Twin-shaft mixer increases yield by 25 per cent

German aggregates business MSW Mineralstoffwerke Südwest GmbH & Co KG operates a quarry at Mönsheim, about 30km from Stuttgart, Germany. It supplies shell limestone aggregates mainly used for road construction.

The shell limestone extracted at the quarry contains high quantities of clay. This used to mean that on average, 30 to 35 per cent of the extracted material could not be sold. The pre-screened material comprising clay and rock mixtures had to be deposited at the quarry’s landfill.

Consequently, MSW started looking for a way to increase the yield of ongoing operations while making it possible for stored waste material to be made usable.

One method for removing clay from pre-screened material is wet processing in a log washer. However, the volume of water available at the site was insufficient for this method. Aside from this, mandatory water treatment and slurry disposal would have been very complex and expensive.

The solution

{{image2-a:r-w:300}}For about six years, a nearby company affiliated with MSW has been running a BHS-Sonthofen single-shaft continuous mixer of type MFKG, for conditioning the dust that is extracted from the aggregate processing plant.

BHS’s mixing technology division produces batch and continuous mixers and offers a full range of process technology services for mixing applications. Its twin-shaft batch mixer, which is considered a global benchmark in the concrete industry, is a key product. Additional fields of application for the mixers include production of dry mortar, cement, sand-lime, concrete paving blocks, asphalt, mineral mixes, landfill construction materials and backfill for the mining industry.

During a visit by MSW personnel to the facility to see the single-shaft mixer, the conversation turned to the planned processing of pre-screened material.

BHS personnel discussed the successful processing of clay and rock mixtures in France, where BHS mixers use the dry Combimix process to make about 90 per cent of the contaminated feed material usable again. BHS developed the method for this application at French companies that were seeking an economical, environmentally friendly solution suitable for industrial processes. These companies include international market leaders such as Cemex Granulats and LafargeHolcim, and medium-sized companies such as Gachet TP, an innovative producer of construction and recycled materials.

Mixing the clay-contaminated pre-screened material with quicklime in a BHS twin-shaft mixer changes the properties of the clay. The mixing process dries out the clay, allowing for the mechanical stress of the mixing tools and the material itself to detach and separate the clay from the rock. Following this separation, the valuable rock can easily be screened from the clay and fed into the next processing step.

This alternative approach was so attractive to the MSW management that they immediately decided to send material from the Mönsheim quarry to the BHS-Sonthofen technical centre for testing.

{{image3-a:r-w:300}}After just four days, the first tests using a laboratory twin-shaft mixer were initiated at the Sonthofen site in southern Germany. Within minutes, it became clear the mixing process was delivering the desired outcome. This finding was confirmed by several additional test series, which involved analysing the sample at the MSW laboratory. The tests showed that adding between one and three per cent of quicklime delivered promising results, even after short periods of mixing.

The project

However, still missing was evidence that it was possible to consistently clean larger quantities of material with a specified maximum particle size of 120mm.

It was also necessary to test the response of the existing processing system, which includes screens and a finger roller screen. For this reason, MSW and BHS-Sonthofen joined forces with the supplier of the quicklime, Märker Kalk GmbH, to perform a large-scale test at the quarry. This test showed that the processed material was clean and usable, even if the feed material contained large stones.

After this, MSW gave the project the go-ahead and – with the help of BHS – redesigned the plant based on the mixer. It was possible to use existing components for the steel construction, and three large silos for binding material were already available on-site.

{{image4-a:r-w:300}}About one per cent of quicklime is added to the twin-shaft mixer containing pre-screened clay and rock mixtures with particle sizes between zero and 120mm.

The proven three-dimensional mixing principle used by the BHS-Sonthofen twin-shaft mixer is a decisive success factor for achieving good results.

A material retention time of 90 seconds proves sufficient for the lime to be thoroughly mixed and react with the clayey rock. The vigorous movement of material during mixing further assists the cleaning process.

Up to 520 tonnes of stone is fed into the plant every hour. From this initial feed material, the roller screen separates out 240 tonnes of clay and rock mixtures with a particle size of zero to 120mm, and delivers it to the BHS mixer.

The output material is screened using a 22mm finger roller screen. Each hour, 150 to 180 tonnes of clean material with particle sizes larger than 22mm is fed back into production. In this way, raw material utilisation increases to more than 90 per cent from its previous level of roughly 65 per cent.

{{image5-a:r-w:300}}Clay-contaminated waste material with particle sizes of zero to 8mm can be sold as dam-building material, while material with particle sizes of 8mm to 16mm can be monetised as loose aggregates.


MSW officially commissioned the plant in June 2016. Today, it is fully integrated into the quarry’s production processes. Since its commissioning, the mixer has been running reliably, and expectations for the output material have been met in full.

After the additional processing step of crushing and screening, the material was clean enough to be sold. This was confirmed by test reports from the recognised testing bodies that perform third party monitoring of the facility.

Experience gained in the first weeks shows that adding about 0.8 per cent of quicklime to the dry material results in a very good separation of the clay and rock mixture.

About 70 per cent of the material that would previously have been deposited at landfill is now usable. Before implementing this process, the maximum volume of saleable material that could be produced per hour was 280 tonnes. The supply capacity has now increased to 430 to 460 tonnes per hour.

{{image6-a:r-w:300}}In other words, to produce the same quantity of goods per year, MSW would previously have needed to extract about 620,000 tonnes of limestone. The company now needs to extract about 432,000 tonnes, which corresponds to a relative reduction of about 30 per cent.

The new process thus conserves limestone resources and thus extends the usable lifetime of the quarry. Furthermore, the plant’s daily operating period is now considerably shorter. It is in operation for two hours less each day, which results in reduced energy and personnel expenses.

This benefit is reflected in the concrete business figures. In contrast to wet processing, the dry process does not incur costs for supplying water or for reslurrying, filtering and slurry disposal.

Also, since the entire plant is operating for shorter periods of time, energy costs are reduced by about 19.5 per cent. This translates into a potential reduction in carbon dioxide emissions of about 30 tonnes per year.

The shorter production hours lead to cost savings in energy, fuel and personnel, approximating at 240,000 Euros ($AUD371,672) per year. Savings are offset by the cost of buying the quicklime, which amounts to about 120,000 Euros ($AUD185,836) per year. Given annual savings of about 120,000 Euros, the investment will pay for itself in four and a half years.

{{image7-a:r-w:300}}This calculation does not consider the potential of “reactivating” large volumes of material that have been deposited at landfill. At times when there are no quarrying activities, the new plant can clean clay and rock mixtures that were deposited over two whole decades because they were considered unmarketable in the past.

As a result, several million tonnes of additional material can now be processed and sold. Furthermore, valuable landfill space is freed up, and time spent storing and retrieving material and distributing it within the landfill is significantly reduced.

This was the first time this innovative and resource-saving technology had been used on a large scale for the efficient and sustainable use of raw materials in the construction industry. For this reason, Germany’s Federal Environment Ministry provided MSW Mineralstoffwerke Südwest with funding.

After the system was commissioned, BHS-Sonthofen and MSW Mineralstoffwerke Südwest received the German Resources Efficiency Award from the German Federal Ministry for Economic Affairs and Energy for the development and successful implementation of the Combimix process.

Gerd Schuler is the mixing and crushing technology field representative at BHS-Sonthoften. Stefan Kunkel is the plant manager at MSW's Monsheim Quarry.

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