Lecture 27. Potash Fertilizers

Transcription

1 Lecture 27 Potash Fertilizers Introduction: The term potash generally refers to potassium chloride, but it is also loosely applied to other potassium compounds that may be differentiated by the words muriate of potash, sulfate of potash, etc. also, when referring to ores, or in geology, all the naturally occurring potassium salts are called potash ores. The size and economic attractiveness of the world s commercial potash operations vary considerably and their pattern of development and continued production have often been only partially based upon true economic merit. This has led to considerable overcapacity in the industry, primarily because the industry is dominated by nationalistic considerations of jobs foreign exchange, etc. a second distinguishing feature of the potash industry is that the generally poor economics and similarity of mining and processing do not allow for much R&D; therefore, cost reduction and efficiency changes have been comparatively infrequent. Finally, increasing maintenance costs and usually much higher mining costs are incurred as potash operations age and face dwindling reserves, declining ore grade, longer mining distances, and often thinner or deeper ore. Potash in Agriculture Most of the world s potash production is used in agriculture. Over 95% -97% of it is sold to improve the world food, fiber and other from output. Potassium is one of the three major plant nutrients and as such must be added to all intensive farming soils as it becomes depleted. Some potassium-containing minerals such as clay, feldspar, and mica are found naturally in soils and the potassium slowly becomes available with weathering. It then goes into solution or is in an ion exchanged with clays or organic matter near the surface and thus not is very mobile; therefore, the placement is important in many soils. Finally, the function of potassium in plant metabolism is different from that of the other major

2 nutrients. The other nutrients become part of the plant structure, but potassium largely remains as an ion in the cells and sap. The function of potassium is to help control the plant s water intake and metabolism. Potash is as vital to the plant as the other fertilizers and is removed from the soil in the same manner, even though its action in the plant is different. For these reasons fertilization with potash is somewhat more complex than with the other nutrients. Some of the specific effects potash are to increase root growth; improve drought resistance by reducing water loss, wilting, and respiration (maintaining turgor ) and also lower the plant s energy losses. Potash helps form cellulose and reduce lodging, enhances many enzyme actions aids in photosynthesis and food formation, helps in the translation of sugar and starch, helps increase the starch and/or protein content of plant, and helps retard crop diseases. It is sometimes called the quality nutrient because of these many beneficial functions. To attain maximum effectiveness, primary nutrients must be supplied to crops in essentially the same proportions as they exist in most plant life, where the ratio of nitrogen must be supplied to crops in essentially the same proportions as they exist in most plant life, where the ratio of nitrogen to potassium is about 2:1. Thus among the commercial fertilizers, about one-half as much potash is used as nitrogen, except when soil is capable of supplying potash. Thus, the ever increasing use of nitrogen fertilizer necessitates the proportional application of potash. Product Quality The quality and size of particles depend on the flotation and purification methods. Crystallization is the dominant method because of the quality of the raw material. Recent crystallization technology allows production of rounded grains of about 1mm size, which is suitable for compaction. Most potash is sold without detailed quality specifications as to either purity or particle size. Potassium chloride is guaranteed to have at least 60%K 2 O (and potassium sulfate 50% K 2 O), but this is only a 95%-96% purity. The impurity is primarily assumed to be NaCl, but

3 there are no specifications for it. With the technical (industrial) grades, however, the specifications for various uses can be very demanding; for example, limits on heavy metals in KCL that is sold to KOH producers, KCL in low sodium food use, etc. An undesirable aspect of product quality is that potash tends to cake, and there is also some unavailable crystal breakage with shipping and handling, which generally results in dust problems. 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 grade of product before shipment and an oil to help reduce dust problems. For those special grades in which such impurities are not allowed a more closely sized and drier product is shipped. Granular Potash 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. In the case, granulation refers to the dry milling of flake into a prescribed size distribution employing a screening step. The general flowsheet of potash compaction/granulation is shown in figure below. The system is composed of four groups of equipment for the following process steps. Storage and feeding. Compaction. Size reduction and classification Finishing. Storage and Feeding: The 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

4 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 diverging 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 predensify the feed to the compactor. Force feeders facilitate automation of the process by regulation of 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 1g/cm 3 to 2g/cm 3, the amount of air is about 0.5Nm 3 /t of product. The compactors are two counter rotating rolls. One roller is located 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. 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.

5 The product from the compaction process is the flake. After compaction the flake is occurred in the conveying system. The flake is broken into smaller pieces immediately below the roller. Pin-type breaker and coarse tooth roll crushers are used to break the flakes. 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 oversize 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. NPK-Components From Fresh Feed Bins Minus 2 mm M-1 SI-2 SI-3 SF-4 M-1 Mixer SC-9 Double Deck Screen SI-2 Feed Bin D-10 Polishing Drum SI-3 Recycle Bin SC-11 S.D Screen SF-4 Screw feede D-12 Conditioning Drum X 5 BC 8 D 10 SC mm SC 11 SI 13 X-4 Compactor SI-13 Silo BC 6 BC-6 Belt conveyer GR-14 Oversize crushers GR-7 Coarse disc crusher BC-8 Bucket elevator GR 7 D 12 Granular Product General Flowsheet for Potash Compaction/Granulation Plant Finishing section- This section includes: Polishing unit Coating unit Final product storage.

6 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 by direct spraying on the conveyer or mixing in screw conveyer. During the quenching stage the sharper corners of the particles break off and a shell of dissolved salt envelops the particles. After wetting the product is dried in a rotary cirum or fluid bed dryer at a temperature as high as 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 transportations and oils prevent the formation of dust clouds at the heading 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.