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Phosphoric Acid

NFM Horizontal Belt Vacuum Filter Specializes in Phosphoric Acid Production

Phosphoric acid is produced from phosphatic ores mined in various parts of the world.

Phosphate ores are of two major geological origins:

  • Igneous – found in Kola, South Africa, Brazil
  • Sedimentary – found in Morocco, Algeria, Jordan, U.S.A.

The phosphate minerals in both types of ore are of the apatite group, of which the most commonly encountered variants are:

  • Fluorapatite – Ca10(PO4)6(F,OH)2
  • Francolite – Ca10(PO4)6–x(CO3)x(F,OH)2+x

Fluorapatite predominates in igneous phosphate rocks and francolite predominates in sedimentary phosphate rocks.

Principles of the Process

The basic chemistry of the wet process is simple. The tricalcium phosphate in the phosphate rock is converted by reaction with concentrated sulphuric acid into phosphoric acid and the insoluble salt calcium sulphate.

Ca3(PO4)2 + 3H2SO4 –––––> 2H3PO4 + 3CaSO4

The insoluble calcium sulphate is then separated from the phosphoric acid traditionally with filtration on Horizontal Belt Vacuum Filters.

Calcium sulphate exists in a number of different crystal forms, depending particularly upon the prevailing conditions of temperature, P2O5 concentration, and free sulphate content.  See Figures 1 and 2 below:

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The operating conditions are generally selected so that the calcium sulphate will be precipitated in either the di-hydrate or the hemi-hydrate form.

Typical available P2O5concentrations are:

  • 26-32% P2O5 at 70-80°C for di-hydrate precipitation.
  • 40-52% P2O5 at 90-110°C for hemi-hydrate precipitation.

Phosphoric Acid – Calcium Sulphate (Gypsum)Filtration

Vacuum filtration separates the phosphoric acid from the calcium sulphate (gypsum). Five tonnes of gypsum are generated for every tonne (P2O5) of product acid produced. The filter medium must move in sequence through the various stages for continuous operation. The initial separation must be followed by at least two stages of washing.  Often three stages of washing ensures a satisfactory recovery of soluble P2O5. It is only possible to achieve the desired degree of separation at a reasonable rate if the filtration is either pressure or vacuum assisted. 

Traditionally, continuous vacuum is the preferred driving force. The remaining liquid must be removed from the filter cake as far as possible at the end of the washing sequence. The cake must then be discharged and the cloth must be thoroughly washed in order to remove any residual particles which might otherwise build up and impede subsequent filtration. The mother liquor filtrate and cake wash filtrate are kept separate from one another.  They are separated from air under vacuum conditions and then delivered under atmospheric pressure as product unless returned to the process.

Typical Vacuum Filtration Throughput Rates on NFM Horizontal Belt Vacuum Filter

Gypsum cake discharge moisture: 13 – 30 % w/w

Soluble P2O5 in cake at filter discharge: 0,2 – 0,5 %

Up to 11 MTPDP2O5/ m2 (boost the productivity)

NFM Horizontal Belt Vacuum Filter Design for Phospho-Gypsum Dihydrate and Hemihydrate

  • Double Discharge Feed Box – for co- and counter-current optimal slurry distribution
  • Cloudy Filtrate Port Isolation in Feed Zone
  • Three (3) Counter-current Cake Wash Zones
  • Customized Vacuum Box Design -- diamond shape with steam jacket to reduce post crystallization
  • Customized Vacuum Box Lowering Assembly -- ergonomically safe to ease maintenance
  • Customized Drainage Belt Design -- SBR-HT or EPDM with deep grooves for high hydraulic load
  • Automated Self-cleaning Cloth Wash Assembly – no nozzle clogging, extended cloth life
  • Fully Automated  Cloth Tensioning and Tracking Assemblies
  • Custom Designed Filter Cloth Construction – at competitive prices

Additional NFM Services to Phosphoric Acid Producers

  • Process Consultancy
  • Process Optimization
  • Laboratory and Pilot Scale Test Capabilities – for sizing and separation optimization
  • Spare Parts for any brand of existing Horizontal Belt Vacuum Filter
  • Modernization of any brand of existing Horizontal Belt Vacuum Filter
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