The tanning industry is one of the oldest industries in the world – and the demand for treatment and waste disposal is as old as the industry itself. The environmental effects that have to be taken into account comprise not only the load and concentration of the classic pollutants, but also the use of certain chemicals, such as biocides, surfactants, and organic solvents. The separation technology offered by the German company Flottweg offers an appropriate solution to tannery waste treatment.
Flottweg are a leading manufacturer of sedimentation centrifuges. The company were founded in 1932 and have been developing and manufacturing decanter centrifuges (solid-bowl scroll centrifuges) since the 1950s. Over the years, they have gained considerable expertise in many solid-liquid separation applications. Flottweg’s current product line comprises solid-bowl scroll centrifuges, disk stack centrifuges, belt presses, and systems for capacities from pilot plant to large industrial installations.
To date, Flottweg have manufactured and sold thousands of centrifuges and hundreds of belt presses worldwide. Flottweg products are well received in the mining, petrochemical, chemical, pharmaceutical, food, and manufacturing industries as well as in the environmental area for applications such as the treatment of oil sludge or waste-water.
Many systems have been installed in tanneries, especially for sludge dewatering. Different decanter sizes with various capacities are available to dewater primary, biological, mixed and chrome sludge with optimum results.
Flottweg’s fleshing treatment equipment has a track record which goes back nearly twenty years. Traditionally, limed fleshings are sent to landfill, a disposal method which incurs high costs. Flottweg have developed a reliable and successful process to reduce disposal costs while also enabling the recovery of valuable fat.
In cooperation with a German tannery, the company deployed a system to separate the three different components of limed fleshings for further application and treatment. The primary objective in this process is the separation into the following components:
Tallow: approximately 7-15%
Solids: (mainly proteins), approximately 8-15%
Water: 75-85%
In the first – and the most useful – phase, tallow is separated, which can be used as a fuel to generate heat. The second stage separates solids rich in proteins, nitrogen, and calcium, which are suitable for composting. The third phase is the removal of excess water which can then be treated in customary wastewater treatment facilities.
The Equipment and the Process
The fleshings are pumped to a large grinder where their particle size is reduced to about 10mm. The fleshings are then conveyed to a continuous cooker for heat treatment. At 75-85 degrees, the ground fleshings are melted by applying direct and indirect heat for about two or three minutes. This short heat treatment avoids disintegration of the solid particles and provides for an optimum release of tallow from the cells. The melted fleshings are then discharged from the continuous cooker and passed into a homogenisation tank from which they are pumped to the key element, the ‘Flottweg Tricanter’. This machine is a three-phase centrifuge which simultaneously separates the three main components – namely tallow, water and solids – continuously by centrifugal force. Special devices on the Tricanter allow the operator to optimise separation manually or automatically, according to the raw material input.
Following separation in the Tricanter, the solid discharge contains 57-65% moisture. To avoid hydrolisation of the proteins, the solids are cooled. The water phase given to the waste-water treatment plant comprises 0.5-0.9% tallow and negligible soluble solids. The water phase has a fairly high organic load in BOD/COD.
The recovered tallow is discharged to a holding tank where static sedimentation takes place, quickly settling fine solids and water. The clean tallow from the top of the tank is pumped into a storage tank and the bottom layer is continuously sent to be recycled at the processing plant. The recovered tallow consists of 99.3% tallow, 0.4-0.6% water and 0.1-0.2% impurities.
The composition of the raw material is critical for the viability of the operation of this type of plant. Using the recovered tallow as a fuel for steam generation means significant savings in fuel costs can be achieved. Proteins recovered can be utilised as fertilisers, thus eliminating or reducing disposal costs. Water can be sent to the normal waste-water treatment facilities. Economic plant size starts at a capacity of 3 tons/hour. Flottweg offer a capacity range between 2.5 and 9 tons per hour. One operator per shift is required to run the equipment, which needs only a limited amount of space. Individual units are available for integration with existing systems.
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