February 26, 2016

The Combimix system – Creating value from overburden

The operating principle of the Combimix system

The Combimix system (DKXC) transforms the DKX type BHS twin-shaft batch mixer into a mixer that operates continuously according to the established three-dimensional mixing principle.

The Combimix system is an economical alternative to the wet processing of clay-and-rock mixtures. It removes clay adhesions from screen waste produced in quarries and gravel pits. Operators report that they are able to convert more than 90% of contaminated feed material into salable rock grade using the Combimix system. At the same time, the dry process significantly reduces water, energy and material costs.

Many quarries contain large quantities of material contaminated with clay that was previously impossible to recycle economically. In many cases, the material had to be dumped at considerable expense as mining overburden. However, wet treatment is one method that allows this material to be used. Yet washing and subsequent water treatment as well as sludge disposal used to be extremely costly. The process often broke down because many quarries lacked access to the required volume of water. Dry treatment in conventional continuous mixers using quick lime has also been problematic because the residence times of the material in the mixer are so short that the clay cannot blend adequately with the lime.

The solution: Variable residence time

The patented BHS Combimix system developed by BHS-Sonthofen offers an economical alternative, and its effectiveness has already been proven in several quarries. The basic principle has been known for years, but it had not been possible to implement the system due to various limitations in mixing technology: Mixing clay with quick lime changes the properties of the clay, which can then be reliably separated from the rock.

For the first time, DKXC type twin-shaft continuous mixers allow regulation of the residence time of the material in the mixer within broad limits. The clay has enough time to mix intensively with the lime and detach from the rock. The intensive movement of material during the mixing process further assists the cleaning process. After screening, the feed material is so clean that it is no longer classified as waste and can be sold. Grains greater than 5 mm in diameter that are largely free of clay can now be further processed in the gravel quarry. Grains less than 5 mm in diameter form a lime/clay mixture that can be sold to the agriculture industry, for example.

Combination of batch and continuous mixers

The Combimix process is based on the combination of twin-shaft batch and continuous mixers manufactured by BHS-Sonthofen. Unlike traditional continuous mixers, the residence time of materials mixed in the Combimix system can be regulated within broad limits – from less than 30 to more than 100 seconds. With BHS technology, the residence time is regulated via a slide on the underside of the mixing trough. This type of trough design is unique on the market. Extracted material is replaced with an equivalent volume of feed material. Continuous measurements are taken during this process to monitor the weight of the mixer. The quick lime is delivered into the mixer by a screw conveyor according to the clay content in the feed material (less than one or a maximum of five percent of the infeed quantity).

Together with the long residence time, the proven three-dimensional mixing principle of the BHS-Sonthofen twin-shaft mixer makes an important contribution to achieving good results: The lime is mixed intensively with the clay so that the treated clay reliably separates from the rock. Costs for supplying water, reslurrying, filtering and disposing of the slurry are no longer incurred because the material is treated in dry state.

 

Use at a plant in France

The Combimix system of type DKXC in a CEMEX France processing plant for the dry treatment of clay-and-rock mixtures, located in the South of France.

Example of a process diagram with the Combimix system

Example of a process diagram for the dry treatment of clay-and-rock mixtures with the Combimix system (DKXC).

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