Drying potash is a vital aspect of processing the ore into a usable form for the fertilizer production process, as well as in finishing product coming off of the fertilizer production line.
The following article highlights the basics of potash drying, including benefits, equipment, and material considerations.
Drying Potash Ore
Mined potash undergoes several processing steps to separate potassium from gangue. This process differs depending on the type of potash mining used, the characteristics of the specific deposit, and the intended end product. In all cases, however, drying is an essential step, and is particularly important in creating a feedstock suitable for use in the fertilizer production process.
When potash will be used to make fertilizer (which is the majority of the time), the moisture content of the feedstock typically needs to be below 0.5%, depending on the characteristics of the specific source of potash and desired characteristics of the end product.
Since potash has a tendency to cake or clump together, reducing the moisture content is critical to creating a uniform, flowable feedstock that can be metered properly into the granulation process and won’t clog downstream equipment. When potash will be stored prior to granulation, drying the potash helps to reduce the potential for caking during storage.
Regardless of whether or not potash will be used in the fertilizer market, drying potash makes shipping more efficient and cost effective by reducing the amount of moisture being transported.
Drying Potash Fertilizer Products
Granulation is commonly used in the fertilizer industry as a way to create marketable products that handle and spread well, with minimal dust. In the case of potash, granulation is carried out using one of two approaches: compaction granulation or wet granulation (aka pelletizing). In both settings, drying plays an important role in the finishing of the product.
Drying in the Potash Compaction Process
Compaction granulation employs a double roll press, or compactor, to compress the potash into a compacted sheet. The sheet is then fed to a granulator, which fractures the material into coarse granules.
Compaction is typically a dry process, meaning a liquid binder is not used to aid in granule formation, and therefore, no drying step is necessary. However, the jagged and irregular granules produced in the compaction process are known to be prone to attrition, or the degradation of product into fines. This is typically a result of the rough granule edges knocking against each other and breaking off. To combat this, a glazing step is often used.
Glazing Potash After Compaction
Glazing is a finishing step implemented after compaction and is often carried out in a rotary dryer (or in other cases a pugmill mixer). By spraying a small amount of water onto hot granules in the dryer, surface cracks are filled in, granule surface is hardened, and the water is evaporated off, essentially creating a recrystallized surface on the granules that is less prone to degradation.
Drying in the Potash Pelletizing Process
While potash producers have historically favored compaction granulation, many are incorporating or switching over to pelletizing, or wet granulation.
This technique employs a liquid binder, along with a tumbling motion, to form granules by layering or accretion. This process is most commonly carried out on a disc pelletizer (also known as a pan granulator), and produces rounded granules considered a premium product in the industry.
Because this technique adds moisture in the form of a liquid binder, a drying phase is necessary.
In both pelletizing and compaction, drying ultimately reduces the product’s moisture content to a level acceptable to the industry (often producers require their own specific target moisture content). In doing so, the potential for product caking is reduced, mold and bacterial growth is prohibited, and shipping, as well as packaging, becomes more efficient.
Potash Drying Equipment: Rotary Dryers vs Fluid Bed Dryers
Potash miners and fertilizer producers are often faced with a choice when it comes to drying their potash: rotary dryer or fluid bed dryer. While both types of industrial drying system are used, rotary dryers have become the preferred method of drying, both in the case of potash ore and fertilizer products.
Unlike fluid bed dryers, rotary dryers are more tolerant to variation in feedstock. They also offer a heavy-duty option that accommodates a high throughput, and when properly maintained, are incredibly reliable. The tumbling action they employ is also ideal for rounding the sharp edges of potash produced via compaction.
Rotary dryers cascade material through the stream of combustion products for optimal heat transfer efficiency. This cascading motion also helps to reduce material clumping during the drying process.
Material Considerations When Drying Potash
While drying potash is a fairly straightforward task, there are a few considerations to keep in mind when designing a potash dryer.
Dryer Airflow
Potash dryers are best designed with a co-current air flow, meaning the potash and air stream flow in the same direction, parallel to each other. If a counter-current airflow were utilized, product degradation and discoloration would be more likely to occur, as potash would be encountering the hottest air at its driest point. Co-current flow prevents this by putting potash in contact with the hottest gases while it is in its wettest state.
Corrosiveness
Potash is notoriously corrosive, requiring special attention on the dryer materials of construction. Consequently, stainless steel is often used at the front end of the dryer in order to defend against corrosion and help to prolong equipment life.
Potash Clumping
Because potash can clump together and stick to the dryer, dryers are often equipped with knocking systems or “knockers” as a means of dislodging any clumped material on the dryer’s interior.
A screw conveyor may also be used to ‘fling’ or ‘throw’ the potash into the dryer, breaking up clumps in the process.
Conclusion
Drying is a critical step in bringing potash products to market. From processing potash ore, to finished fertilizer products, drying plays a unique role in producing products with the qualities and characteristics the market demands. Rotary dryers are the industry’s preferred choice for drying potash, but they must be designed with potash’s specific characteristics in mind to yield a dryer that performs efficiently and reliably for years to come.
In business since 1951, FEECO is the potash industry’s preferred supplier for custom potash dryers and fertilizer production equipment. We also offer extensive potash process development and testing services, as well as comprehensive parts and service support. For more information on our potash dryers or services, contact us today!