The general flow sheet of a fertilizer compaction/granulation plant comprises four groups of machinery corresponding to the following process steps:
The material is transported within the plant by various means such as bucket elevators or belt and trough chain conveyors.
Storage, Dosing and Mixing
The number of bins used depends on the number of fertilizer components to be incorporated in the final product. Batch mixing is the preferred method in a system designed for the production of different mixed fertilizers. Components are metered into a weigh bin. After formulation, the contents are discharged into a batch mixer. A surge bin between the discontinuous formulation part and the rest of the system allows continuous operation of all other equipment.
Systems granulating a single multi-component fertilizer are normally equipped with continuous metering and mixing devices such as belt scales for feeding into mixers. Whichever operation is used, selection of the mixer type depends on the fertilizer composition and requires careful consideration.
Using Köppern roller presses based on the principle of press-agglomeration, particulate material is introduced into the nip of the two counter-rotating rollers by means of vertical feed screws. As the material is compacted, pressure within the material increases, reaching a peak just above the line of closest approach between the rollers, and subsequently drops rapidly to zero.
During this process, the apparent density of the mixture increases by a factor of 1.5 to 3 due the decreasing void volume of the bulk material. The resulting product is typically a flat sheet or web of 5-20 mm thickness.
Size Reduction and Classification
The sheet produced in the roller press must be broken up into granules and classified into the desired particle size range. Often, the first crushing step takes place in a specially designed flake breaker installed directly below the roller press. The sheet is broken into smaller pieces (flakes) which make perfect feed for subsequent crushers.
Using a multi-deck screen, the material is then separated into oversize, product and undersize classifications. The undersize (which passes directly through to the lowest screen deck) is re-circulated for compaction, while the oversize forms a residue on the upper screen deck which is fed back into the size reduction process.
Many mixed fertilizers, particularly those containing phosphates and/or urea, initially produce relatively soft, plastic sheets that harden during aging. For this, curing bins are installed between screen and secondary crushers.
The economics of a fertilizer compaction/granulation system depend strongly on the appropriate roller press design and the correct selection of crushing and sizing equipment. In addition to the particle shape and the size ranges, the compaction process as well as the crushing and screening circuits determine the yield and output of a plant.
Often the product is subjected to either one or two finishing steps. The irregularly shaped particles can be tumbled in an abrasion drum to round off sharp edges and corners, whereby the fines produced are removed by single-deck screening and then re-circulated. Other finishing steps are for example moistening and subsequent drying in order to improve the abrasion resistance of the individual fertilizer grains, or coating with special anti-caking agents.
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