Ro-Star™ Vertical High Intensity Ultra-fine Grinding Mill
Background to the Ro-Star Development
As ore grades continue to decline and free milling ore bodies become depleted mining companies are being forced to treat more refractory ores often requiring finer grinding than has traditionally been associated with conventional ball mills. These finer grinds have resulted in a new generation of stirred bead mills which are much more efficient than the traditional ball mills, as the desired product size drops below 75 microns. Due to limited competition in this ultra-fine grinding mill market these mills are generally very expensive and thus increasing the overall capital cost of such projects. The Ro-Star mill development originates from MMS’s requirement to grind refractory gold concentrates to very fine sizes as part of its Leachox™ process. As such MMS has gained valuable experience in investigating and operating, in its own right, ultra-fine grinding mills in the mining industry.
The Ro-Star is a suspended, vertical high intensity wet grinding mill with a vertical drum and rotor-stator grinding disc mechanism. Hence the name “Ro-Star” The mill body consists of a shell equipped with internals that are in a specially designed segmented disc arrangement and then an agitator shaft with a similar arrangement. The rotor-stator configuration has been developed with flexibility in mind. The feed slurry is pumped into the mill from the base and exits by means of an overflow arrangement at the top of the mill which is exposed to the atmosphere.
Flexibility of Operation
The Ro-Star can be run in open or closed circuit and can be combined with a classification stage of hydro-cyclones, either before or after the mill, to reach a final product quality. Adjustment of the mill speed, solids content, residence time, mill filling and media type allows matching of the operation with the required product quality. The fineness of product can be automatically controlled by means of on-line particle sizing technology.
Key Design Advantages
The mill has a compact design giving high power density (kW/m3) connected with a small footprint. The design of the Rotor-Stator arrangement is unique amongst the range of ultra-fine grinding mills currently offered. The concept does not require a high tip speed of the grinding discs to transfer energy into bead movement as typified in other mills, with their consequential higher energy usage and wear rates of components.
Due to the lower rotational speeds required, generally less than 10 m/s, minimal wear rates are envisaged with the use of hard steel mill components. Due to the design of the Rotor-Stator concept the mill can be equipped with only hard steel wear components and thus the use of plastics, such as polyurethane can be avoided. The history of operations undertaking ultra-fine grinding has shown that the high temperatures that are generated in the mills can degrade polyurethane which leads to excessive wear. The suspended mill concept facilitates rapid removal of mill internals as the floor space below the mill is now designed for bead retention and re-loading of the mill. A complete spare agitator shaft can be on stand-by to maximise mill availability.
The mill is specifically designed for high density (up to 6.3 g/cm3) ceramic media and can accommodate high bulk densities of media down to sizes of 0.8mm in all mill sizes.
Ro-Star Mill Sizes
|Installed power (kW)||45||75||160||355||630||1000||1500|
|Mill volume (litres)||55||120||300||700||1200||1950||3000|
Typical Applications or Ro-Star Ultra-fine Grinding
Ultra Fine Grinding of Refractory Mineral Concentrates- Leachox process™
In this application a refractory gold orebody, typically a pyrite/arsenopyrite ore, is ground to approximately 80-100% -74 microns in size in a conventional milling circuit. Imhoflot pneumatic flotation is then conducted to produce a low mass, high grade sulphide concentrate. This concentrate is then ultra-fine milled in the Ro-Star™ mill to <20 microns or even as low as <10 microns, depending upon the mineralogy, before being subjected to partial oxidation using Aachen Reactors and the Leachox process. Finally depending upon local regulatory requirements, the MMS cyanide destruction process CN-D can be used alone or in conjunction with the Air/SO2 cyanide destruction process to reduce residual cyanide tenors to below the discharge requirement.
Fine Grinding of Base Metal Flotation Concentrates and Subsequent Flotation
In this application the Ro-Star™ mill is used to regrind flotation concentrates to improve metal liberation and hence recovery. The grind size varies according to mineral type and orebody but would typically be down to a P80 of 45-20 micron i.e. finer than that for conventional ball milling but slightly coarser than ultra-fine grinding.
The next stage of the process is to use Imhoflot G-Cells to float the liberated mineral. The use of G-Cells here is particularly important as they recover fines which cannot be recovered by conventional tank cells leading to a significantly higher metal recovery.
Continuing Ro-Star R&D, Testing and Technical Development
To assist in the development of the Ro-Star mill for industrial applications and to aid scale-up calculations MMS, in conjunction with academia and the mining industry, are undertaking an EU funded European Institute of Innovation and Technology Raw Materials project. This project, which commenced in 2016, is investigating the scale-up of the Ro-Star mill from laboratory and pilot size to an industrial sized mill placed at an operating base metal mine. To this end a containerised Ro-Star pilot plant has been assembled which can undertake on site testwork at client’s operations. In addition, laboratory units have been manufactured for laboratory testing at Maelgwyn Mineral Services Africa metallurgical laboratory in South Africa and other selected commercial laboratories.