Filter presses and their function in the washing process
In today’s world there is more demand for the earth’s natural resources than ever. For companies to succeed and grow, sustainability and the circular economy are a vital consideration in the business plan. As planning permissions for natural aggregates are taking longer to grant with escalating costs, and landfill space for construction materials once regarded as a waste is reducing so we have the perfect storm brewing between supply and demand.
Many companies in the aggregate supply chain are now considering and purchasing aggregate wash plants suitable for the processing of construction, demolition, and excavation waste to recover the valuable sand and aggregate from this material for reuse in construction. This is a very sustainable approach to the finite resource of construction sand and when done correctly can also add significant value to the materials produced from the wash plant.
An important consideration for washing aggregates is how the dirty water from the wash plant containing the silt and other contaminates from the washing process are to be dealt with. Lagoons are rarely an option today due to space constraints and the fact that the Environment Agency will want any water from the plant managed in a controlled and measurable way to guarantee no pollution.
The washing of recyclable aggregates is often regarded as relatively straight forward when a tried and tested process is followed. However, with such a variable input material the key to a successful washing operation is the water treatment plant and ultimately what is produced from the silt fraction that has been washed and scrubbed out of the feed material.
There are several technologies currently available for receiving a thickened sludge from a water treatment plant, removing the solids from the water to produce a manageable cake, and returning the clean water back to the washing process. When deciding which of these to use consideration must be given to the initial capital cost versus operational costs, civils requirements, space available and ultimately what is going to be done with the silt cake that is produced at the end of the process.
Most companies today need to produce the driest possible cake that can be stockpiled, transported, and potentially used as a product from the end of the washing process. To achieve this filter press technology offers the best solution to process a variable feed and produce the driest possible cake. Typically, the moisture content of the filter cake is below 25% moisture compared to 40% moisture from other alternative technologies. This guarantees the maximum return of water into the washing process and the lowest loss of water out of the process.
Filter presses are machines used to dewater slurry. They separate solids from the waste water or slurry and recover the water, which can be re-used in the processing cycle. Filter presses are used in a wide range of applications - aggregates washing, mining concentrate and tailings, recycling, concrete batching plants and virtually in any process where water/solid separation is involved.
Tyrone International work exclusively in the UK market with our Italian partner Fraccaroli & Balzan, who have been designing and manufacturing industry leading filter presses for more than fifty years as one of the world leaders in this technology.
A common misconception:
The filter press is basically constructed from a heavy-duty steel chassis carrying a series of recessed plates covered by a filtering medium (cloths and sometimes backing cloths). The plates are kept tightly closed by hydraulic rams creating a seal around the perimeter of the plates. A high-pressure slurry pump forces slurry into the empty chambers between the plates which have an inner cavity holding the solids, while the water drains through the clothes and is recovered for the process. It is a common misconception that the hydraulic rams squeeze the water out of the sludge when it is the pump that feeds the sludge into the press which creates the pressure to force the water out of the sludge. The result is dry solid cakes containing a low percentage of moisture, which are dropped beneath the filter press at the end of the filtration cycle. This process is fully automatic and does not require the presence of an operator.
The feed pressure to the press is also an important consideration. If the feed pressure is too high (for example 20 bar) then this will result in high energy consumption, accelerated wear on the filter cloths, accelerated wear on the pump impellors and liners and damage to pump seals. The goal is to achieve filtration at the lowest possible feed pressure which in our industry can be found between 10 and 14 bar pressure. To achieve the driest possible cakes, factors other than feed pressure are considered such as the density of the feed sludge, the thickness of the filter cake and the choice of filter cloths.
When considering which filter press to purchase customers have a choice between a side beam press or overhead beam press. Side beam presses are manufactured from rectangular steel box sections which run along either side of the filter plates whereas overhead beam presses are manufactured from heavy duty I section beams located at the top of the filter plates. Therefore, side beam presses are cheaper to construct and tend to be used for smaller applications as they cannot support as many filter plates as overhead beam presses without the addition of further steel frame supports. Overhead beam presses offer unrivalled access to the filter cloths whereas side beam presses obstruct access and maintenance to the cloths must be done from the top of the filter press.
To meet the growing demand of the market for larger throughput capacity, the size of the filter presses has constantly increased and now Fraccaroli & Balzan manufacture some of the largest filter presses in our industry that can discharge up to 32 tons of cake per press cycle.
Even though the basic concept of filtration is quite simple, Fraccaroli & Balzan has enhanced many features to improve the performances of the filter press, reducing operational costs, increasing the production rate, and making the management of the filter press much easier.
The larger filter presses are equipped with dual over-head beams, to provide a high structural integrity, plate stability and easy access to the plates for maintenance or any other intervention. In fact, the structure of the filter press is completely open from the sides and from above, so that the operator can enter directly into the filter press and the plates can be removed from the top.
The filter press is equipped with a robot for high-speed plate shifting and shaking and for allocating the automatic cloth washer. There are no chains, sprockets, or any other complex and unreliable mechanisms for moving the plates. It is particularly important to highlight the scope of the plate shaker. It makes sure all cakes are dropped at the end of the cycle. Fraccaroli & Balzan was the first company to introduce this mechanism in the filtration business.
A full-length drip tray or a shorter moving drip tray is integrated in the filter press structure. Either way the collection of water during the washing cycle and access to the filter press for maintenance is guaranteed.
The electronics and software used for the filter press has been constantly improving using the most reliable and advanced technologies. The filter presses are fitted with a PLC and all operations can be visualized and controlled remotely. With a local WIFI, the customer can use a tablet to control the filter press. As an alternative, the customer has the possibility to control the filter press remotely using any connection, for example their personal PC. With a dedicated app, we can control the touch screen from our smartphone.
If you are considering the washing of recycled aggregates or are looking for a solution for the dewatering of sludge from your existing process, please contact the team at Tyrone International.
Author – Fintan McKeever is MD of Tyrone International.