Technology report: Compound interest
7 Dec 2023
HF analysis shows how tandem mixing technology can reduce energy costs and increase efficiency
London - In a recently issued report, HF Mixing Group detailed the results of a joint project with Kraiburg Austria to analyse the potential energy-savings and other efficiencies offered by the tandem mixing process.
The tire industry has traditionally used several mixing stages due to the use of highly active carbon blacks and the need to reduce the compound viscosity.
The individual stages – including dispersion and incorporation of fillers; reduction of viscosity; distribution of small and cross-linking chemicals – are performed in different mixing processes that are separated in time and, if necessary, in location.
With two internal mixers arranged with one unit located below the other, however, the tandem mixing process makes it possible to significantly reduce the number of mixing stages.
Typically, an upper machine is designed as a ram kneader and does not usually differ from a standalone machine. The lower machine is designed as a ram less kneader, with a larger mixing chamber than the upper machine.
By dividing the mixing processes between two internal mixers, optimal conditions for dispersion, distribution, viscosity reduction and cooling can be achieved.
Moreover, by designing the upper machine as a ram-type kneader, correspondingly high shear forces can be applied for enhanced dispersion and incorporation of the fillers.
The batch is then transferred to the lower machine, where the larger mixing chamber volume results in a lower fill-factor at a constant batch weight – making it possible to focus on distributive mixing.
The larger mixing chamber also offers the possibility to achieve additional space by pushing the compound up into the shaft, resulting in a very good distributive mixing. In addition, pre-mixed material and/or the crosslinking system can be added via a feeding belt on the lower mixer.
Overall, therefore, the assignment of tasks to the upper and lower machine allows each mixer to be optimised for the specific mixing application/task – unlike standalone machines.
Furthermore, opportunities for consolidating mixing stages and even eliminating others lead to reduced preparation times, offering significant advantages in terms of efficiency and throughput.
In particular, this stage-reduction and optimisation significantly lower specific energy consumption in the mixing line, as well as in the downstream machines.
To determine the cost-reduction potential of tandem mixing, the HF-Kraiburg Austria team analysed the results from processing an industrial recipe – a blend of natural rubber and polybutadiene, active carbon black, small chemicals and a sulphur-based cross-linking system.
Equipment used included a GK320E standalone mixer with PES5 rotors and a semi-automatic system for polymer and chemical addition in the upstream. There were two mills and a batch-off in the downstream. An energy consumption analysis of the entire line was carried out for each individual stage, with the results calculated separately for each stage.
Analysis showed that a cost-reduction of up to 50% could be achieved through the reduction of stages and the elimination of energy costs and work, while tandem mixing also reduced the mixing time by up to 63%.
This extra capacity, noted HF, could be used to increase in-house production and/or supply additional compounds to external parties, thereby enhancing economics and profitability of the site.
Moreover, energy-savings of up to 57% were identified: highlighting the potential for tandem mixing technology to improve efficiency and reduce CO2 emissions in the tire and rubber industry.
