Water soluble fertilizers

Purified concentrated phosphate-containing fertilizers, such as monoammonium phosphate (MAP), diammonium phosphate (DAP) and monopotassium phosphate (MPP) are completely water soluble and therefore represent a special class of products used for fertigation systems, foliar feeding of field plants and crops, including greenhouses.

Depending on the final product requirements (the content of the main nutritional components), as well as used WPA composition, purified water soluble MAP is produced:

  1. From wet-process phosphoric acid (WPA) by chemical precipitation of impurities;
  2. From pre-purified wet-process phosphoric acid with organic extractants.

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Fig. 1. Purified crystalline MAP

Table 1. Water soluble MAP fertilizer specifications

Specification

Standard

Physical form

White powder product

Mass fraction of total nitrogen (N), %

12±1

Including ammonium nitrogen, %

12±1

Mass fraction of water soluble phosphates recalculated to P2O5, %

61±1

Mass fraction of water, %, not more than

0,3

Mass fraction of insoluble residue, %, not more than

0,1

Friability, %

100

NIUIF employees together with Voskresensky JSC NIUIF have been developing and improving the technology of purified phosphates since the 90s of the last century.

Water soluble MAP technology from WPA by chemical precipitation of impurities

WPA is neutralized with ammonia in a pipe reactor to the desired NH3:H3PO4 ratio. The resulting slurry after aging in a tank enters the filter presses to separate the precipitated impurities.

The filtrate is fed to the evaporator and then to a vacuum crystallizer. Suspension from crystallizer is fed to the centrifuge separator (cyclone). Wet MAP crystals are sent to the drying drum (or fluid bed dryer). Dried MAP is sent for cooling and then packing. The mother liquor from centrifuges (centrate) is sent for mixing of the residue, precipitated on filter presses. This residue is used for granular fertilizers manufacturing.

Water soluble fertilizer manufacturers are faced with the problem of eliminating the turbidity of solutions obtained by MAP crystals dissolution. To solve this problem, it is proposed to carry out additional purification by dissolving crystals, filtration of the resulting solution, evaporation and secondary crystallization. Taking into account the laboratory studies results, it is assumed that with the use of the described additional stages, it will be possible to obtain a water-soluble MAP with a high degree of purity, including feed quality. On top of that, an additional stage of recrystallization and filtration makes it possible to minimize the influence of the parent WPA quality, associated, for example, with fluctuations in the impurity composition of the used phosphate raw material.

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Fig.2. Purified MAP manufacture flowsheet with chemical precipitation of impurities

PR - pipe reactor; F - filter presses; VEU - vacuum evaporation unit; VC - vacuum crystallizer; Ce - centrifuge; DD - drying drum; С - cooler; V - vessel; P - pump.

Water soluble MAP technology from pre-purified WPA

WPA pre-purification technology has several advantages:

  • possibility to use WPA with high content of impurities;
  • possibility to manufacture purified commercial phosphoric acid;
  • possibility to manufacture purified phosphates (MAP/DAP/MPP) with a higher nutrient content;
  • possibility to manufacture liquid complex fertilizers based on polyphosphates 11:37;
  • possibility to manufacture feed phosphates and bring them to food grade quality.

WPA purification stage

WPA is desulphurized with apatite in a tank reactor and then fed for clarification in a thin layer settler. After the settler, the condensed part is used for fertilizers’ manufacture.

The desulphated clarified phosphoric acid is sent for purification of impurities through liquid extraction using organic extractant (TBP). The extraction process is carried out in pulsed columns. After purification with tributyl phosphate (TBP), phosphoric acid is sent for evaporation and additional purification from fluoride through fire evaporation. Part of the acid, contaminated with impurities, is used for fertilizers’ manufacture.

MAP manufacture

Purified phosphoric acid is neutralized with ammonia in pipe reactor. The neutralized solution is evaporated and then fed to a vacuum crystallizer. Suspension from the crystallizer is fed to the centrifuge separator. Wet MAP crystals are sent to a drying drum. Dried MAP is sent for cooling and then packing. The mother liquor from the separator is sent for evaporation and recycling to the neutralization stage (in a mixture with purified phosphoric acid).

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Fig.3. Purified MAP manufacture flowsheet with WPA prepurification

S - settler; PC - pulsed column; FEU - fire evaporation unit; PR - pipe reactor; VEU - vacuum evaporation unit; VC - vacuum crystallizer; Ce - centrifuge; DD - drying drum; С - cooler; V - vessel; P - pump.

Water soluble MCP technology

The main method of obtaining monocalcium phosphate (MCP) is the neutralization of phosphoric acid with potassium carbonate (potash) or potassium hydroxide. Depending on the method of WPA and potash (or potassium hydroxide) impurities separation, there are two options for setting the process of obtaining water soluble MCP, similar to the water soluble MAP technology:

  • with purification from impurities by chemical precipitation without preliminary WPA purification;
  • with preliminary WPA purification by liquid extraction and subsequent purification from impurities by chemical precipitation.

The first option requires fewer capital costs, however, it will most likely be impossible to obtain a food-grade product according to this flowsheet. Process flowsheet for obtaining water soluble MCP with purification from impurities by chemical precipitation without preliminary WPA purification is shown in Fig. 4.

Figure 4. Process flowsheet for obtaining water soluble MCP with purification from impurities by chemical precipitation

The second option allows to obtain water soluble MCP of high quality (including food-grade), since the preliminary WPA purification by liquid extraction allows to effectively remove harmful impurities (heavy metals, arsenic, etc.), but significantly increases the cost of the process. To obtain a water soluble product, additional purification from insoluble impurities may be required after neutralization of the purified WPA. Process flowsheet for obtaining water soluble MCP with preliminary WPA purification by liquid extraction is shown in Fig 5.


Figure 5. Process flowsheet for obtaining water soluble MCP with preliminary WPA purification by liquid extraction