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Millions of litres of oil recycled from grinding sludge

German automotive company ZF Friedrichshafen AG has recovered around 1.75 million litres of valuable oil in the past 12 years by processing grinding sludge in two RUF briquetting presses.

ZF is one of the world’s largest automotive suppliers with around 230 locations in about 40 countries. At the two plants in Passau, it manufactures around 200 000 axles and transmissions, as well as other drive components. At its Thyrnau site, employees produce bevel gear sets as well as spur gears, including for the company’s own electric vehicle drives: in total about two million per year.

Thanks to RUF briquette presses at both sites, which have been operational for around 100 000 hours, the company has achieved return on investment in six months and less than two years, respectively. Around 250 000 litres of oil from grinding sludge builds up during production and these quantities were ultimately saved when purchasing fresh oil. At current prices, this equates to a net purchase value of EUR 2.2 million.

In Thyrnau, 35 grinding machines currently produce between 250 and 300 tonnes of grinding sludge per year. About half of it is made up of steel particles, microscopically fine steel chips, and grinding wheel abrasion while the other half is grinding oil. Until 2006, technicians used their own constructions to press some of the oil from the sludge.

‘However, we were nowhere near recovering the large quantities of oil that the RUF press enables,’ says Johann Zinöcker, who is in charge of environmental and energy management in the plant. This development in oil recovery was initiated by the RUF sales manager, Andreas Jessberger, who read about the Thyrnau plant in a trade journal in 2006 and subsequently pointed out the potential of the briquetting plants to the plant management.

‘By pressing with the RUF machine we recover between 100 and 120 tonnes of oil per year from the grinding sludge,’ Zinöcker points out. Since the oil has a density of only about 0.85 g/cm3, these masses correspond to a volume of about 120 to 140 cubic metres. ‘The press is simple and uncomplicated – if only all machines would run so reliably and economically,’ Zinöcker adds.

At the current price per kilogram for oil (just under EUR 1.50), the press currently generates annual savings of up to EUR 175 000. Furthermore, disposal costs for the residual grinding sludge have been significantly reduced.

The RUF system, driven by a four-kilowatt electric motor, compresses the grinding sludge and presses out most of the oil. What remains are largely de-oiled briquettes consisting of fine metal abrasion and a small amount of residual oil. Two-stage filtering system automatically purifies the oil. The oil separated during the pressing process is collected and purified in a two-stage filtering system designed by ZF in Thyrnau.

For the plant in Passau, RUF integrated this filter solution directly into the briquetting system. A coarse filter retains elements larger than 160 micrometres. This is followed by a second stage, which also separates particles larger than 40 micrometres. From there the oil flows into an IBC container holding one cubic metre. Full containers remain motionless for four days so that any remaining solids sink to the bottom. Following this sedimentation phase, ZF pumps out the oil from the top and returns it to the grinding machines without further treatment.

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