What twin-shaft mixing can do for you: the technology is gaining market share throughout the...

Everyone has a general idea what mixing means. But describing this is not so easy. The goal of any mixing process is to distribute materials with different characteristics, and usually in differing quantities, as evenly as possible. A new homogeneous material with different properties results.

Introducing relative movement of the correct intensity into the materials achieves mixing. A simple movement in only one direction with inadequate acceleration of the materials would obviously not result into a homogeneous mix of the desired properties. This would be more like stirring than mixing.

How much relative movement is introduced is a decisive criterion. Conditions for successful mixing include all of the material volume to be mixed and also achieving the highest probable likelihood that a particle being in a specific position at the beginning can be found at any random location in the mixing chamber at the end of the process. This is the only way to ensure that a consistent result is achieved every time mixing is repeated.

Three-part cycle

The mixing cycle includes three phases: the feeding, the actual mixing, and discharging. The sequence in which the individual materials to be mixed are filled into the mixer can make the mixing process easier or more difficult. To achieve optimum results, the discharging should take place so there is no risks of segregation. Costs for energy and wear should be as low as possible. Finally, it is very important that the mixing cycle takes place as quickly as possible. All of this results in successful mixing.

In concrete production, selecting the best mixing technology is crucial for both the quality of the concrete produced as well as the economy of the production process. It must be ensured for each batch that the total volume of cement in the mix can react with the mixing water and that the aggregates are enveloped with the cement slurry as completely as possible.

Most concrete formulas also involve some small quantities of chemical additives, which must also be evenly distributed. The time required per mixing cycle and the costs of energy, wear, and maintenance determine the economy of the concrete production.

Twin-shaft technology

The twin-shaft batch mixer has become the widely preferred technology for concrete production in many countries.

The twin-shaft mixer for concrete production was invented at the end of the 19th century and registered for patent by BHS-Sonthofen in Germany. The starting point for the invention was mechanizing the manual mixing with a shovel in which a line of aggregate/cement/water being on the ground had been repeatedly shoveled from right to left. This concept was later abandoned and, over a number of development stages, was replaced by today's highly dynamic movement process.

Today, there are mixing blades on both mixing shafts that are geometrically arranged so they follow the pattern of an interrupted spiral. This transports the materials to be mixed in a screw-like movement along the mixing shafts and on each shaft in opposite directions.

Toward the end of each shaft, the mixing blades are positioned in counter direction so they can transport the mix onto the opposing shaft. This way, the materials are constantly rotated around the mixing vessel. At the same time, this material rotation process takes place in an inward turning spiral. This results in an intensive three-dimensional movement of material.

Intense mixing

The two mixing circuits overlap in the middle to further increase the intensity of the relative motion. This creates a high turbulent zone in the middle of the mixing vessel and further intensifies the mixing significantly. Thanks to the proven design of a BHS twin-shaft mixer, it's possible to achieve 95% of homogeneity with only 30 seconds of mixing time. This can be achieved with a relatively low speed of the mixing shafts of only about 20 to 30 rpm. This saves energy, reduces wear, and avoids stress on the particles to be mixed.

Cement is the most important and most costly raw material in concrete production. Only an excellent mixer can guarantee the most efficient use of this high-value raw material. Various tests have shown that a twin-shaft mixer can produce concrete of a given strength with less cement then a drum tilting mixer.

Discharging is accomplished in a twin-shaft batch mixer along the middle of the vessel underneath the two shafts using a rotary sliding gate. A major portion of the mixed materials empties when opening the gate because of gravity. The rest is pushed out of the opening by the broad mixing blades, leaving almost no remaining residue. The risk of mix segregation is therefore extremely low. The rotary-gate system allows several opening positions to increase or reduce the outflow during discharge. A further advantage of this rotary system is the possibility to close the gate without the risk of particles sticking in between.

The twin-shaft mixer is also characterized by its relatively high filling level compared to other concrete mixing systems, resulting in some decisive advantages. The mixing vessel can be built very compact, saving space. Also, the surface of the mixing vessel being exposed to wear is much smaller than for other mixing systems, allowing only limited filling levels.

A comparison with some other mixing systems also shows advantages in terms of installed drive power. Because of the short mixing time, the twin-shaft mixer uses less power per batch. The BHS mixer is furthermore distinguished by an extremely robust drive system. It can even be restarted when fully loaded.

Different applications

The twin-shaft mixer is also a good solution for unusual demands or operation situations. From a technical point of view, it is not a problem to add ice for cooling the concrete or hot steam for heating it. A twin-shaft mixer is completely insensitive to under-filling batch size situations. Good mixing results are achieved in such cases. Concrete formulas with coarse aggregate sizes (up to 180 mm) can be processed. Scientific studies have confirmed that even self-consolidating concrete can be reliably mixed within 30 seconds.

The principle advantages of the twin-shaft mixing technology become even more obvious the larger the batch size of the mixer becomes. For example, at first glance, mixing a batch of 1, 3, or 6 cubic meters in a given mixing time obviously places different demands on the mixing machine. The BHS twin-shaft batch mixer achieves equally good mixing results using the same mixing time regardless of batch size. This is the result of a careful upsize design of the mixing tools.

In addition, the operating costs fall degressively per cubic meter of concrete the larger the mixer becomes. The large number of advantages of the twin-shaft mixer demonstrates that it is an ideal product to produce all kinds of concrete. This is why twin-shaft mixers are capturing a greater share of the market.

The author is managing director, construction machines division, at BHS-Sonthofen GmbH. Visit www.bhs-sonthofen.de or telephone 49 8321 802 200 for more information on the subject.