disaggregated material; and re-pelletising the disaggregated material.

3. A method as claimed in claim 1 or 2 wherein the granule fraction is directed through an oversize screen to separate product granules from non-product granules.

4. A method as claimed in claim 3 wherein the product granules are directed to a drier to form pellets.

5. A method as claimed in claim 3 wherein the non-product granules are directed to a secondary mixer which feeds the wet screening.

6. A method as claimed in claim 3 comprising disaggregating the pellets of the oversized and/or undersized fraction and or the non-product granules in an operating chamber of a mixer/granulator and re-pelletising the disaggregated material in that same operating chamber.

7. A method as claimed in any preceding claim, comprising mixing the powder with the binder in an operating chamber of a mixer/granulator and processing the mixture to form pellets in the operating chamber.

8. A method as claimed in any preceding claim, wherein the wet screening involves the application of a rotary drum screen.

9. A method as claimed in claim 8 wherein the rotary drum screen is a self cleaning trommel screen.

10. A method as claimed in any preceding claim, comprising disaggregating the pellets without substantially drying them subsequent to the processing step.

11. A method as claimed in any preceding claim wherein the binder comprises starch.

12. A method as claimed in any preceding claim wherein the binder is flour.

13. A method as claimed in any preceding claim, wherein the liquid comprises water.

14. A method as claimed in any preceding claim, wherein the pellets of the quantity of pellets contain greater than 90% by mass of the evaporite mineral.

15. A method as claimed in any preceding claim, wherein the evaporite mineral is polyhalite.