Multotec Rubber was founded to service the specific target market for rubber and composite mill and scrubber linings. Other products include: smooth or profiled rubber impact wear plates; rubber screen panels; rubber chute liners; conveyor belt loading points; and composite liners. “We have a similar philosophy to Multotec Manufacturing in that we make consumables. We are not a mill manufacturer. The mill lining comprises a whole range of different components, shell plates, lifter bars, head plates, grate plates and such like, and these are our products,” Taylor tells MechTech.

He tells us about the use of rubber for lining grinding mills and scrubbers: “The shell plates protect the steel of the mill and the lifter bars do two things; they clamp down the shell plates so they don’t fall out and they lift the balls and the charge to create the tumbling action needed for grinding.

“In the old days, they only used steel but rubber has been used now for about 40 years, particularly in the smaller ball mills.” Rubber companies throughout the world have expanded into supplying rubber composite liners for SAG and Primary Ball mills, as well as rubber linings for AG mills in the diamond industry. He explains the difference between these mills: “Autogenous Grinding (AG) doesn’t use any grinding balls, Semi-Autogenous, (SAG) uses a reduced number of balls and ball mills use a full complement of tumbling steel balls to do the grinding.”

Using rubber instead of steel for mill and scrubber linings results in less wear and a longer life with substantial cost savings. Other advantages include:

• Reduces downtime, both during initial installation and replacement.
• Reduced weight and hence less wear on components like bearings and reduced power consumption;
• Rubber is more user friendly and safer to handle and install.
• It has excellent sealing properties, reducing the risk of flushing behind the lining and leakage through bolt holes.
• Rubber virtually eliminates clogging at the grate plates.

“Rubber is also excellent because it reduces noise and is popular in the diamond industry because it doesn’t cause any degradation of the diamonds. It is light, easy to handle and cut to size,” says Taylor. “However, as you get to the bigger mills, you get to a certain size where it makes more sense to use steel,” he adds.
“We have to design certain face angles, heights and thicknesses into our components which are integral to the design of a mill. We specify for a customer, what we believe is the best dimensional arrangement for a particular grinding operation. We also deal very closely with OEMs in this country, FLSmidth Minerals, Krupp Polysius, Outotec and Bateman,” he explains, “specifying things like the drilling pattern for them.”

We ask about the difference between a mill and a scrubber. “A scrubber uses no charge and there is a much higher water to ore ratio. They also generally run at a slower speed,” he responds. “A scrubber generally breaks up the clay mass into something that is easier to screen in the downstream process – to liberate minerals like diamonds. In a mill you are specifically looking for size reduction, usually a very fine fraction of around 75-100 microns. In a scrubber, some size reduction does occur but this is a secondary benefit,” Taylor explains.

In extending the market for rubber to bigger mills, Multotec Rubber has a composite range of lifter bars. “Composite lifter bars have a 75 mm thick Hardox 500 quench and tempered plate embedded into the rubber on the leading face of the lifter bar of a mill. We also supply a heavy duty lifter bar which has a steel casting embedded in the rubber, called the HD MultoMet,” he informs us. Composites enable rubber to be used for larger diameter mills. “We can now do mills of up to 6,4 metres in diameter with composites,” explains Taylor. There comes a size that is too large to do with either pure rubber or composite, and then you will need to go for steel liners.”

Where rubber is unsuitable, Multotec Rubber has an Africa-wide licensing agreement with ME Elecmetal for cast-steel grinding mill components. “A restriction you have when using rubber, is the percentage critical speed. At 100% critical, the centrifugal force and the gravitational force balance each other out, so the charge stays on the shell. Your ideal range for rubber is between 72 and 75%. As the mill runs, the charge forms a kidney shape and the balls fall into the right area for effective and efficient grinding. For any percentage critical speed above 80% though, you have to use steel liners, because there is slippage, which causes much more abrasion. Then, no matter how big your mill is, you must use metal,” he explains.

We ask about the manufacturing process. “We use a compression moulding process – you close the press, hold it under pressure and supply steam and you get cross-linking of your rubber formulation which gives you the properties and strengths that you need for the application.”

The rubber used by Multotec is a blended compound made to a proprietary Multotec formulation. “We use a limited range of very high quality rubber compounds,” he tells MechTech. We have developed one quality compound – called system 3000 – that we can manufacture economically and we use it for all of our mill linings and screen panels.

“Our formulation has a very good blend of high impact, abrasion and cut resistance. You can vary these properties easily though, for example in a chute liner you might need more abrasion and cut resistance but less impact,” he says.

Multotec manufactures all of the components loosely, which then they get bolted into the mill at the customer’s site. “We have two installation teams who fly all over Africa to fit linings to mills. We have just sent a team to Argentina too,” says Taylor.

The recession? “Obviously, we all have to tighten our belts, chase business and be more active in the market. We are still maintaining our turnover, though, and if we analyse our customers, 29 out of our top 30 are still active but six of those are in the seriously affected diamond industry,” he concludes.

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