Schwing Bioset Application Report 18, Allan, Saskatchewan
Written by Larry Trjoak, Trojak Communications
Version also published in Powder & Bulk Engineering Magazine, September 2013
Successfully getting any product to its final destination is often all about product protection. Fail to offer adequate protection and a company or supplier runs the risk of that product suffering damage and losing value. For most, an effective, packaging system alleviates those concerns. For others - shippers of corn or grain, for example - nature provides something of its own packaging. Some products, however, must be shipped in bulk (such as by rail car) and do not enjoy the luxury of this natural protection. In those cases, they need to be prepared in such a way as to ensure they remain intact throughout the shipping process. Potash (potassium chloride, KCI), a mineral ore and one of the principal components in commercial fertilizer, is one such product. Left untreated after mining and processing, granulated potash would break down during shipment to such a degree as to lose effectiveness or cause problems in its target applications. To avoid this, many major potash producers, including Potash Corporation of Saskatchewan (PCS), expose the granulated product to a mist of water late in production (called “conditioning”), then quickly dry it - essentially creating a “case-hardened” shell around each potash granule - in what is called a “glazing” step. The end result of this glazing operation is better resistance to degradation, a higher quality product and, by extension, a more satisfied customer. While a number of different drying methods have effectively achieved those goals, PCS reports that its Allan, SK plant, by combining higher concentrations of water with the use of fluid bed dryers from Schwing Bioset (Somerset, WI), has significantly lowered material degradation rates.
While homeowners throughout North America regularly apply fertilizer to their lawns each year, few realize that potash, one of the key components in that product, most likely came from the Canadian province of Saskatchewan (SK). More than 80% of the world’s potash, in fact, comes from this rich source and SK-based PCS is one of the largest suppliers to that market. The process of getting it from its origin (half a mile below the Earth's surface, or deeper) to a finished product is complex, involving mining, crushing, de-sliming, flotation, skimming and initial drying. However, getting it from that point to one where it is ready for shipment is equally challenging, according to Trent MacDonald, process engineer at PCS’s Allan facility.“Once the mined material has been through the primary production process, it enters the compaction circuit,” he says. “There, we subject it to crushing forces of 3,000 p.s.i., creating a briquette-like material, which we then break down into a granular product in a process called de-flaking. After that, the granular material is sent to a glazing screw where it is sprayed with water. Because the product temperature is approximately 160°C at that point, the water essentially flashes-off, hardening the outside of each granule and protecting it during shipment.”
Beating the Drum for Productivity
While that water flashing action removes much of the water from each granule, some moisture still remains - and that can be problematic from a number of different perspectives. A wetter product, while cooler, tends to stick together. To further remove moisture, a secondary drying process is undertaken. The methods by which the potash granules are dried vary, but generally include either rotary drum units or fluidized bed dryers. MacDonald and his team at PCS Allan, have experienced both and prefer the fluidized bed approach.
“I see a real disadvantage to using a drum,” he says. “Just imagine you are trying to dry your clothes and you spray them immediately before they go into the dryer. They will tend to stick to the outside of the machine’s drum, really slowing down the drying process. The same holds true for potash granules, with the added disadvantage that they also tend to congeal into a large mass. In a fluid bed dryer, on the other hand, the product is virtually suspended all the time it is moving through the unit. There is much better movement of air around each granule and less likelihood that granules will stick together.”
He adds that he has seen the impact (literally) using rotary drum dryers has had while visiting other PCS divisions. Hammer marks on the side of the drums provide visible proof of material sticking to the sides and having to be beaten out.
“I’ve been at drum dryer-based operations in which they’ve been taking huge chunks of potash out by wheelbarrow each hour - material that has to be sent back for reprocessing. We, on the other hand, generally have about ½ wheelbarrow of such product every 12 hours. It’s not hard to see that our process efficiencies are much better.” Combining this process efficiency with the greater overall thermal efficiency of a fluid bed dryer operation, it is clear to see that the Schwing Bioset drying system is a clear winner for PCS.
Situation is Fluid
Although the advantages are obvious, currently, only PCS’s Allan and Lanigan divisions use fluid bed dryers (both from Schwing Bioset) and changes with regard to maximizing the effectiveness of each unit are ongoing.
“We are working with Lanigan to see about adjusting the amount of water they add, the temps at which they run their dryer, and so on,” says MacDonald. “The operating [air] temperature of our dryer is 240° to 250°C. They run theirs at 190° to 200°C. We feed the dryer at about 120 metric tons per hour (MTPH), while they can do a bit better at 140 MTPH."
Along the way, however, MacDonald and his team at Allan have gained some key insight which has helped them avoid downstream issues that have plagued many others. That includes keeping the temperatures at their dryer’s exhaust outlet - the point at which fluidizing air is removed from the unit and routed to a bag house to remove entrained particulate matter - at about 120°C.
“We’ve found that dropping below 100°C tends to leave the air too wet, creating problems for the bag house. Maintaining that temperature threshold optimizes bag house performance by avoiding any differential pressureissues which can result from material caking [on the filter media] as it enters. So, we add more water in our glazing step, but are careful to compensate and avoid those bag house issues.”
MacDonald adds that they have had a great working relationship with Schwing Bioset - a plus when it came to requesting some modifications to the dryer installed in their plant. “The company as a whole, and Joe Scholl, their product manager for this market, have been great in support of our effort - quickly and effectively dealing with any issues we’ve had. Initially the inlet of this particular fluid bed dryer was designed for an air temperature of 250°C. But, we asked for it to be moved up to 350°C, again because we add more water but still need to maintain that outlet air temp of 120°C. They worked with us to make that happen and the result has been very solid for us.”
Sharing the Wealth
As mentioned, the reason for re-wetting and glazing the product is to reduce the attrition rate - the amount of product that is damaged from the time it leaves the plant until it reaches its destination. The majority of end-users...
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