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Hemihydrate

Processes with an initial hemihydrate reaction

When energy costs increase significantly, if the water balance allows, it is sometimes worth producing a more concentrated acid directly from the filter.

In order to meet these requirements, and to produce a self-drying hemihydrate, we have developed the PH3 process. This is a three-crystal process which produces phosphoric acid with a high P2O5 content and phosphogypsum that can be used as a base for manufacturing building materials. Having developed this process, we modified it by cutting out stages to meet certain customer requirements. This led us to develop two other hemihydrate processes: the PH1 (**) and PH2 processes (***).

Prayon processes stand out from others in that they include a reaction section that is split into two separate zones. In the first zone, the high temperature and a low sulfate provide good conditions for dissolving. On the one hand, monocalcium (MCP)/dicalcium (DCP) phosphate displays maximum solubility in solution due to the temperature, and on the other, at such a temperature, the low sulfate prevents the formation of anhydrite.

In the second zone, the temperature is reduced by using a low-level flash cooler (or a air cooling system for smaller plants) and the sulfur content is increased by adding sulfuric acid. This creates conditions under which MCP/DCP precipitate in solution in the form of alpha hemihydrate that can be filtered easily. This also means that the filter can be fed by a solution with a substantially lower dissolved fluosilicate content on account of the low temperature. Units can therefore work faster than those using other non-Prayon processes.

The PH1 process

Processes with an initial hemihydrate reaction

LThe reaction takes place in two stages. The first stage takes place with a low — albeit positive — sulfate content, while the second stage uses a higher level of sulfate. By accurately controlling both the sulfate and the temperature, it is possible to filter the pulp which contains a highly stable hemihydrate and which has few scaling properties.


The main characteristics of this process are :
  • simple, low-cost process;
  • high acid strength (39-45%)

Recommended for sites with :
  • low phosphate rock costs;
  • low sulfuric acid costs;
  • high energy costs;
  • a gypsum discharge facility.

The PH2 process

Hemihydrate mode

After the first reaction in hemihydrate mode, the resulting acid is separated. It has a low sulfate content and contains 46% P2O5. Once it has been washed, the remaining alpha hemihydrate is mixed with the sulfuric acid, under conditions in which the alpha hemihydrate is unstable and recrystallized like gypsum, producing a co-crystallized and unattacked gypsum with a low P2O5 content.


The main characteristics of the process are :
  • high phosphoric acid content;
  • high P2O5 yield (98%+).

Recommended for sites with :
  • medium/high phosphate rock costs;
  • high energy costs;
  • a gypsum discharge facility.

The PH3 process

Alpha-hemihydrate attack filtration This process consists of 3 stages :

Stage 1 - Alpha-hemihydrate attack filtration :
The phosphate reacts under conditions that guarantee acid production with a 40-46% P2O5 strength and a low sulfate content. By controlling the operating conditions, it is possible to produce alpha hemihydrate which can filter rapidly and which rehydrates during the second stage.
Stage 2 - Conversion of the alpha hemihydrate into a dihydrate :
y modifying the operating conditions, the alpha hemihydrate is converted into dihydrate, thereby providing initial purification of the substance by reducing the content of unattacked and co-crystallized P2O5 acid in the gypsum.
Stage 3 - Conversion of the dihydrate into alpha hemihydrate and final filtration :
The operating conditions are modified to transform the dihydrate into alpha hemihydrate. The resulting pulp is then filtered. This stage is similar to the second stage of the Central Prayon Process (CPP) and is designed to produce a very pure calcium sulfate that is separated by filtration and washed with water. If requested, the hemihydrate may then be beneficiated (either by neutralization, or by rehydration for producing solid form) to produce a substitute for natural gypsum.

The main characteristics of this process are :
  • high P2O5 yield (98%+);
  • high phosphoric acid content;
  • self-drying gypsum.

The last two properties reduce energy consumption costs.
Recommended for sites with :
  • medium/high phosphate rock costs;
  • high energy costs;
  • no gypsum discharge facility.


Back to Phosphoric acid production


Sales contact

Prayon Technologies (PRT)
144, rue J. Wauters
4480 Engis
Belgium
Tel: +32 4 273 93 41
Fax: + 32 4 275 09 09
E-mail : prt@prayon.com

 

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