- Published: Wednesday, 17 August 2011 13:49
- Written by CP Staff
Fast cycles plus high powder and power economy
The objective of any mixing process is to distribute and blend materials with different characteristics and quantities as evenly as possible, resulting in a homogenous mixture. Introducing proportional movement with the optimal intensity yields effective, homogeneous mixing results. Much like stirring, simple one-directional movement with inadequate acceleration of the materials would not achieve the same mixing results.
The amount of relative movement introduced to a batch is a decisive factor. Circumstances for successful mixing involve distribution of all materials, and achieving the highest probability that particles and other mix constituents located in a specific position at the beginning can be found at any random mixing chamber point at the end of the process. This is the only way to ensure that a consistent result is achieved every time mixing is repeated.
The mixing cycle occurs in three phases: charging the mixer, mixing process and discharging. The sequence in which individual materials are introduced can have a huge impact on mixing cycle efficiency. The cycle overlaps charging, followed by mix discharge. To achieve optimum results, effective discharging methods should be taken into consideration to avoid risks of segregation. Costs for energy and equipment wear should be as low as possible, and it is important the mixing cycle commences as quickly as possible.
In concrete, selecting the best mixing technology is crucial for quality and production efficiency. The optimum system rapidly disperses all ingredients evenly throughout the mixing trough and completely surrounds all coarse and fine aggregate with cement slurry. Most concrete mix designs also contain small quantities of chemical admixtures, which also must be evenly distributed. The time required per mixing cycle, plus equipment energy, wear and maintenance costs determines production economy.
The twin-shaft batch mixer has become the widely preferred technology for concrete production in many countries. It was invented at the end of the 19th century and registered for patent by BHS-Sonthofen in Germany. The initial reasoning was to replace manual mixing with a shovel in which a line of aggregate, cement and water—positioned on the ground—was repeatedly shoveled from right to left. This concept was later abandoned and, over a number of development stages, replaced by today’s highly dynamic process. Today’s twin-shaft mixers have blades on both mixing shafts that are geometrically arranged to follow an interrupted spiral pattern. This motions the materials to be mixed in a screw-like pattern both along the mixing shafts and on each shaft in opposite directions.
On the ends of both shafts, the mixing blades are positioned in a counter direction so they can transport the mix onto the opposing shaft. This way, the materials are constantly rotating around the mixing trough. At the same time, the material rotating process also takes place in an inward-turning spiral. This effects an intensive, three-dimensional movement of material.
The two mixing circuits (Figures 1 & 2) 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 trough and significantly intensifies the mixing process. Certain twin-shaft models can achieve 95 percent mix homogeneity at a mere 30 seconds’ mixing time (Figure 3). This can be achieved with a relatively low shaft speed, between 20 to 30 rpm, reducing energy consumption and wear, while avoiding stress on the particles to be mixed.
Cement is the most important and costly raw material in concrete production, typically 10 percent of the mix design and up to 90 percent of the (pre-delivery) cost. Various tests have shown that a twin-shaft batch mixer can produce concrete of a given strength with less cement than other mixing methods and types.
The twin-shaft mixer discharging process is accomplished along the center of the mixer between both mixing shafts using a rotary gate running the trough length. A major portion of mixed materials is discharged through the opening created by gravity; broad mixing blades force out the remainder leaving almost no residue. The risk of mix segregation is therefore extremely low.
The rotary discharge gate system is also equipped with adjustable opening positions that increase or decrease the outflow during discharge, plus internal adjustable ledges for sealing. This design also allows the door to be closed without the risk of interference from mix constituents.
The twin-shaft batch mixer is also characterized by its relatively high filling level compared to other concrete plant mixer models; this allows for a much more compact mixing trough, simplifying retrofits and replacements of other mixer types. Further, due to the smaller mixing area, the twin-shaft has less wear than other mixing systems with lower filling levels. A comparison with other mixing systems also shows advantages towards drive power and reduced energy consumption.
The twin-shaft mixer suits standard and unique operational requirements, including the addition of ice or steam for concrete with tight temperature specifications. It can also accommodate small batches, delivering the same mixing performance and results from one to the next, or mix designs with aggregate up to 7 in. Studies have confirmed that self-consolidating concrete mixes can also be processed within the same 30-second window as more conventional, lower-slump formulations.
The principle advantages of twin-shaft mixing technology increase with larger mixing batch size and capacity, which place different demands on the equipment. Consistency in a range of mixer capacities results from the precise upsize design of the mixing tools. In addition, the operating costs are significantly reduced per cubic yard as the mixer becomes larger. The numerous advantages of the twin-shaft mixer demonstrate that it is an ideal unit to produce standard or speciality concrete mixes, including SCC, roller compacted and ultra high performance.