- Nongranular material
- Granular materials
- Storage and feeding
- Compaction
- Size reduction and classification(“granulation”)
- Finishing section
During much of the history of potash production from the 19th century to the early 1950s, particle size was of little consequence. The material called “standard” had an average size of 0. 8 mm. In the 1960s when bulk blending was increasing in the United States, producers developed ‘”coarse” material with a typical size range of 0.8-2.0 mm. Introduction of granular fertilisers for bulk blending required an increase in the size of potash particles. The “coarse” material was hardly compatible with other fertilisers. Therefore, the average size was increase to one matching the regular fertiliser sizes (D50 around 3.5 mm) and granules were often smoothened to prevent segregation when blending. Standard product is still used, mainly as (cheaper) raw material for PK/NPK granular production.
Nongranular material
Obviously, the quality and size of particles depend on the flotation and purification methods. Crystallisation is the dominant method in Germany and in the brine lakes because of the quality of the raw material.
Most potash is sold without detailed “quality” specifications as to either purity or particle size. Potassium chloride is guaranteed to have at least 60% K2O (and potassium sulphate 50% K2O), but this is only a 95-96% purity. The impurity is primarily assumed to be NaCl and MgCl2. With the technical (industrial) grades, however, specifications for various uses can be very demanding; for example, limits on heavy metals and bromine in KCl that is sold to KOH producers, NaCl in low-sodium food use.
An undesirable aspect of product quality is that potash tends to cake, and there is also some unavoidable crystal breakage with shipping and handling, which generally results in a dust problem. An interesting effect from the strong adsorption of amine groups to potash is the fact that the flotation reagents used are also effective anticaking agents. However, to enhance the effect most companies include amounts of amines in the different grades of product before shipment and an oil to help reduce dust problems. Industrial users normally prefer a product without coating.
Granular materials
Compaction/granulation is now an important stage in the overall potassium chloride production/beneficiation process. Compaction implies the agglomeration of particles under force to produce a densified and coherent sheet-like material referred to as flake. In this case, granulation refers to the dry milling of flake into a prescribed size distribution employing a screening step. Rounding off / polishing of the broken material can be performed in an additional drum. Compaction/granulation of potash began in the 1950s. Currently, about 60% of the total world potash production for fertilisers application is compacted.
The general flowsheet of potash compaction/granulation is composed of four groups of equipment for the following process steps, Figure 1:
• Storage and feeding.
• Compaction.
• Size reduction and classification.
• Finishing.
Storage and feeding
This section usually includes:
• Material storage hopper allowing the undersize material from the granulation section to be recycled.
• Feeding hopper system, possibly with magnet and rough screening system.
• Control feeding system.
Compaction
The section usually includes:
• Feeder.
• Compacting rolls.
• Flake breaker.
There are two types of feeders: gravity and force-type feeders.
A gravity system is composed of a slightly converging chute located above the rolls and an adjustable vertical feed control tongue to keep the level constant.
The force feeders are equipped with single or multiple screws. These screws may be positioned vertically, at an angle, or horizontally. Tapered screws deaerate and pre-densify the feed to the compactor. Force feeders facilitate automation of the process by regulating the speed of the screw. Both gravity and force feeders are deaerated through ports connected to the dust system. The amount of the air removed depends on the size of material and compression ratio. In case of compacting potash from specific gravity of 1 g/ cm3 to 2 g/ cm3, the amount of air is about 0.5 Nm3/t of product.
The compactors are two counterrotating rolls. One roller is in a set of fixed bearings; the other is a floating-bearing unit. The feed passed between the two rollers is progressively transformed from a loose to a dense state. The rolls can be smooth or corrugated, waffled, or have pocketed surfaces. The chevroned surfaces increase flake production and allow the nip width to be increased. The nip width also establishes the compression ratio. Compression ratios range between 1.5 and 3, Table 1.
The selection of equipment depends on the material and its properties, including particle shape, size, size distribution, and compaction force needed. For compaction the general rule is that the larger particles should be accompanied by fines to fill in the larger pores or open space. The van der Waals attraction forces are often not sufficient to produce flake with adequate strength. Sometimes binders are added, or creation of crystalline bridges is used.
The temperature of potash feed determines its plasticity. In comparison with colder feed the warmer feed may require less pressure to obtain product with a given strength and density, or, at the same energy input, produce material with greater strength and density. Elevated temperatures are beneficial for the formation of crystal bridges.
In general compaction is defined as a dry agglomeration process. Sometimes the addition of water may be beneficial; however, it is rare to observe compaction of material with a water content of more than 2%. The compaction of KCl is accomplished essentially in a dry state, Table 2. The product from the compaction process is the flake. The thickness of the potash flake varies from plant to plant but on average is about 20 mm. After compaction the flake is cured in the conveying system. For potash the normal design of the plant provides about 3 minutes of curing time. The flake is broken into smaller pieces immediately below the roller. Pin-type breakers and coarse tooth roll crushers are used to break the flakes.
Size reduction and classification(“granulation”)
The granulation-section is essentially composed of crushing and screening equipment. Potash granulation mainly consists of primary and secondary crushers in closed circuit with a multi-deck screening system. Two-stage milling decreases the recycle ratio. The primary crusher grinds large materials from the compactor, and the secondary crusher is fed with over-size from the screen. Hammer mills, cage mills, or chain mills are used as the primary crushers. Roll crushers with toothed rolls are often used as secondary crushers.
The roll crusher parameters are important, the rotational speed, size, and shape of the crusher teeth. Usually, the mills are fitted with variable speed drives. The discharge from the compression (roll) crusher is controlled by the gap setting. Evaluation of circuits using roll crushers indicates higher yields than from those equipped with impact crushers.
Finishing section
This section includes:
Polishing unit.
Coating unit.
Final product storage and dust collection.
The particles after granulation have sharp edges and are of irregular shape. To diminish the crushing of the material during further handling and dust creation, the potash particles are quenched, dried, and passed through the polishing screen. Quenching consists of wetting particles with water or brine or steam by direct spraying on the conveyer or mixing in screw conveyers (maximum 5% but usually about 1%). During the quenching stage the sharper corners of the particle break off and a shell of dissolved salt envelops the particles. After wetting the product is dried in a rotary drum or fluid bed dryer at a temperature as high as 200°C. Dry product is passed over a polishing screen to remove the fines.
In the final steps the product is cooled and coated in a rotary drum with anticaking agents (amines and oils). Amines depress the caking tendency of potash during transportation, and oils prevent the formation of dust clouds at the handling points.
Dust Collection System – Major dust sources are the hopper in which the payloader dumps the raw material, the crushers, the transfer points, the mixer, and the screens. The amount of air needed in the dust collection system depends on the number and types of collection points. The system consists of a fan, cyclones, and filter bags connected by air ducts.
The velocity of the air in the ducts must be kept high, to avoid the deposition of solids. The fan is sized based on the volume of air required and the pressure drop through the system. The fan is installed on the clean air side of the cyclone or filter bags.
References
Potash Corp. of Saskatchewan. (1991). “Product Specifications,” PCS Sales, Canada.
Pietsch, W. 1991. “Granulation of Fertilizers by Compaction,” In Urea-Based NPK Plant Design and Operating Alternatives, Workshop Proceedings , pp. 89-98, Schultz, J. J. and Hoffmeister, G. (Eds.), SP-15, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A.
Fayard, C., and Hervieu, P. 1991. “Compaction / Granulation of Fertilizers,” In Urea-Based NPK Plant Design and Operating Alternatives, Workshop Proceedings, pp. 99-108, Schultz, J. J. and Hoffmeister, G. (Eds.), SP-15, International Fertilizer Development Center Muscle Shoals, AL, U.S.A.
Klein, M.O. 1991. “The Manufacture of Fertilizers by Compaction/Granulation,” In Urea-Based NPK Plant Design and Operating Alternatives, Workshop Proceedings, pp. 119-132, Schultz, J. J. and Hoffmeister, G. (Eds.) , SP-15, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A.
Zisselmar, R. 1988. “Compacting/Granulation of Fertilizers,” In NPK Fertilizer- Production Alternative, Proceedings of an International Workshop, pp. 46-56, SP-9, Madras/Bombay, India, International Fertilizer Development Center Muscle Shoals, AL, U.S.A.
Links to Related IFS Proceedings
297, (1999), Tailoring Potash to the Needs of the Fertiliser Industry, H. Rug, K. Kahle
857, (2021), Mineral Sizing Journey from Pit to Port and its Influence on Fertiliser Products, R McConnell
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