BPD is a bi-product produced during the manufacture of cement. It is a free flowing alkaline powder that has a severe exothermic reaction when mixed with water. Historically, CEMEX treated BPD as a waste product and sent it to landfill. However, after extensive trials, CEMEX discovered that it could be reintroduced, at a controlled rate, back into the manufacturing process and there would be no degradation on the final cement product.

CEMEX already had unused storage silos that they planned to use for storing the BPD but didn’t have the technology to meter it back into the mills. Thermo Fisher Scientific determined that normal weighing equipment was not a suitable solution due to the nature of the product and limitations on the head room within the mill house, plus the system needed to be totally enclosed.

Thermo Fisher Scientific devised a solution, based around two screw conveyor weighing systems, for removing the BPD from the silo outlet and feeding it back into the mill, at controlled rates from 0-3 tph. Firstly, the company fitted PLC controlled air operated silo discharge aids to ensure the BPD flowed from the silo in a ‘mass flow’ and controlled way. Associated isolation valves were also fitted.

The BPD flows into the first screw conveyor, known as the ‘extraction screw’, and then proceeds into the second ‘weighscrew’ - the outlet of which is fed to the inlet of the mill. The weighscrew runs at three different preset speeds depending on the BPD feed rate while the extraction screw speed is made variable to control the feed rate.

The Thermo Scientific system can operate in both local and remote modes. In remote operation, it is controlled automatically from the main control room. To ensure there are no product blockages in the system, Thermo Fisher Scientific fitted a number of its microwave-based flow/no flow switches. It also uses a Thermo Scientific Microtech 3000 weighing controller. To shorten and simplify the installation phase, the systems were designed as skid mounted units.

“We have undertaken many projects with Thermo Fisher Scientific so knew they were capable of coming up with the right solution,” said Nicholas Clewes, project engineer at Cemex. “The support from the company’s engineering manager during design, installation and commissioning phases was truly exceptional. This was a particularly challenging exercise for Thermo Fisher Scientific as the screw conveyor weighing systems have to operate under the arduous conditions that exist in a mill area, but as far as I’m concerned, everything is functioning really well.”

Ian Lee, project and engineering manager at Thermo Fisher Scientific comments: “Although we have a number of these types of installations throughout the world, this is the first of its type in the UK handling a product as aggressive and difficult to control as BPD. It demonstrates Thermo Fisher Scientific’s depth of experience in offering both standard and bespoke solutions to almost all weighing applications in the bulk solids industry.”