RU2230050C1 - Method for preparing chlorine-free npk-fertilizer - Google Patents

Abstract

FIELD: chemical technology, fertilizers. SUBSTANCE: invention relates to technology for preparing complex mineral fertilizers, in particular, chlorine-free fertilizers by method of nitric acid decomposition of natural phosphates. Method involves the following stages. Ammonization of nitrate-phosphate solution is carried out firstly to pH 2.5-3.0 and then to pH 5.8-6.0. Prepared solutions are evaporated to the residual moisture value 0.4÷1.0% to obtain homogenous melts with a low viscosity index. NPK-fertilizer is granulated by prilling method using potassium sulfate as a potassium component that is added before prilling stage immediately. Method provides preparing NPK-fertilizer containing water-soluble sulfur and displaying the high agrochemical effectiveness. EFFECT: improved preparing method. 3 tbl, 1 ex

Description

The invention relates to a technology for producing mineral fertilizers and may find application in the production of prilled complex fertilizers.

A known method for producing complex NPK fertilizers by decomposing natural phosphates with nitric acid, separating insoluble impurities by sedimentation, freezing and separating calcium nitrate from an NP solution, feeding with phosphoric acid or ammonia, mixing the solution with potassium sulfate, removing calcium sulfate, evaporating the solution, followed by granulation or prilling [Patent, India, No. 168731, IPC C 05 D 11/00, 12.16.1986].

The disadvantages of the method:

- the bulkiness of the technological scheme;

- production waste (precipitate of insoluble impurities, calcium sulfate);

- two-stage removal of calcium from the decomposition solution (by freezing and precipitation with potassium sulfate, which contributes to the complexity and cost of the scheme);

- removal of calcium in the form of calcium sulfate or gypsum, which is not widely used;

- the use of an additional reagent - phosphoric acid.

The closest to the proposed method according to the technical essence and the achieved result is a method for producing chlorine-free NPK fertilizers, in which the phosphate feedstock is decomposed with nitric acid, a part of calcium in the form of calcium nitrate is isolated from the decomposition solution by crystallization and filtration. The filtrate thus obtained — nitrate-phosphate solution — is ammoniated in two stages. In the first stage, gaseous ammonia to a pH of 1.5; on the second, with ammonia water and / or gaseous ammonia to pH 5.0-5.5, the neutralized solution is evaporated to a residual pulp moisture of 10-20%, potassium sulfate is added and granulated while drying the finished product [RF Patent No. 2141462, IPC C 05 V 11/06, C 05 V 7/00]. According to the prototype, the resulting product is granulated in drums, the essence of this method is to spray the pulp onto a finely ground, substandard product (retur) in a rotating drum with the formation of granules of irregular shape.

The disadvantages of the method:

- inefficient ammonization of nitrate-phosphate solution, namely: ammonization in the first stage to a pH of 1.5, leads to intensive ammonization in the second stage, which contributes to the occurrence of local supersaturations and increase the viscosity of the solution. This is due to the deposition of impurities (compounds of calcium, aluminum, iron, etc.) under adverse conditions, which leads to the formation of finely dispersed crystals with a distorted (semi-amorphous) crystalline structure. Calcium under conditions of supersaturation, as a rule, precipitates in the form of crystalline tricalcium phosphate;

- at the second stage, it is impractical to use ammonia water, since an additional amount of water is introduced with it, which then needs to be evaporated;

- high moisture content in the pulp after mixing with potassium sulfate before granulation. This contributes to ion exchange processes between potassium sulfate and calcium compounds with the formation of calcium sulfate. These interactions lead to the transition of the water-soluble form of sulfur (potassium sulfate) to the general form (calcium sulfate), which reduces the agrochemical effectiveness of the fertilizer;

- product drying is required;

- granules of irregular shape are formed.

The present invention is to develop a method for producing chlorine-free NPK fertilizer containing sulfur in a water-soluble form with high agrochemical efficiency according to a simplified and rational technological scheme, followed by granulation by prilling.

To solve this problem, a method is proposed that differs in a more rational distribution of a neutralizing reagent, namely gaseous ammonia, over the stages of ammonization, evaporation of the resulting ammonized solution to obtain a low-viscosity homogeneous melt, mixing it with potassium sulfate and granulating NPK fertilizer by prilling. An essential role in the prilling granulation method is played by the physicochemical properties of the melt. The pulp obtained by the method specified in the prototype cannot be used for granulation by prilling in towers due to its high moisture content and high viscosity. In the proposed method, the obtained low-viscosity homogeneous melt is sprayed into the barrel tower with cooling of the droplets by the oncoming air flow with the formation of spherical granules.

According to the claimed method, chlorine-free NPK fertilizer containing sulfur in a water-soluble form is obtained by nitric acid decomposition of apatite concentrate, with freezing of calcium nitrate and its further processing into production chalk, neutralization of nitrate-phosphate solution in two stages with gaseous ammonia: in the first, to pH 2.5 -3.0, in the second - up to 5.8-6.0, by evaporation to a residual moisture content of 0.4-1.0 wt.%, Mixing with potassium sulfate and granulation by prilling.

Technical result achieved:

- a more rational distribution of the neutralizing reagent, namely gaseous ammonia, over the stages of ammonization, which avoids local over-ammonization and increase the viscosity of the solution;

- the use of gaseous ammonia in the second stage of neutralization instead of ammonia water allows you to reduce the load on the evaporation system or to use additional water at other stages of the process, for example, for washing calcium nitrate or chalk;

- nitrate-phosphate solution is evaporated to a residual moisture content of 0.4-1.0 wt.%, which allows to obtain a uniform salt melt;

- good rheological properties (viscosity, uniformity) of the resulting melt allow to obtain fertilizer by prilling;

- the resulting product does not require drying;

- receiving the product in the form of spherical-shaped granules allows to improve its presentation;

- the resulting product contains sulfur in a water-soluble form, which is an additional nutrient and increases the agrochemical value of the fertilizer;

- obtaining chlorine-free NPK fertilizer with high agrochemical efficiency.

Example 1. Industrial testing of the proposed method was carried out at the current nitroammophos production according to the following scheme.

Nitric acid decomposition of apatite concentrate was carried out with 58% nitric acid at a temperature of 55-60 ° C with a flow rate of 1.3 tons of 100% acid per 1 ton of apatite concentrate. Next, the decomposition solution was cooled to (-7) ÷ (-10) ° С, the crystals of calcium nitrate formed were separated and processed into chalk. The nitrate-phosphate solution was neutralized with gaseous ammonia in two stages: in the first, to a pH of 2.5-3.0, in the second, to a pH of 5.8-6.0 with simultaneous dilution with an amcelpeter solution. The ammoniated solution was evaporated to a residual moisture of 0.5-0.7 wt.%, Mixed with potassium sulfate in the ratio NP: K ₂ SO ₄ = 2.5: 1 and granulated by prilling in a tower. In this case, a product was obtained whose chemical composition is presented in table. 1.

In the field, the effectiveness of NPK with potassium sulfate in sod-podzolic soils was tested. The use of this fertilizer increases the yield and quality of various crops compared with the control by 15-35%, compared with the use of NPK with potassium chloride by 4-6% (Table 2). During agrochemical tests, a high payback of fertilizers by crop for a number of crops was noted (Table 3).

Claims (1)

A method of producing a chlorine-free NPK fertilizer, including nitric acid decomposition of phosphate feedstock, freezing and separating calcium nitrate, ammonium nitrate phosphate solution, evaporation, mixing with potassium sulfate, granulation, characterized in that ammonia nitrate phosphate solution is carried out in gaseous ammonia in two stages at the first stage to pH 2.5 ÷ 3.0, at the second to pH 5.8 ÷ 6.0, evaporation is carried out to a residual moisture content of 0.4 ÷ 1.0 wt.% to obtain low-viscosity homogeneous melts, NPK is granulated fertilizer containing sulfur in a water-soluble form, prilling method.

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