Vacuum filtration is the process of creating negative pressure within the filter drum of a filter machine through the action of a vacuum machine, resulting in a pressure difference between the two sides of the filter medium layer (filter cloth). As the filter rotates, the mother liquor in the alkali solution is extracted, while the heavy alkali is adsorbed on the filter cloth and finally scraped off by a scraper.

    Vacuum filter generally refers to a rotary drum vacuum filter. Its main body is a cylindrical filter drum, which rotates once. The functions of each part during the filtration process are shown in the figure on the right. About two fifths of the lower half of the filter drum is immersed in the alkali solution tank. When the filter drum rotates, all filter surfaces alternately come into contact with the alkali solution in the alkali solution tank. The filtrate is sucked into the filter drum, and heavy alkali crystals adhere to the filter cloth. The function of rotating the filter drum once can be divided into the following five processes:

    (1) The suction of the filtrate starts slightly to the left (around 14 °) at the bottom center of the alkali liquid tank, until it reaches the liquid level. The liquid is sucked into the filter drum, and heavy alkali crystals adhere to the felt filter cloth (or metal filter cloth). This is the true filtering function.

    (2) The first time it is sucked dry starts from the liquid level and ends at about 300-400mm below the alkali liquid tank, and the liquid (filtrate) in the filter cake is sucked dry.

    (3) Washing, the first time the position is sucked dry starts from about 150mm above the upper washing sink, which is the stage of washing with washing water. Wash the filtrate with water of about 0.2-0.25

Soda ash, the washing water left in heavy alkali is about 0.36-0.4

    (4) The second suction drying starts from the washing tank and ends at the scraper. The residual mother liquor and washing water in the heavy alkali continue to be sucked dry, which is the true suction drying stage, accounting for about a quarter of the filter drum area. At the same time as drying, there are three pressure rollers to help squeeze out the moisture in the heavy alkali. To minimize the moisture content, the filter drum speed should not be too fast.

    (5) The lower part of the blowing scraper (below the horizontal line) sucks in the filtrate, and then compressed air is introduced (partially below the liquid level). The 3-4mm thick alkali layer remaining on the filter cloth is blown down by air, restoring the filter cloth's ability to absorb alkali solution.

    The filter drum repeats the functions of these five stages during continuous rotation to complete the operation of heavy alkali filtration.