Build-up versus throughput
Even well-designed processes can experience accumulations, which have a significant impact on output and profitability. Changes in process conditions, raw materials or weather can all have an effect on material flow, and even small amounts of accumulation can grow into a serious blockage. Beyond moisture content, there are many causes of raw material buildup on vessel walls. Some metals contain naturally occurring magnetic properties. Nearly 90 % of the earth’s crust contains silica, and the sharp crystalline structure can contribute to buildup. Other factors can include the surface friction of the silo walls, the shape of the vessel, the angle of the slope and the size of the material being loaded.
“While some large facilities choose to make the capital investment to purchase their own cleaning gear to clear process equipment and storage vessels – as well as train personnel – others are finding it more sensible to schedule regular cleanings by specially-trained contractors,” said Pronschinske. There are a few types of equipment used for this purpose. One operates like an industrial-strength “weed whip,” rotating a set of flails against the material in the vessel. This approach eliminates the need for confined space entry and hazardous cleaning techniques, typically allowing the material to be recaptured and returned to the process stream.
The whip can be set up quickly outside the vessel, and it’s portable enough to move easily around various bin sizes and shapes. Technicians lower the device all the way down through the topside opening, then start at the bottom of the buildup and work their way up. In extreme cases, a “bin drill” can be used to clear a 30.5 cm pathway as deep as 45 m to start the process.
One method for reducing the need for cleaning or even eliminate it is through industrial vibrators designed for bin and chute applications. Electric vibrators are generally the most efficient, delivering the longest life, low maintenance and low noise. The initial cost for an electric vibrator is higher than for pneumatic designs, but the operating cost is lower. Turbine vibrators are the most efficient and quietest of the pneumatic designs, making them well suited to applications in which low noise, high efficiency and low initial cost are desired.
Air cannons are another approach to maintaining good material flow, particularly in larger vessels. In the mining industry, air cannons are frequently specified to eliminate build-ups in hoppers, storage vessels, transfer chutes, bins and other production bottlenecks. They can also be found in mineral processing plants where metals are extracted using processes creating slurries and other wet, tacky tailings. The timed discharge of a directed air blast can prevent accumulation or blockages that reduce process efficiency and raise maintenance expenses. In underground mines with potentially explosive dust, manual firing of cannons without the use of electrical solenoids is an option.
“There are a number of approaches that can help resolve those problems caused by material accumulations and blockages before they turn into expensive downtime, lost material and safety hazards,” Pronschinske concluded.