Energy from waste: the recycling of limed fleshings
Leather products have been around a long while - there are those who claim that it is the second oldest business in the world. Going back a few millennia, when our earliest ancestors decided that sitting on hard rock was not a soft option, they turned to other materials in order to create more comfortable seating, as well as warmer bedding and improved clothing.
Animal skins became the fabric of choice and, at some time or another, they discovered that various treatments applied to the raw hides and skins helped to stop the degeneration, through bacterial action, of the by-product of their food supply.
The tannery operation consists of converting the raw hide or skin, a highly degradable material into leather, a stable material, which can be used in the manufacture of a wide range of products. The whole process involves a sequence of complex chemical reactions and mechanical processes. Among these, tanning is the fundamental stage, which gives leather its stability and essential character.
Therefore the tanning industry is one of the oldest industries of the world – and the demand for treatment and waste disposal is as old as the industry itself.
Manufacturing leather, leather goods, leather boards, and fur produce numerous by-products such as solid waste, wastewater, and emissions into the air.
It is obvious that waste treatment is becoming increasingly important for the tanning industry. The environmental effects that have to be taken into account comprise not merely of the load and concentration of the classic pollutants, but also the use of certain chemicals, such as biocides, surfactants, and organic solvents.
The FLOTTWEG Approach
The separation technology offered by the German company FLOTTWEG offers an appropriate solution to tannery waste treatment.
FLOTTWEG is a worldwide leading manufacturer of sedimentation centrifuges. The company was founded in 1932 and has been developing and manufacturing decanter centrifuges (solid-bowl scroll centrifuges) since the 1950s. Over the years, FLOTTWEG has 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 has 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.
In tanneries, there are many installations in service, 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 going back nearly 20 years. Limed fleshings are by-products generated during the traditional tanning process. Once the hides are taken out of the processing vessels, they are slippery, alkaline-swollen, and translucent. At this stage, the residual flesh and tissue remaining after skinning can easily be cut away from the corium by using a fleshing machine. Most of the fleshings are sent to a landfill site for disposal. This method of disposal generates very high costs.
FLOTTWEG developed a reliable and successful process in order to reduce disposal costs while providing a very good side benefit in recovering valuable fat.
In cooperation with a German tannery, the company deployed a system to separate the three different components of the 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%
In the first – and the most useful – phase, tallow is separated. Tallow can be used as a fuel to generate heat. Additionally generated is the solids phase, 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 mincer or grinder where their particle size is reduced to about 10 mm. The fleshings are then conveyed to a continuous cooker for heat treatment. At 75-85 degree the minced fleshings are melted by applying direct and indirect heat for about two or three minutes. This short heat treatment avoids desintegration of the solids 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 homogenizer 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 FLOTTWEG TRICANTER® allow the operator to optimize separation manually or automatically, according to the raw material input.
Following the separation in the TRICANTER®, the solids discharge contains 57 -65% moisture. To avoid hydrolization 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 recycled to 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, significant savings in fuel costs can be achieved. Proteins recovered can be utilized as fertilizers, 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. The capacity range offered by FLOTTWEG is 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 of the equipment are available for integration with existing systems.