High performance screening machines in the potash industry, Saskatoon
Meike Schulz, RHEWUM GmbH
Custom-made RHEWUM screening machines grant high production rates in the range of 0.2 to 40 mm and low maintenance intervals at the same time.
Potassium constitutes an important component of life. It plays a key role in many areas of nutrition. Together with nitrogen and phosphorus potassium it is the third essential plant nutrient and represents an important part of plant and animal life - it supports plant growth, enzyme activity, protein formation, photosynthesis, respiration and the improved absorption of nutrients. Almost 95% of all potassium produced is used as fertilizer . Even the supply of potassium to plants may be suspended short-term potassium remains an essential mineral in the long term.
A versatile mineral
|Sylvinit||KCl * NaCl||
|Langbeinit||K2SO4 * 2MgSO4||
|Kainit||KCl * MgSO4 * 3 H2O||
|Carnallit||KCl * MgCl2 * 6 H2O||
Table 1: The most important potassium-containing minerals with their content of water-soluble potassium oxide (K2O)  Around the world, there are many potassium-bearing deposits, as chloride, sulphate and nitrate compounds. The chloride-containing form plays the most important role in industry. Potassium is available in various compounds, e.g. with sodium, magnesium and calcium (Table 1). Just four countries account for three quarters of the worldwide production – Canada, Russia, Germany and Belarus. The worldwide production capacity reached around 38 million tons in 2005. The four most important consuming countries are USA, China, Brazil and India. Canada disposes of about half of the worldwide known potassium deposits but the production capacity in Canada only reaches 30% of the amount needed worldwide. The most important producers worldwide are:
Potash Corporation of Saskatchewan Inc. (Canada) According to their own information PCS is the third-largest enterprise in the nitrogen and phosphate market and a producer of animal food and industrial chemicals as well. The company produces more than 15% of the potash production worldwide. Potash Corp was founded 1975 by the government of province Saskatchewan. In 1989 the company passed into private hands and got public. In the 1990`s PCS acquired several US-American producers of potassium chloride: Potash Company of America, Florida Favorite Fertilizer, Texasgulf and Arcadian Corporation. The main markets are North America, Brazil and Asia including China. Website: www.potashcorp.com
Belaruskali (Belarus) The state owned coal mining business named “Production Union Belaruskali” is the largest producer of potash fertilizer in the CIS area. The headquarter is located in Salihorsk/Belarus where the four mining plants of the company are stated (potash deposit of Starobin). Belaruskali got 20.000 employees. The production is delivered to more than 50 countries in Europe, Africa, Asia, North America and South America. Every sixth ton of potash fertilizer worldwide is produced in Belarus. Website: www.kali.by
Mosaic Company (USA) The Mosaic Company from Plymouth/Minnesota in the USA has the largest phosphate production capacities worldwide and is the third-largest potash producer at the same time. More than 15 % of the worldwide potash production is generated by Mosaic. The phosphate is mined and processed in five mining plants and four production facilities in Florida and Louisiana. Like the Potash Corporation the Mosaic Company is part of the export consortium Canpotex. The Mosaic Company owns potash mines in Saskatchewan (Canada), New Mexico and Michigan (USA) and has 7.400 employees worldwide. The main activities are stated in North and South America as well as in Asia. Parts of the Mosaic Company are IMC Global and Cargill Nutrition. Website: www.mosaicco.com
K+S Kali GmbH (Germany) The company with it´s headquarter in Kassel/Germany is the fourth-largest potash producer worldwide and the leading supplier of potash and magnesium raw salt in Europe. Potassium chloride is the top-selling product of K+S Kali GmbH: an universal fertilizer used e.g. for the cultivation of cereals, corn, rice and soy beans. Parts of the K+S Kali product range are approved for ecological cultivation. In the year 2008 a quote was submitted for recommissioning of the potash plant near to Rossleben/Germany. The business activities of K+S consist of four groups: Fertilizers, Industry, Health Care & Food and Nutritients.
Uralkali (Russia) Uralkali is the second-largest mineral fertilizer producer in Russia. It is headquartered in Beresniki in the Perm area. The company got more than 15.000 employees and predominantly mines potassium chloride which is mainly exported. It was founded in 1930 as public enterprise and since 1992 is an incorporated company. One of the biggest shareholders is the billionaire Suleiman Kerimow. The world market share of potassium amounts 10 %. More than 90% of the products are exported, mainly to China, Brazil, South-East-Asia and India, but also to Europe, Africa, Middle East and the USA. Low costs of labour and energy allow Uralkali to offer low priced products. Website: www.uralkali.com
DSW, Dead Sea Works DSW is the abbreviation of Dead Sea Works. After the Israeli war of independence in 1948 the Palestine Potash company – founded in 1920 by Moshe Novomeysky in the British mandated territory Palestine – was nationalized and redefined. The company now belonging to Israel Chemicals Ltd. (ICL) is the main important economic center in the forbidding southern Israel. Its raw materials arose from the Dead Sea which lies 400 metres below sea level. In 1992 ICL was passed into private hands. 90% of the sales are made abroad, many of the to Europe. ICL is one of the largest companies in Israel, the largest supplier of fertilizer and the leading industrial chemistry company in the country. Website: www.icl-group.com
China The Chinese potash- production amounts 3.5 million tons per year. The mining activities are state-owned. The Chinese potash-production does not play an important role regarding the global trade because China consumes the potash-production itself and purchases on the world market to cover own demands. According to Germany Trade and Invest in 2008 87 Chinese companies were producing potash fertilizer. The most important producer is Sinofert, belonging to the chemistry giant Sinochem.
APC, Arab Potash Company (Jordan) The Arab Potash Company also mines their potash at the Dead Sea in the north-western area. The Jordanian company headquartered in Amman was founded in 1956. The extraction of potash was started by APC in 1980. Since then the company expands its activities itself as well as with its subsidiary, the Jordan Dead Sea Industries Company. APC diversifies its activities with chemistry and fertilizer projects. In 2003 the Canadian Potash Corporation of Saskatchewan took a share in APC. With this joint venture the production shall be increased. APC sells its products mainly in the Mediterranean area and Asia. Website: www.arabpotash.com
Agrium Inc. (Canada) The focus of Agrium´s activities is the production of nitrogen, besides it is producing phosphate and potash. It was founded in 1931 as Cominco Fertilizers Ltd. and changed over to Agrium Inc. in the year 1995 The company is headquartered in Calgary/Alberta in Canada. Agrium got about 6.600 employees (in 2006) and operates several potash and phosphate mines in Canada. The annual nitrogen capacity is 5 million tons and as by their own information the company benefits from the low gas price in Alberta. Agrium sells its products in Canada, USA and Argentina. Like the Potash Corp and the Mosaic Company Agrium Inc. is part of the North American export consortium Canpotex. For several times the company was avenged to breach of environmental specification. Webseite: www.agrium.com
Potassium deposits which lie in the form of solid strata at depths of up to 1.400 m are usually extracted by means of conventional mechanized methods of underground mining. In case that this is not longer possible owing to the depth of the deposits and/or if problems with incoming water prevent conventional mining the salt-domes are solution-mined. Solar evaporation of brines containing natural potassium is the third method of extraction but does not play a significant role.
Screen technologies in the potash industry
Screens are used at different points in potassium processing, starting with extraction – the mineral extracted from the mines is first crushed and then sized – and then in compaction, granulate screening and loading. Screens are also found in wet processing, i.e. wet sizing after the flotation process, as well as dewatering. As screens have much lower specific energy consumption than crushers, besides the straight production of saleable product, they can be used to relieve the load on crushing stages (selective crushing) in order to avoid cost-intensive overgrinding 
The screens used are mainly vibrating and cloth vibrated screens. Vibrating screens are used for high throughput rates at medium to coarse cut-points. For critical product properties and separations < 3 mm cloth vibrated screens demonstrate their advantages compared to vibrating screens. Double frequency screening machines (DF) are a special type of vibrating screens. The drive principle of screening machines type DF is based on the use of two out-of-balance motors operated at different speeds, which are mounted at feed and discharge end of the screening machine. In the feed section, the feed material is loosened up by the bigger amplitudes and sorted by size (segregation). In the discharge section, small amplitudes are generated, which improve screening efficiency by effectively separating the near-mesh particles. By changing the direction of rotation of the motors, it is possible to selectively influence the conveying speed and therefore the feed rate as well as separating sharpness. Compared to conventional screening technology thanks to this drive principle this screen type is designed as a compact unit with high capacities.
The screen consists of a vibrating inner part and a static housing. This allows fixed connections without sensitive flexible connections and lower vibrating mass. Together with the external drives this enables high-temperature screening up to 600°C. The screwed construction allows extraordinary long operating times and a very easy to maintain operation. For example, after the first and second crushing stage, two lines based on this technology are operating a throughput rate of 1.600 t/h. Following scalping of the feed at 40 mm on a grizzly – the feed is then separated on the screen at 25, 12.5 and 5 mm. Separation at 1 mm is then done on WAU screens with direct excitation of the screening mesh, described in the following.
DF screening machines for screening of compacted KCL
A further application area for DF screening machines are screens for production in compaction of potash. Powder product is fed together with binding material into roller presses. The rolled flakes are then crushed and screened. Usually only around 15 to 23 wt.% of the desired product fraction 2 to 4 mm are contained in the crushed material. Therefore this material needs to be screened with a high throughput rate and precisely defined limits for out-of-spec particles to de-load the compacting circuit. While the fines coming from the crusher are screened and re-fed to the compaction the screened oversize particles are crushed again.
|4 и 2 мм||
|4 и 2 мм||
|Load out screen||
Table 2: Screening performance of the different RHEWUM screening technologies
Because of the high screening performance and compact dimensions at the same time as well as its technology the DF screening machine became established as loading screens at almost all well-known potash producers worldwide.
Load out screens for compacted KCL
At the plant of a well-known French potash producer the dust content of compacted KCL has to be reduced from up to 10 % to 0.5 % and agglomerates larger than 5 mm have to be removed. For the feed rate of 300 up to 360 t/h a three-deck double-frequency screening machine was selected. Feeding of the material is realised by an integrated co-vibrating distributor. This allows compact installation dimensions and maintenance free operation compared to a vibrating conveyor. By installing a further screening deck in the screening machine a de-loading for the finest separation can be realised. Doing this the required screening area for the finest screen cut which determines the size of the screen can be minimized and therewith the whole machine will become smaller in size.
Screening machines with direct excitation of screen cloths
For finer separation smaller than 2 mm down to 0.16 mm screening machines with direct excitation of screen cloths are used. They are suitable for effective separations at high throughput rates.
This machines are throw screens with direct excitation of the screening surface. The screen aggregate itself is static. This allows fixed connections at the inlets and outlets. An advantage of this type of construction is the limitation of dust emissions as well as the reduction of maintenance costs which will occur compulsory with dynamic sealing systems. Outside of the screen housing rugged vibrators generate vibrations which are transferred directly into the screen cloth by means of vibrating axis. As a result of the high-frequently vibration the screen material is thrown at a right angle from the screening surface - only the pitch of the screen cloth determines the transport. A thinlayer screening at a high transport rate takes place. Fine products become fluidised on the screen cloth. Because of the high frequency despite of the high transport rate a high quantity of contacts take place between screen cloth and particles. An adjustable cleaning cycle reliably avoids the blinding of the screen mesh. Because of the low masses that have to be moved (only the screen cloth vibrate) high accelerations on the screening surface are achieved. This helps, for example, to overcome adhesion of moist or sticky material (e.g. pellets). Besides this the power consumption is very low (0.05-0.10 kWh/t).
In several industrial operations this screening technology is used for pre-screening, the cold crystallisation process and flotation as well as dedusting. Due to the static screen housing there is no transfer of dynamic forces into the building. This allows to operate this type of screen even in high buildings without increasing the dimension of the steelwork caused by undesired vibrations.
Wet-screening generally requires short screen lengths, because with the separation of the liquid no more classifying takes place. In fact the width of the screen is much more important to reach high throughput rates and a precise separation at the same time. By feeding the pulp as thin layer on a width instead of a long machine higher federates and more precise separations are achieved and the lifetime of the screen cloth is increased considerably. In general for wet screening free water and vibrations are necessary to transport the fine particles through the screen mesh. As soon as the water fed as pulp onto the machine has passed the screening surface the classifying process ends. The solid matter left on the screen cloth is carried to the outlet of the screen by means of vibration and gravity.
The RHEWUM screening machine type RIUS 1+1 with a double-feed system is a compact machine with two screening units as an one-deck machine. This allows high specific feed rates and the maintenance friendly operation of a single-deck screening machine at the same time. The drive principle of the screening machine is realised by high frequent vibrations adopted to the screen material. Because the screening machine only needs one drive for two separation steps investment costs are reduced. Besides the screening of potash the RIUS 1+1 can also be used for feldspar, garnet, iron ore, kaolin, phosphate, quartz sand as well as tantalum. It is an economic and efficient alternative to hydraulic classifiers and hydrocyclones.
Dewatering screens (RHEWUM ES) are used for the de-watering of products coming from wet sorting steps and for reclaiming of process liquid. Compared to conventional screening machines at this type of machine the pitch of screen cloth ascends in direction of the outlet. This leads to considerably higher screen material layer and an improved separation of solids and fluid. High accelerations of the screen cloths are very important for the dewatering effect. Therefore drives with high rotation speed are used to generate a linear vibration and to improve the dewatering effect. The retention time of the material on the machine is influenced by the setting angle of the machine. This should be chosen appropriate to the viscosity of the liquid. Important for the customer is the high dry content of the solid matter after the screening machine.
Screening machines are often used in granulation circuits resp. crushing and grinding circuits. The product leaving the screening machine often passes the screening machine again after another processing step. To de-load these circuits and increase the production rate the online determination of the grain size distribution of the material coming from a granulator, a mill or a crusher is very helpful. The interaction of both components can be improved and the re-circulation of product will be reduced. Depending on the process chosen for the production of saleable particle sizes considerable variations of the particle size distribution can occur. They increase the amount of material to be re-circulated and decrease the production rate at the same time. In this case an online particle measurement becomes very important. For the online determination of particle sizes certain amount of the material is separated by a sample taking system from the product stream and then fed to the determination unit – the SizeChecker.
A vibrating conveyor carries the product to a detection-unit which optically determines the particle sizes during a free-fall. Therefore a CCD line-camera takes a shadow image of the respective particles in front of a LED illumination unit with up to 4096 pixel. From this endless picture which allows to determine the size of each single particle the contours and dimensions of the particles are analysed by “Blob-Analysis”. By the relation of minimum and maximum dimensions as well as the contour different shape classifications are calculated online. As result the particle size distribution, the particle shape as well as long-term trends connected to previously fixed limits of permissible errors are given. Control signals are send to external control devices. Besides the actual data of the particle size distribution the D50-values resp. MAs can be calculated and further processed. Due to the SizeCheckers interface to the OPC Server which is very common for process control the system can be used flexible for many applications. Process control circuits can be build up to regulate a crusher or mill resp. compaction units or granulators to increase the production rate with only a little effort. Based on RHEWUMS experience of decades in screening technology, especially in potash industry, RHEWUM develops more and more effective solutions for the processing of potash. New material grades and technologies disclose new machine sizes and types which seem to be feasible just a few years ago. The aim is to develop an even more effective and easy to operate processing technology for raw materials. With regards to the worldwide limited resources and the increasing demand for fertilizer we consider this as very important.
Bibliography 1.CEH Report, Potash, by Bala Suresh, November 2004 2.Potash Case Study, International Institute for Environment and Development (IIED) and World Business Council for Sustainable Development (WBCSD), No. 65, February 2002 3.Südwestdeutsche Salzwerke AG Mitarbeitermagazin 2 / 2002 4.Schmidt, P.; Körber, R. u. Coppers, M.; Sieben und Siebmaschinen, Wiley-VCH Verlag, 2003 5.Coppers, M. und Schütz, S; Optische Online-Partikelanalyse als Produktionsüberwachung im industriellen Einsatz. Aufbereitungs-Technik 46 (2005) Nr.7 S. 42-47