Gravity separation techniques are used to separate minerals of different relative densities. The dominant competing force in gravity separation is usually a fluid drag force. Pneumatic tabling and winnowing have been included as dry forms of gravity separation, although in these cases particle shape will contribute significantly to product quality.
Bench Scale Capabilities
- Heavy Liquid Separation (HLS)
HLS utilises the differences in particle density to concentrate material into various density classes. Separation is facilitated by a separation medium as follows; for densities greater than 2.96g/cm³, an unstable suspension of TetraBromoEthane (TBE) and atomised ferrosilicon is used as the medium. This method of fractionation is time-dependant and hence only particles coarser than 1mm can be treated. For densities less than 2.96g/cm³, a purely liquid medium of TBE and acetone is used. Density classes with intervals of 0.05g/cm3 can be fractionated with the data being used to construct washability grade-recovery curves. Typical ores treated by HLS include kimberlites, PGMs, hematite, chromite and base metals.
- Shaking Tables
Shaking tables utilise both particle density and to a lesser extent particle size to fractionate fine ore samples (<1mm) into product, middlings and waste fractions. The grade-recovery profile obtained on the shaking table is also used as a benchmark for modelling performance of a spiral circuit. A standard one-eighth size laboratory scale Wilfley shaking table with an Metquip deck is employed for bench scale testwork. Chromite, tantalite, heavy minerals, ferrous and non-ferrous ores and PGM-bearing ores are typically characterised on shaking tables.
- Mineral Density Separator
The MDS is a laboratory scale unit that utilises both differences in particle density and to a lesser extent size to fractionate coarser ores (<40mm >1mm). MDS testwork is often conducted in conjunction with HLS to evaluate an ore’s potential to be upgraded by dense medium separation or by jigging. The MDS is capable of fractionating an ore into different density fractions exceeding 4g/cm3. Mintek does not have pilot jigging capabilities. Typical applications of the MDS are to characterise hematite ores, manganese ores and ferrochrome slags.
- Mozley Super Panner
The Mozley Super Panner is used for quick and efficient separation of mineral groups with distinct density differences and is generally used as a cleaner unit where the mass available for upgrading is very small: typically for Au applications.
- Teeter Bed Separators
A teeter bed separator (TBS) or hydrosizer is a hindered settling classifier in which an upward flow of teeter water is used to stratify and separate coarse, dense particles from smaller, lighter particles. Since the separation in a TBS is based on differential settling rates, particle size, density and shape all contribute to the quality of separation.
A laboratory TBS for samples of ~6kg in mass and a pilot TBS unit capable of treating up to 1t/h solids are employed for testwork. Typical applications of a TBS include coal, hematite, chromite, heavy minerals and uraninite ores.
A winnower separates particles based on shape and relative density by exploiting the differential drag of particles in an airstream. Mintek has a horizontal and vertical feed laboratory and pilot scale unit that has in the past been modified for specific applications. Micaceous ores such as vermiculite are upgraded using winnowers and more recently the removal of dolomite from char has been successfully installed in industry.
Pilot scale Capabilities
- Dense Medium Separation (DMS)
Bulk samples in the size class -20+1mm are treated in a 2-5t/h DMS cyclone plant which operates with either 250mm or 350mm diameter cyclones.
The facility is able to treat samples from 100kg to 100t in the larger pilot applications. Density tracers and HLS analyses on the sink and float fractions (or MDS analyses at densities above 4g/cm3) are used to confirm the separation efficiency of the DMS testwork. Projects on diamondiferous gravels and kimberlitic ores, andalusite, hematite, PGMs, gold, base metals, and chromite ores have been run successfully providing design information as part of bankable feasibility studies.
DMS-cyclone plant at Mintek
Spirals operate using density differences to concentrate material into respective high grade and low grade (waste) products. For spiral applications, the feed material must be less than 1mm and deslimed at 38µm should an excessive amount of fines be present.
A variety of spiral concentrators, including large diameter and high capacity spirals that can be run either in batch mode or continuously at solids feed rates from 1t/h to 8t/h are available. Tantalite, hematite, phosphates, heavy minerals, coal, PGMs, base metals and chromite ore fines have all been evaluated at pilot scale, typically in conjunction with hydrocyclones.
- Pneumatic or air tables
Air tables operate in a similar manner to shaking tables with air used as the medium instead of water. Air is continuously injected through the porous bed of the table to facilitate separation. Unlike shaking tables where particle size increases and density decreases from concentrate to tailings, on air tables both particle size and density decrease from concentrate to tailings. A pilot air table at a feed rate of ~100-300kg/h is employed for testwork. Air tables are typically employed to clean zircon concentrates, remove shredded metals from plastics, upgrade vermiculite and staurolite
Enhanced Gravity Separation:
Bench Scale Capabilities
- Mozley Multi-Gravity Separator (MGS)
The Mozley MGS unit is suitable for the treatment of fines with a top size of 500micron but is most efficient below 300micron. The MGS utilises a similar working procedure to the shaking table with an additional centrifugal force applied to enhance the separation of fine particles. As the MGS is a low capacity unit, it is typically employed for the cleaning of precious metals or valuable minerals such as cassiterite from pre-concentrates.
- Falcon Centrifugal Concentrator
The falcon concentrator is essentially a spinning fluidised bed concentrator. The feed (<2mm) flows up the sides of a cone-shaped bowl, where it stratifies according to particle density before passing over a concentrate bed fluidised from behind by back pressure water. The concentrate bed retains dense particles and lighter gangue particles are washed over the top. Falcon automated batch concentrators are typically applied for the recovery of free gold where low mass pull (<1%) is required. High mass pulls of up to 40% to concentrate are also achievable on the continuous units. Base metals, industrial minerals and tantalite ores have also been successfully treated on falcon concentrators.
Magnetic & Electrostatic Separation:
Magnetic separators utilise the differences in magnetic susceptibility to separate either valuable minerals from gangue. In general, minerals can be classified as diamagnetic, paramagnetic or ferromagnetic. Different types of separator are used depending on wet/dry applications, particle size and magnetic susceptibility.
Electrostatic separation utilises the difference in electrical conductivity between the various minerals. It is typically applied to heavy mineral beach sands.
Bench Scale Capabilities
- Davis Tube
The Davis Tube is used for the separation of fine ferromagnetic particles from non-magnetic particles. Material is fed into a glass tube that lies at an angle between two magnets, where the magnet strength can be varied. Separation is facilitated by the reciprocating motion of the tube. Tube rotation of 120° and stroke of 2 inches (51 mm) are fixed standards. This wet method of separation is used as a benchmarking tool for Low intensity Magnetic Separators (LIMS).
- Frantz Isodynamic magnetic separator
The Frantz Isodynamic magnetic separator is a precision laboratory instrument and consists of a chute passing through two electromagnetic poles. The magnetic field and/or chute slope can be varied to separate fine, dry mineral samples. The method of separation is on a particle by particle basis and is hence used as a benchmarking tool for magnetic separation. The Frantz is typically used to provide detailed, accurate fractionation of mineral samples with similar magnetic susceptibilities such as titaniferous magnetite and mineral sands.
- Ultrastat separator
A strong static electric field is used to selectively induce charge onto the conductive mineral particles (Conductive Induction Charging). The mineral passes beneath a charged electrode which induces a polar opposite charge on the conductor particles; as a result these charged particles are electro statically attracted to the electrode and are drawn away from the grounded surface. A splitter located further in the separation zone separates the conductor particle and non conductor particle trajectories dividing the feed into mainly non-conductor and conductor fractions.
Bench Scale and Pilot Scale Capabilities
- Low Intensity Magnetic Separators (LIMS)
All LIMS separators are used to recover ferromagnetic minerals such as magnetite or pyrrhotite from paramagnetic and non-magnetic minerals. This method utilises a drum with permanent magnets which generate a magnetic field of ~700 to 3000 Gauss. Recovery of ferromagnetic is usually very efficient down to <5micron sizes as long as operating conditions are varied to ensure selective magnetic agglomeration can take place. Mintek has a variety of LIMS units from laboratory scale to pilot scale.
- Permanent roll magnet (Permroll)
The Permroll is a dry separator and is most suited for samples coarser than about 100 micron. The unit operates with a graphite impregnated belt passing around a permanent magnet and tension pulley. Three magnet strengths can be utilised namely the Barium Ferrite (Low), Samarium Cobalt (Medium) and Neodymium Born Iron (High). Both a laboratory unit and a pilot separator (300kg/h) are available. These types of magnetic separators are usually used in the latter stages in minerals processing circuits as their capacities are not high and their feed material must be dried and cooled to ensure no decay of the magnetic field strength. They are commonly used in mineral sands and andalusite cleaning.
- Wet High Intensity Magnetic Separator (WHIMS)
The WHIMS is suited for particles less than -1mm, and is capable of separating material with paramagnetic properties from non-magnetic material. WHIMS is done at both laboratory (Eriez) and pilot scale (Jones pilot-scale separator). The main application fields of WHIMS are recovery of ilmenite, hematite, rutile and manganese products as well as removal of Fe contaminates from fluorspar and other industrial minerals.
Ore sorting is mainly utilised as a pre-concentrating method whereby barren waste is rejected thereby aiming to reduce downstream energy, water, capital and operating cost requirements. Sufficient liberation of the ore is required to prevent significant loss of values. Additional requirements include:
- Sizing of the feed into top:bottom size ratios of about 2:1
- Preparation of a clean particle surface
- Particles are required to be fed in a mono-layer.
- Commodas Ultrasort
The unit on site is capable of handling up to 100t/h at a feed size range of 20mm to 80mm, depending on the mineral composition. The double camera optical sorter unit has been successfully tested for the cost-effective upgrading of gold, platimum-group metal (UG2), base metals and industrial minerals.
- X-Ray Fluorescence sorter
The RADOS XRF Sorter uses well established XRF technology to distinguish between concentrate and waste material. The full scale unit at Mintek can accommodate a maximum throughput of 30 tph which is dependent on particle size and density. The single unit can process particles that are -150mm with a minimum size being 30mm. The RADOS XRF sorter has been successfully tested as a waste removal option for PGM ores (Merensky and Platreef), hematite, chromitite and uranium ores.
Modelling & Simulations:
Bench scale characterisation data from HLS, MDS, shaking tables, spirals and LIMS is scaled up to pilot plant and full scale applications. Realistic performance criteria such as Ep, are included in the modelling and flowsheet simulations to ensure predicted plant performance is not overstated. The models are specific per unit operation but are based on modified Weibulls equation depicting partition number versus both particle density and size. A further enhancement has been the inclusion of mineralogically derived characterisation data for fine samples (<1mm) that produces particle by particle data including chemical composition, liberation, density and shape parameters.