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BHS-Sonthofen introduces method for recycling bypass dust

Published by
World Cement,

BHS-Sonthofen’s indexing belt filters allow the bypass dust accruing during clinker manufacturing to be completely reincorporated into the process through the use of up to 100% more of alternative fuels in cement kilns.

Reincorporation of the salvaged bypass dust increases cement production by up to 3%.

Fuels such as coal, oil or gas are used increasingly being replaced by more cost effective alternative fuels, such as car tires, plastics and municipal and industrial waste, in the manufacture of cement. The percentage in Europe has already increased to around 60% and will rise even further in the years to come.

However, when using alternative fuels, more chlorides are released into the combustion chamber. They settle on the bypass dust during the cooling of the gas flow extracted from the gas bypass, meaning that this dust can have a chloride content of up to 20%. It cannot be added back into the cement, because it would exceed the maximum permissible chloride content of 0.1% in the end product.

The enable cement plants to do away with disposal costs for bypass gas, the system manufacturer A TEC developed a patented process in cooperation with Holcim, which was trialled on a complete industrial scale, with a BHS belt filter system, at the cement plant in Rohoznik, Slovakia. Even when starting with high concentrations, it reduced the chloride content of dust to below the desired threshold. The BHS belt filter assumes an important process step in this system, achieving a chloride wash-out efficiency of 95%, meaning that the bypass dust can be re-added to the cement, even when using secondary fuels.

The core element of the systems is the BHS indexing belt filter, type BF. The gas flow extracted from the burning process – with which the bypass dust is discharged from the system – is cooled and made free from dust. The dust separated in the filter system is sludged with washing liquid and then filtered, washed efficiently in multiple stages counter-current wise and mechanically dewatered to the greatest possible extent. The material flows extracted from the belt filter are a mother filtrate concentrated with chlorides, which is treated or disposed of, as well as the washed out filter cake which is added back to the cement. BHS uses indexing belt filters, because they are clearly advantageous for typical flow rates of approximately 3 to 9 tons dry substance per hour – in comparison to filter presses, for example. They are especially suitable for high flow rates because they work continuously. Using a combination of indexing belt filter and an additional pressing device, BHS reduces the residual moisture in the bypass dust to 25 to 30 percent – i.e. a value that could never be achieved using normal suction. The value also falls below the thixotropic point and handling is improved, because the dried filter cake is crumbly and can be transported away by the conveyor belt and directed back into the process. Once the bypass dust leaves the belt filter system, its chloride content is far below the level of the accruing dust from the gas bypass (wash-out rate > 95 percent) and all the dust can be directed back into the process.

BHS provides a complete solution for treating the bypass dust. The scope of supply includes the dosing and mixing station, the filtration system with filtration of the suspension and cake washing using a multi-stage counter-current method and the cake dewatering, all pumps and the separators as well as the complete array of measuring and control technology. The system is therefore easy to integrate into the existing process as a skid-mounted module – as a “black box”.

The initial pilot plant provided proof that the process works reliably and on a commercial scale, with an annual capacity of 20 000 t.

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