Reduction of fluoride and sulfate ions in Syrian phosphoric acid by extraction with tertiary amins

General Fertilizer Company GFC Homs produces a lot of wet phosphoric acid, but it is unsuitable for food or pharmaceutical uses. Reduction of fluoride and sulfate ions contents in wet phosphoric acid by liquid – liquid extraction with tertiary aliphatic amins was studied. The effect of solvent concentration and temperature on the extraction was studied. The effect of diluent on the extraction yield and raffinate acid specifications was also studied.


Introduction
There has been a wide interest over the last few years to develop a process to purify commercial wet phosphoric acid to obtain a pure acid.Most of the works are still in the developmental stage.Wet phosphoric acid contains 1 to 2%wt fluoride and about 2% sulfate that makes this acid unsuitable for food or pharmaceutical uses.R. Baidoun et al [1] stated that fluoride reduction can be achieved to be about 97% using silica-gel under different conditions of pressure and temperature.In other works, processes for defluorination and purification of wet phosphoric acid were described.Partially immiscible organic solvents such as ethers, alcohols and amins were used [2][3][4].Other investigators [5] reported the conditions for the fluoride removal from wet phosphoric acid by precipitation with sodium carbonate, sodium sulfate, potassium carbonate and potassium sulfate.MEAB report [6] describes a process to precipitate sulfate in wet phosphoric acid by adding barium carbonate.The efficiency of precipitation increases by increasing barium carbonate concentration and temperature.Canadian patent [7] claims the method of extracting fluoride and sulphate from an aqueous solution containing the fluoride.Ahron Eyal et al [8] stated the Defluorination of WPA to feed grade level can be achieved by extraction by amine with acids of low Al content.In earlier papers [9,10].We have studied the precipitation the sulfate ions from wet phosphoric acid using barium carbonate and we have studied the investigation of the synergistic effect of some alcohols on the extraction of phosphoric acid by TBP.The extraction of fluoride, sulfate, by these reagents is relatively small.Khorfan [10] found the extraction of H 3 PO 4 by alcohols is selective and the extraction of heavy metals is negligible while that for iron and fluoride is small.Mousa [11] investigated the extraction yield of phosphoric acid after the addition of an optimized amount of some mineral acids.

Materials
Tributylamin 99%, trioctylamin (DEE) 93 % and tridodecylamin 95 % from Merck were used .A commercial wet phos-phoric acid 29% wt.P 2 O 5 and a density of 1.28 g/cm 3 and containing 65 ppm uranium as supplied from the General Fertilizer Company GFC Homs/Syria were used after being treated from solids and organic.Kerosene (flash point 76 0 C -< 5% aromatics -3 mg/kg sulfur) as supplied from Pemco Chemical Co. was used as inert diluent.

Apparatus and procedures
Extraction was carried out in a beaker with a magnetic stirrer placed in a thermostat to control the temperature.The aqueous and organic phases were stirred for 5 minutes and were allowed 20 minutes to separate in a funnel.The concentrations of sulfate and chloride in the aqueous phase were determined by ion chromatography model Metrohm 792 basic IC.The concentration fluoride was determined by ion selective electrode model Metrohm 692 pH/Ion Meter.The concentration of the acid was measured by a potentiometric titration with 0.1 NaOH.The potentiometer type of E536 from Metrohm Co. was used.The concentration of sulfate and fluoride in the solvent was calculated from material balance.The yield was calculated from the equation: where [X]= sulfate or fluoride.
The uranium concentration was determined by complexing it with ammonium thiocyanate and measuring the absorption at 365 nm by a UV spectrophotometer type of the Spectronic 601 from Bausch&Lomb Co.

Results & discussions 3.1 Effect of the amine concentration on fluoride and sulfate extraction
The effect of the amine concentration was investigated by extraction fluoride and sulfate from wet phosphoric acid with different (tributylamin, trioctylamin, tridodecylamin) under the same experimental conditions (T=25 0 C, O/A=1, P 2 O 5 =29% wt).The concentration of the amine was varied from 0.1 to 0.5 mol/l in kerosene.The results are plotted in Fig. 1 in the form of the sulfate extraction yield versus the concentration of the amine and in Fig. 2. in the form of the fluoride extraction yield versus the concentration of the amine.The results show that the sulfate and fluoride extraction yields increase rapidly by an increase in the amine concentration for the three above solvents according to the following: tributylamin< trioctylamin < tridodecylamin.Ease of separation of the two phases decreases significantly with an increase in the amine concentration.A compromise should be made between ease of separation and extraction efficiency.So, experiments were further carried out on 0.3 mol/l amine in kerosene.

Effect of the extraction by tridodecylamin on the acid specification
The effect of the fluoride and sulfate extraction from wet phosphoric acid by 0.25 mol/l tridodecylamin in kerosene on the acid specification was studied using phase ratio O/A=1 at 25 0 C. The results are represented by Table 1.The results show that the fluoride and sulfate extraction from wet phosphoric acid has no effect on the density and concentration of the acid and a slight effect on the vanadium and uranium concentration in the acid.

Conclusions
The previous results show that : 1 Sulfate and fluoride extraction yields increase rapidly by an increase in the amine concentration and the tridodecylamin in kerosene has the highest extraction yield.
2 The extraction yield increases by an increase in the amine concentration and the ease of phase separation decreases by an increase in the amine concentration.So, the concentration of 0.25 mol/l tridodecylamin in kerosene was found to be the best as a result of the trade-off between the extraction yield and the ease of phase separation.4 The extraction has no effect on the phosphoric acid specification and a slight effect on the vanadium and uranium concentration in the acid

3
The temperature has a positive effect on fluoride extraction for trioctylamin and tridodecylamin and a slight effect on the sulfate extraction for the three tertiary amines and on the fluoride extraction for tributylamin.
Tab. 1. Shows the effect of extraction on the acid specification