Twin mixing plant: Two different mixing procedures in one system

1 Introduction

Müllerkalk supplies both special binders for ground stabilization and agricultural lime from the Üxheim-Ahütte plant. The raw materials required for this mainly come from the lime works’ own quarries.

The company was founded in 1834 and has recently developed “Müllermix”, the new product for ground stabilization; it consists of a mixture of various lime products and Portland cement. It avoids several work steps in road construction, thereby cutting costs: “Müllermix” is milled into the ground and hydraulically bound – the ground no longer has to be excavated.

Apart from standardized products, Müllerkalk also offers customized mixtures: The customers sample their ground and receive individually configured mixtures from Müllerkalk depending on its composition. Although both product lines have only been on offer for a short time, the demand for the products has been steadily growing in the meantime.

The second product line consists of earth-moist agricultural limes for agriculture and forestry. They are used to neutralize the pH value of the soil. The moist – and therefore heavier – particles of the agricultural lime spread evenly over the ground and thereby have a direct effect.

Müllerkalk has used a single-shaft continuous mixer of the type MFKG 0520 from BHS-Sonthofen to moisten agricultural lime for more than ten years. With a throughput rate of around 50 t/h, it could no longer cope with the continuously growing demand and the requisite shortest possible truck loading times. The company management therefore planned a ­major expansion of the production capacity.

2 The goal: Exploitation of synergy effects

Since the sale of the various lime products is subject to seasonal fluctuations, Müllerkalk was also looking for solutions which enable constant utilization of the production facilities as far as possible.

The expansion of the product range with the new special binders finally tipped the scales in favor of investment in a new mixing system. It includes both product lines – both dry mixing in batches as well as continuous moistening. Another synergy effect for the company as a whole was provided by using the cement products of the parent company, Portlandzementwerk “Wotan”.

The homogeneity of the products and a consistent quality were of prime importance in the selection of the mixers.

Since the plant had grown over decades from small beginnings, the extremely cramped conditions and the space requirement for the construction of the new mixing system were a particular challenge.

Owing to the excellent experience with the existing single-shaft mixer, Müllerkalk contacted BHS-Sonthofen, but only on the subject of “mixing technology” to begin with. It quickly became clear that the dry powder batch mixers of the type DMX were the perfect solution for the planned special binders. These mixers were therefore “set” for the project, irrespective of the potential suppliers of the overall mixing system.

However, the discussions were gradually extended from the mixers to the complete plant, because the concept was so far from a standard solution that the special application and engineering know-how of BHS was needed for the special plant technology.

3 Mixers in a twin-package

In the course of the project, the specialists at BHS-Sonthofen developed a concept for a twin mixing plant together with the personnel of Müllerkalk. It combines the fundamentally different mixing procedures for agricultural lime and mixed binders in two mixers:

The engineers from Sonthofen recommended the dry powder batch mixer of the type DMX 2600 for the mixed binders; this provides an output of 2.2 m³ per batch, which corresponds to an hourly output of approximately 100 t.

The MFKG 0728 single-shaft continuous mixer, which achieves an output of between 80 and 90 t/h, was planned for the earth-moist agricultural lime.

Apart from the BHS mixers, the system includes two binder scales for the batch process and a volume weighing unit for continuous metering. Both systems are supplied from ten 150 m³ silos. The silos are filled with raw products via a newly installed pneumatic conveying plant – firstly, directly from the lime plant and, secondly, with high-quality cement products from the adjoining Wotan cement plant.

The experience of the operators was also incorporated in this special construction. For example, a wide stairway was built up to the filters of the binder silos to provide optimum accessibility for maintenance and service work.

4 Just-in-time delivery

This overall package was the key to awarding the contract to BHS-Sonthofen in December 2013. The construction and assembly phase began in July 2014, the first mixture was produced as early as November.

The engineering and project planning experts at BHS planned and implemented the complete mixing plant with the exception of the binder silos, as well as the weighing terminal and the associated automated loading. Apart from the mixers, the scope of the project therefore included the weighing, metering and materials handling technology of the mixing system right up to the loading bellows and the dust removal. Owing to the cramped space conditions mainly caused by the works traffic and the continuous production, the individual components were delivered “just-in-time” depending on the progress of the construction work.

The control of the two independent production lines was one of the challenges for the concept of the new mixing plant. It constitutes both dry mixing in batches as well as continuous wet mixing including the complete metering systems. The precise proportional control of limestone powder and water is of critical importance particularly in continuous mixing, because the mixing ratio must be configured to provide a dust-free, moistened, lump-free final product at all times. Both driving lanes have calibrated vehicle weighbridges which provide the control system with the precise loading quantities for optimum filling of the trucks, in order to systematically exclude the danger of overloading.

5 Loading 24/7

The two mixing lines work in parallel, so that loading can take place simultaneously on the wet and the dry side.

Apart from the customized plant concept, the loading process is also user-friendly: The re­spective material can be loaded virtually on a 24/7 ­basis owing to the complete automation of the system. This means that the customers of Müllerkalk can also flexibly collect urgently needed material at short notice at night and at weekends.

Loading is basically carried out by the truck drivers themselves. The truck driver is briefed on the overall loading process by an employee of ­Müllerkalk at the first visit. The drivers also receive a smart card for independent loading, on which the customer and order data is stored. A weighing terminal with a touchscreen is located at the mixing system. The driver uses this to start the job and single-handedly control it via the respective loading area of the two product lines. Owing to the uncomplicated operation, acceptance of the automatic loading is very high, also among non-German speaking drivers.

6 Initial experience

The new plant enables Müllerkalk to occupy new market niches and strengthen the position of the company as a whole: With Müllermix, Müllerkalk now offers a unique product over a wide area.

The excellent homogeneity of the mixed ­binders and earth-moist agricultural limes is repeatedly praised by the end customer. Apart from scheduled maintenance work and plant extensions, the twin mixing plant has operated stably since being ­commissioned. Unscheduled downtimes have not occurred so far.

The short mixing times enable rapid loading of the silo vehicles and semitrailers. The loading of approximately 25 t of finished product takes around 20 minutes on average.

Müllerkalk will additionally offer “Müllermix” in Big Bags in the near future, in order to be able to serve customers with smaller construction projects. An individual filling station for this is already ­under construction.

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