A controlling hand for batch mixing

In a manufacturing process for fireproof fibre boards used in the automotive industry, five ingredients - a polymer flocculant, latex, alum, antifoaming agent and cellulose containing 4 per cent solids - had to be mixed by hand and dosed into a mixing vessel.

Automation of this process was preferred but was particularly demanding as the cellulose does not flow. The solution was found by using a range of equipment from ProMinent Fluid Controls in an automated procedure that mimics the manual method as closely as possible.

One of the most challenging aspects of automating the process was to move the extremely viscous, solids-containing cellulose mixture. A ProMinent Orlita DR Series pump was the answer. This valveless precision metering pump can move fluids of viscosity up to 106mPa over a wide temperature and pressure range. The pump features a rotary control piston mechanism with a synchronised oscillating and rotating movement. This design means that the piston itself opens and closes the inlet and outlet lines so no valves are needed.

The pumping action is very gentle, so the fibres in the cellulose mixture remain intact. The pumping mechanism also gives extremely precise dosing capabilities with an accuracy of ±0.5 per cent. The pump head is manufactured from stainless steel and the piston and piston shaft liner are treated with a special wear-resistant coating. The pump head has been designed to be easily dismantled for cleaning of all parts that come into contact with the fluid.

To automate the polymer preparation part of the process, an Ultromat MT 500 flocculant preparation system is used. This produces a batch of 440 litres of polymer solution and is ideally suited to this application where batching operations are intermittent. The prepared solution is ready for use after it has aged for the appropriate length of time and is then dosed into a catch pot using a Sigma/2 motor-driven dosing pump.

Latex is dosed into a separate catch pot using another Sigma/2, while gamma/L solenoid-driven pumps are used for dosing the alum and antifoaming agent. All the pumps are pre-assembled on a polypropylene skid for ease of installation and each chemical has its own storage tank with the pumps on flooded suction for ease of priming.

Catch pots are used for the polymer, latex and cellulose to mimic the hand-dosing procedure and each pot is equipped with a motorised valve on the bottom. These open to dump the chemicals into the mixing tank, while the alum and antifoaming agent are dosed directly into the mixing tank under batch control.

A remote control panel with an integral Mitsubishi PLC controls the entire process. There is push button operation for the motorised valves which open to empty the catch pots, then close with a signal sent to start the motor pumps to refill the pots for the next batch. A start signal is also issued to the other dosing pumps. Feedback monitoring of the DR cellulose pump is carried out via a stroke counter to ensure that the correct quantity of cellulose has been transferred.

As two different types of board are produced at this site, two different recipes are programmed into the PLC. The system has been in operation continuously for more than six months and has so successfully reproduced the manual conditions that a second system has now been commissioned.

Bob McAlster is with ProMinent Fluid Controls (UK), Ashby de la Zouch, Leicestershire.