Currently, the process lines, where granulation and drying processes are combined in one apparatus - a drum granulator-dryer (DGD), are the most widely used in the Russian Federation and the post-soviet space.
Mineral fertilizer technology with DGD apparatuses is one of the key activities of JSC NIUIF. Our inventions, including those describing structural solutions in DGD apparatus (RF patent 2450854), are covered by patents.
Phosphate fertilizer technologies with DGD apparatuses are widely used and, in some cases, their use is preferable compared to the technology with ammonizer-granulator. The advantages of granulation in this apparatus are the high efficiency of heat and mass transfer, a small ratio of external recycling (1÷2), good quality, close grain-size distribution of the final product and process automation possibility.
The widespread use of the fertilizer technology with DGD is due to the following reasons:
Flowsheets with DGD apparatuses include less equipment pieces, therefore the power consumption is reduced, and fewer capital investments are required for their manufacture. A shorter process (recycle) line can provide fewer equipment failures, which increases the effective operating time and reduces repair costs. Process lines with DGD apparatuses are traditionally considered to be low recycle, which also reduces energy consumption and allows the use of lower capacity process equipment (and, consequently, lower cost).
Figure 1. Drum granulator-dryer (DGD)
1 – nozzle; 2 – loading chamber; 3 – screw feeder; 4 – reverse screw; 5 – flat lifting flight; 6 – discharge chamber; 7 – jet; I – granulation section; II – drying section.
DGD is a rotary drum with an internal flat lifting flight, installed with inclination of 1÷3° to the horizon line (Fig. 1). A distinctive feature of the DGD apparatus is a reverse screw, which allows returning of a certain amount of granulated material to the head part of the apparatus (internal recycle). When the drum rotates, the blades of special design, installed inside the drum, pass through the product layer at the bottom (“obstruction”) capturing it and then create the “curtain” by pouring over the whole section. The slurry is fed onto this curtain through nozzles and dispersed with compressed air (or by steam, generated at pipe reactor (PR)). In addition, the flue gases and external recycle are fed into the drum head part. To transfer recycle to the curtain area, head part of DGD is equipped with a screw feeder. During spraying of slurry onto the curtain particles, the granules are formed and then dried. Maximum amount of slurry, sprayed onto the curtain, depends on the granules’ moisture limit, at which the cakes in “obstruction” start to form.
The apparatus dimensions (diameter and length) are determined by the capacity of the process line, as well as the sprayed slurry moisture. Process lines with DGD are commonly used for phosphorus-cointaning fertilizer manufacture (DxL): 3.5x14m; 4,5x16m.
Ammonium phosphate-based fertilizer technology (MAP, DAP, NPK) with DGD
Figure 2. Flexible production process of phosphorus-containing fertilizers with DGD
1 – preneutralizer; 2 – bins; 3 – acid tank; 4 – tank for mixing of slurry with potassium chloride; 5 – pipe reactor; 6 – combustion chamber; 7 – DGD; 8 – elevators; 9 – screen; 10 – mill; 11 – fluid bed cooler; 12 – Solex cooler; 13 – coating drum.
The flowsheet for manufacture of ammonium phosphate-based fertilizers using DGD is shown in Figure 2. It includes neutralization of phosphoric acid (or a mixture of sulphuric and phosphoric acids) with ammonia in neutralization vessel (or high-speed ammonizer-evaporator), ammoniation in pipe reactor, granulation and drying in DGD, classification of granules with the recycle of crushed oversize and fines, and also a portion of the product fraction in DGD in the form of recycle through recycle process. The final product is cooled, coated with conditioned mixture to improve consumer properties and sent to the final product storage. Preliminary mixing of a portion of ammoniated slurry and potassium chloride should be done for granulation improvement, highest conversion degree of chemical processes and reduction of the final product caking. The use of magnesium-containing additives (magnesite, brucite) allows to further reduce the caking and dustiness of complex fertilizers. This method involves mixing the magnesium-containing additive with phosphoric acid and then introducing the feed for neutralization.
Depending on the WPA concentration and fertilizer grade, there are various options for an optimal ammonia neutralization method (ammonium phosphate-based fertilizers manufacturing):
Depending on the process conditions, self-engineered high-speed ammonizer-evaporators (invention is covered by RF patent 2503495), preneutralizers and pipe reactors can be used for neutralization (ammoniation). High-speed ammonizer-evaporators, where mixing depends on heat change of neutralization reactions, are efficient when manufacturing one type of fertilizer (or two/three types max). For a wider range, a PN reactor (preneutralizer) is recommended. There are also high-speed ammonizer-evaporators with forced slurry circulation. To neutralize a more concentrated WPA (≥ 43% Р2О5), it is better to use self-engineered pipe reactors (PR) (invention is covered by RF patent 2360729). During processing of concentrated acids, the use of PR for mineral fertilizers manufacturing allows to obtain slurry with a moisture content of 6÷10%, due to which the process is highly efficient, the recycle and specific consumption of energy resources are reduced, production capacity increases, etc. In some cases, it is relevant to use a two-stage neutralization flowsheet (or a combined one), in the first stage of which a PN reactor is used (or a high-speed ammonizer-evaporator), and in the second - a PR.
In the process of sulphur-containing fertilizer manufacture sulphuric acid is addditionally used, adding to the available in WPA. Usually, sulphuric acid is neutralized by ammonia together with WPA.
Calcium phosphate-based NPK fertilizer technology with DGD
Figure 3. Calcium phosphate-based NPK-fertilizers manufacture process flowsheet with DGD
B1-3 – raw materials bin; F1-3 – feeders; R1-3 – reactors; P1-4 – centrifugal pumps; C – conveyer; E1-2 – elevators; T – scrubber liquid tank; HFS – high-speed foam scrubber; VS – Venturi scrubber; Cy – cyclone; S – screen; M – mill; CC – combustion chamber.
Figure 3 shows the flowsheet of the calcium phosphate-based fertilizer manufacture process (invention is covered by the RF patent 2551541). Dry components from bulk storages are added using belt conveyer feeder. In the first reactor, neutralization of WPA and sulphuric acid occurs with carbonate calcium (mined chalk). Acids can be fed either separately or as a mixture. In addition, scrubbing liquid is fed to the first reactor. When using unfiltered slurry, produced from WPA, it is also fed into the first reactor. The degree of neutralization is controlled by the reaction slurry pH value.
Acid neutralization process is completed in the second reactor. Besides, various additives may be fed to the reactor (nitrogen-containing components, micronutrients, etc.).
In the third reactor, the reaction slurry is mixed with potassium chloride, after it is fed to the DGD, where the granulation and drying processes take place. After DGD, product is fed onto the screen, where it is screened in three fractions. The fines and crushed oversize are returned to the DGD as an external recycle. Final product is sent for cooling and coating.
Scrubbing liquids are fed to the first reactor to dilute the neutralized phosphate-sulphate slurry to a moisture value that will ensure its feed. To maintain liquid volume, that circulates in the scrubbing system, the recirculating water is supplied to the HF scrubber.
An example of this technology implementation is the phosphorus-potassium fertilizer production plant with capacity of 100 thousand tons per year according to JSC NIUIF technology, which was put into operation at Metakhim CJSC (Volkhov) in 2013.