Aluminium Fluoride from fluorosilicic acid / (AlF3 high density)

 

Aluminium fluoride is manufactured from anhydrous hydrofluoric acid generated either from fluorspar or fluosilicic acid.

The process for the manufacture of Aluminium Trifluoride (ATF) from fluorspar is based on following chemical reactions:


   CaF2 + H2SO4 CaSO4 + 2 HF
   Al2O3.3H2O Al2O3 + 3 H2O
   Al2O3 + 6 HF 2 AlF3 + 3 H2O
 

The process involves two main stages: manufacture of Hydrofluoric Acid from Sulphuric Acid and Fluorspar and then reaction of HF gas (or HF liquefied and evaporated) with dried Alumina Hydrate in a fluidized bed reactor

 

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 Process simulation of aluminium fluoride reactor system (material and heat balance)

 

alfreakt 

 

1.    From fluorspar, dry process

 See also HF from fluorspar

 

2.    From fluosilicic acid dry and wet process

 

2.1  Wet FSA process, first generation: production of aluminium fluoride by crystallization (Chemie-Linz Process)

FSA1G LBD-Aluminium fluoride from FSA

FSA1G Technologies based on FSA of the first generation (1G) known also as wet FSA process, wet AlF3 process from FSA or HF/fluorspar wet process and cryolite process also.

See also Aluminium fluoride from FSA - Wet process

 

2.2  Dry FSA process, second generation: production of HF, aluminium fluoride by sulphuric acid decomposition (Tennessee Process)

FSA2G
HBD-Aluminium Fluoride
from FSA and AHF from FSA

 

FSA2G Technologies based on FSA of the second generation (2G) known also as HF from FSA or dry process for AlF3 from FSA or fumed silica process by the fluorine route

 

A process for manufacturing AHF / HF from fluosilicic acid was disclosed first by Tennessee Corp., USA maybe 50 years ago and further disclosed by Wellmann-Lord, etc and more lately by Flemmert (Nynaes Petroleum, Sweden), and Lubon Works, Poland, the latter operated a small pilot plant for diluted HF. In 2008 Wengfu, China commissioned a first commercial plant for AHF with the technology of Buss Chemtech AG, Switzerland based on know-how from Lubon Works.

Chemistry:

H2SiF6.SiF4(aq) + H2SO4 → 2 SiF4 + 2 HF(aq) + H2SO4

5 SiF4 + 2 H2O → 2 H2SiF6 .SiF4 (aq) + SiO2 (s)
Al2O3.3H2O   → Al2O3 + 3 H2O
Al2O3 + 6 HF → 2 AlF3 + 3 H2O

The process is based on the mixing of strong fluosilicic acid with strong sulphuric in a stirred reactor and separating silicon tetrafluoride gas and extracting the anhydrous hydrofluoric acid using sulphuric acid as dehydrating agent into separation columns as per the principle shown on the flowsheet below. The evaporation can be with one single stage or two stages. Presently AD Process Strategies Sarl proposes an improved process of this technology to suit the water balance of the phosphoric acid plant, DH PA Process and especially HH process not suitable to receive large amount of water. Sulphuric acid containing water that is generated from this HF plant has to be re-circulated to the phosphoric acid plant. The sulphuric acid recirculation normally 30 T/T AHF as 100% H2SO4 can be reduced to 15 T/T. Water is reduced from more than 10 T/T AHF down to 5 T/T.

The FSA2G technology is proven, resolves environmental issues, is very profitable as raw material costs are low (low cost of fluorine, no cost for sulphur or sulphuric acid), capital cost is reasonable and it offers access to promising markets like HBD aluminium fluoride and anhydrous hydrofluoric acid, both being large volume chemicals.

This process is less sensitive to impurities contained in the FSA as an AHF purification stage is provided in the process.

 

Aluminium_fluoride_and_AHF_from_FSA_FSA2G

 An optional process is a process with hydrolysis of STF in the gas phase under high temperature to produce silica, Fumed silica as per the Nynaes process or silica as per the Reed process. STF silicon tetrafluoride can be produced from this process as well.

 Plants built with this technology: Lubon, Grace, Wengfu for AHF (usual name)

  

2.3  Dry SSF process: Hydrofluoric acid, aluminium fluoride, silicon tetrafluoride from fluosilicic acid (3rd generation) by the SSF Process.
       SSF Process is an exclusivity from AD Process Strategies Sarl.

 

FSA3G
HBD-Aluminium Fluoride from fluosilicates (Sodium Silicofluoride) (from SSF / FSA) and AHF

 

FSA3G (FSA-based technology of the third generation). This is the advanced process for stand-alone plants providing many alternatives for having not to recycle the water and sulphuric acid to the phosphoric acid plant or optional re-use of the materials available in streams in the process cycle.

When diluted sulphuric acid stream is not returned to the phosphoric acid plant or can not be re-circulated due to technical reasons or not and in particularly in the above FSA2G process, AD Process Strategies Sarl proposes a new technology for stand-alone HF plants. The proposed process uses fluosilicate as an intermediate (solid) raw material, which is transportable not like FSA and reaction of this fluosilicate with strong sulphuric acid. The silicon tetrafluoride and hydrofluoric acid obtained are treated as per the state of art in a similar manner as mentioned above using absorption and desorption of HF in sulphuric acid. The diluted sulphuric acid stream resulting from this process can be pre-concentrated and recycled to the phosphoric acid plant or used in production of fertilizers like single superphosphate SSP, dicalcium phosphate DCP, ammonium sulphate AS, etc or concentrated and recycled to the HF reaction or purified for sale.

AHF_and_AlF3_from_FSASSFFSA3G

 

Chemistry: The process does use the same reaction of sulphuric acid / sodium fluosilicate in opposite direction under different conditions, aqueous, low temperature and anhydrous, high temperature:

Step-1 Production of sodium fluosilicate (SFS) or silicofluoride (SSF) by one of these process route: (reaction 2 is the usual process to manufacture SSF)

H2SiF6(aq) + Na2SO4(s) Na2SiF6(s) + H2SO4(aq)
H2SiF6(aq) + 2 NaCl(s) → Na2SiF6(s) + 2 HCl(aq)

 

Step-2 Decomposition of SSF by Sulphuric Acid and recycle of SiF4 to generate additional FSA and SSF. (Reaction was tested and is working, incompleteness is not so important as the salt is recycled or can be purified as required)

 

Na2SiF6(s) + H2SO4 2 HF(g) + SiF4(g) + Na2SO4(s)
3 SiF4 + 2 H2O → 2 H2SiF6(aq.) + SiO2(s)

 

Step-3 Reaction of HF with alumina trihydrate in a fluidized bed reactor (same process as fluorspar process or FSA2G)

 

Al2O3.3H2O → Al2O3 + 3 H2O
Al2O3 + 6 HF → 2 AlF3 + 3 H2O

 

Step-4 Pre-concentration of sulphuric acid (pilot unit operating since almost 10 years on concentration of sulphuric acid up to 85% in presence of fluorine) and recycle to process or reused by other consumers.

  

Plants built with this technology: First Prefeasibility Study underway. Acceptance for FSA3G is higher than for FSA2G

Using the PSF Potassium Fluosilicate process instead of the SSF still needs to be investigated.

 

Related productions and chemistries

Many possibilities are available with the aim of reducing recycling and improving benefits and economics of projects.

 

Production of Dicalcium Phosphate (DCP)

 

Production of DCP from HCL (and optionally from sulphuric acid and sodium chloride or sea water) for production of DCP (Feed grade) or as neutralization unit with separation of solids and recycling.

Dicalcium Phosphate dihydrate CaHPO4.2H2O, is produced by a wet process that comprises the following phases:

 

Ca3(PO4)2 + 4 HCl → Ca(H2PO4)2 + 2 CaCl2

Ca(H2PO4)2 + Ca(OH)2 → 2 CaHPO4.2 H2O + 2 H2O

Reaction of phosphoric rock with hydrochloric acid, main raw materials, a process from which a monocalcium phosphate liquor is obtained.

 Purification of the remaining solution by means of the removal of the inert matter and undesirable compounds.

 Production of dicalcium phosphate by means of calcium salts precipitation and product filtration.

 Drying of dicalcium phosphate at moderate temperature to keep its two water molecules

  

Production of Sulphate of Potassium (SOP)

 

 Production of SOP by the double salt decomposition, Glaserite process. In this process sodium sulphate is reacted with potassium chloride to yield potassium sulphate.

 This reaction occurs in two steps as follows:


4 Na2SO4 + 6 KCl = Na2SO4. 3 K2SO4 + 6 NaCl

2 KCl + Na2SO4.3K2SO4 = 2 NaCl + 4 K2SO4

This process route is cheaper than the Mannheim process and it makes sense if the sulphate is available and cheap.

  

Production of Silicon Tetrafluoride (STF)

 

 STF is dried, purified and compressed into pressure cylinder for transportation to the polysilicon plant.

  

Production of Sodium Sulphate (SSF) and Purification

 

 Sodium sulphate can be purified according to the crystallization process as required for the final application of SSA detergents, paper, glass, etc

  

Production of Silica

 

 Byproduct silica can be used in the defluorination process of phosphoric acid or sulphuric acid, production of water glass, of precipitated silica, zeolite, of cements, as soils conditioners.

 Fluorine chemistry is basically a good solution for higher added-value to the downstream products:

 SSF FSA AlF3 AHF STF HFC PTFE NF3 LiPF6

 Continuation of this paper will deal with the introduction of economics of all these technologies. No doubt, economics of all these technologies are good. It is only a matter to discuss how good they are? What are the options to be selected?

 Presently as costs of raw materials are escalating, expertise is available, technologies are affordable and of course through us, the barriers between various industries are not anymore a brake between fluorine and other industrial sectors, like fertilizers, aluminium, cement, solar and silicon , etc, great opportunities for cooperation are in front of us. We hope that all of you will cooperate with us regarding technology and other business matters.

 All these technologies are offering high flexibility and can be implemented with confidence in a cost effective manner. They are real innovations for the fertilizer industry and surely they will be implemented progressively. More and more projects are under study and hopefully some projects may be executed in the near future….

 

The ammonia route to HF / AlF3 is not discussed in this paper as it is not significant yet.

 

This article was first presented at Symphos, First symposium on Innovation and Technology in the Phosphate Industry, held from May 9th to 13th, 2011 in Marrakech and sponsored by OCP.

 

Aluminium fluoride (LBD & HBD: Low & High Bulk Density) from FSA

 

Fuosilicic acid finds its main application in the manufacture of aluminium fluoride being a large volume chemical mostly produced from fluorspar as high bulk density (HBD) aluminium fluoride and as well from fluosilicic acid as low bulk density (LBD) aluminium fluoride. Hereby is disclosed new processes for manufacturing anhydrous hydrofluoric acid (AHF) from fluosilicic acid (FSA) from which (HBD) aluminium fluoride can be produced. Aluminium fluoride is essentially used as a flux for smelting aluminium by adding it volumetrically to the cells of aluminium smelters in order to regenerate the cryolite bath. HBD aluminium fluoride is the preferred material and it is fully produced from fluorspar; none of this material being currently produced from FSA through AHF which process technology often referred to as the Dry/FSA Process is available, feasible and proven at this time. An opportunity to invest in profitable projects does really exist. Additionally the manufacture of other downstream products of HF: refrigerants, fluoropolymers, etc and downstream products of FSA: silicon metal, silicon tetrafluoride, silicas may offer extra opportunities.

           The first process known for manufacturing (LBD) aluminium fluoride from fluosilicic acid was patented by Chemie-Linz, Austria and many plants were built based on this technology or similar technologies. This process uses the direct neutralization of the fluosilicic acid with alumina hydroxide in a stirred reactor. It is often referred to as the Wet/FSA Process . Although this technology tends to be abandoned due to the low density and low fluidity (flowability) of the product, the high capital cost of the plant and environmental issues resolved partially only as mother liquors may have to be neutralized, further developments of this process would refresh this technology being still accepted by very few players only. Improvements of crystallization, water neutralization, equipment design and costs,.. are under progress... 

            A process to produce AHF / HF from fluosilicic acid was disclosed first by Tennessee Corp., USA and further by Wellmann-Lord, etc and more lately by Flemmert (Sweden), and Lubon Works, Poland who operated a small pilot plant. More recently Wengfu, China built a first commercial plant with the technology of Buss Chemtech AG, Switzerland based on know-how from Lubon Works. The process is based on the mixing of strong fluosilicic acid with strong sulphuric in a stirred reactor and separating silicon tetrafluoride and extracting the anhydrous hydrofluoric acid using separation columns. Presently AD Process Strategies Sarl proposes a similar type of process with major improvements completed to suit the water balance of the phosphoric acid plant (PAP); sulphuric acid containing water that is generated from this HF plant has to be recirculated to the phosphoric acid plant without affecting the performances of the PAP. The technology offered by AD Process Strategies Sarl uses a minimum of sulphuric acid to perform the reaction of decomposition as water fed to the process maybe variable and utimately nil. The technology is proven, resolves environmental issues, is very profitable as raw material costs are low, capital cost is reasonable and it offers access to promising markets for HBD aluminium fluoride and anhydrous hydrofluoric acid, both being large volume chemicals. 

            For situations described above when the diluted sulphuric acid stream is not returned to the phosphoric acid plant or can not be recirculated due to technically reasons or not, AD Process Strategies Sarl proposes a new technology for stand alone HF plants. The proposed process uses fluosilicate as intermediate raw material, which is transportable not like FSA and reaction of fluosilicate with sulphuric acid. The silicon tetrafluoride and hydrofluoric acid obtained is treated as per the state of art mentioned above. The diluted sulphuric acid stream from this process can be preconcentrated and recycled to the phosphoric acid plant or used in production of fertilizers, concentrated and recycled to the HF reaction or purified and sold. 

These technologies are offering high flexibility and can be implemented with confidence in a cost effective manner. They are real innovations for the fertilizer industry and surely they will be implemented progressively. More and more projects are under study and hopefully some projects may be executed in the near future….