Understanding the interrelationships and tapping potentials of mixing technologies

Liebherr-Mischtechnik GmbH has been involved with mixing technology for 65 years and has experienced employees, modern testing facilities and a worldwide sales network. In the following contribution the interrelationships of mixing technologies and possible potentials are discussed.

Concrete mixing is an interesting system of process engineering with still many exciting, open questions and great potential for new solutions. Liebherr-Mischtechnik GmbH, which has been involved with the task of mixing technology for 65 years, has committed employees, superbly equipped testing facilities and a worldwide sales network. As a result, it is in an excellent starting position.

The further development of mixed products and/or their formulations – to obtain new properties and enhanced qualities and to make manufacturing processes more efficient – are challenges for which new solution approaches are needed. A better understanding is necessary of the interrelationships of process engineering that must be mastered for a mixing task for product-specific application. In this context, Liebherr-Mischtechnik is able to draw on a development base that combines modern computerized simulation techniques with testing and practice. The objective is to increasingly better understand the processes that take place in a mixer and the process engineering required for attaining the desired product properties. Here, the fact is often overlooked that the mixing process itself is only one of four basic operations of mechanical process engineering. In addition, the thermal or chemical processing techniques that accompany or even dominate the attainment of the desired product need to be taken into account.

In order to speed up and succeed in realizing the insights gained into the different mix technologies, research at Liebherr is taking place in parallel in the following areas:

Simulation (digital mixing processes)

Laboratory scale (scalable mixing tests) and

Practice (mixing tests and practical experience).

One of Liebherr’s core competences are technical solutions for mixing with tasks where solid phases represent the major part of a formulation. The mixing systems used here enable high mix qualities, both for distributive mixing of components difficult to blend, and for dispersive mixing with intensive material dispersion.

The ring-pan mixer

The application focus of this mixing system is on dispersive mixing (i.e., the fine dispersion and fine distribution of cement, fillers and additions), for which the ring-pan mixer with integrated whirler system is very well suited. With this mixing system, the mixing paddles that revolve in a ring trough perform the task of achieving rough blending, material transport and discharging the mix. In addition, 1 to 2 whirler systems rotate on a circulating path within the ring trough. The whirler speed of rotation correlates with the rotational speed of the mixing tools. The whirler rotational speed, however, is considerably higher and can be infinitely variably adjusted if necessary.

This subjects the material mix, during every circulation, to highly efficient stress that leads to the desired fine dispersion (dispersion effect.) Owing to the low water content in the mix, this results in pronounced agglomeration of the material mix during the mixing phases: in particular, in the manufacture of fine-grained self-compacting high-strength and ultra-high-strength concretes. Here, the ring-pan-mixer system demonstrates its full advantages and marks, according to manufacturer, the current benchmark with respect to mixing time efficiency.

The twin-shaft mixer

The twin-shaft mixer focuses on distributive mixing: i.e., the uniform distribution of components difficult to blend and satisfies the challenging demands placed on the quality of the mix. This is achieved by an asymmetrical helical arrangement of the mixing tool on both mixer shafts, which ensures circulation of the material mix throughout the entire mixing space. Between the mixing shafts, in the highly turbulent mixing zone, an intensive horizontal exchange takes place. In addition, due to the rotation of the mixer shafts, the material mix is moved in the vertical direction. The combined movement results in a genuine three-dimensional process and consequently in a high statistical probability that any given particle reaches any given position in the mixing trough. Very good mix qualities are also achieved at low (product-friendly) rotational speeds. However, a twin-shaft mixer system can also be operated at higher rotational speeds if additionally greater shear stresses are required.

Accordingly, high mix qualities can be efficiently obtained in a large range of rotational speeds.  Owing to these functional properties, construction materials with wide differences in particle-size distribution, different densities (e.g., intermixing of lightweight materials) and different forms (e.g., intermixing of fibers) can be manufactured in maximum mix quality. Uniform distribution of coloring admixtures within a short time also presents no problem. Rapid discharge thanks to the large discharge opening increases the hourly output.

The laboratory mixer

Laboratory mixers reduce the time and effort of testing, enable finding systematic solutions and can provide sound bases for decisions. The objective of Liebherr Mischtechnik is to enable developers of new formulations and quality surveillance facilities with this technology to perform their practice-related experiments. This laboratory mixer features the same engineering processes that actually occur in practice. In other words, mixing tests at the laboratory scale of 1:1 realize the same loading and motion sequences as in practice. These aspects were included in developing the new Liebherr laboratory mixers. The DW 0.06/100 twin-shaft mixer is primarily designed for distributive mixing. The DWI 0.06/100 twin-shaft mixer is designed for higher shear rates. It enables mixer shaft speeds of up to 600 RPM. The RIV 0.06/100 intensive laboratory ring-pan mixer features rotational whirler speeds that can be separately and independently adjusted according to the speed of the mixing tools. It is available for complex dispersion processes.

State-of-the-art control enables, among others, sub-division of the experimental process into individual sections, a well-organized presentation of the experimental procedure and integration of various sensor systems, such as for temperature, moisture, rotational speed and forces. The data can be read out in a neutral format for further analyses. The control can also be integrated into its own network.

The technical center

In its professional technical center, Liebherr Mischtechnik GmbH offers its customers excellent possibilities in their search for a suitable mixing technology for upcoming investments or new product ideas. The mechanical testing facilities available include the described laboratory mixers. In addition, there is the possibility to conduct validation tests on practice-sized mixers (from 750 l to 3800 l).

The tests are accompanied by qualified Liebherr personnel with extensive experience and support for finding a target-oriented experimental system.

Simulation

The development of mixing technology until now has been strongly characterized by empirical data. With regard to understanding the functional principles of mixing systems, there is still much to be learned. This explains why development processes have slowed. New methods for computerized simulation and visualization enable deeper insights into the technical processes involved in mixing. The utilization of these possibilities is an important key to new developments. In seeking to tap these potentials, Liebherr Mischtechnik GmbH is systematically working on developing its own in-house simulation competence. These possibilities not only help to further develop in-company internal mixing technology, but will also help customers to better understand the technical processes of mixing – and in this way provide a building block for securing an investment decision. This is an important step towards visualizing process engineering processes and thus towards digitalization.  

In summary, Liebherr offers: the experience for solution of specific technical mixing tasks, tried-and-tested mixing system, extensive experience in the construction of mixing systems, a new laboratory-mixer construction series, utilization of modern simulation techniques as well as the possibility to perform internal and external experiments on working scale.

CONTACT
Liebherr-Mischtechnik GmbH
Im Elchgrund 12
88427 Bad Schussenried/Germany
+49 7583 949-0

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