Removal of Uranium from Crude Phosphoric Acid by Precipitation Technique

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1 Removal of Uranium from Crude Phosphoric Acid by Precipitation Technique M. A. Mousa, 1 H. S. Gado, 2 M. M.G. Abdelfattah, 2 A. E. Madi, 2 M.H. Taha, 2 and O. E. Roshdy 2 1 Chemistry Department, Faculty of Science, Banha University, 2 Nuclear Materials Authority. P.O. Box 530, El Maddi, Cairo, Egypt Received: 4/6/2013 Accepted: 27/6/2013 ABSTRACT Investigations on the removal of uranium from crude phosphoric acid by precipitation method were carried out using iron powder and ammonium fluoride. The influence of various factors affecting the precipitation process as precipitation time, ammonium fluoride/ phosphoric acid mass ratio, acetone/ phosphoric acid mass ratio, precipitation temperature, free sulfate concentration and phosphoric acid concentration, has been investigated. Citric acid was used successfully for the uranium dissolution from the produced ammonium iron phosphate hydrate. Key Words: Uranium, Crude phosphoric acid, Removal, Precipitation technique INTRODUCTION Phosphate rock is the major source of phosphorus in nature. It exists mainly in the form of hydroxy- and fluoroapatite, Ca 10 (PO 4) 6(OH) 2 and Ca 10 (PO 4) 6F 2, respectively or a mixture of both and is used mainly for the production of fertilizers and elemental phosphorus (1, 2). The most commonly used process for the production of phosphoric acid is: thermal and wetprocess. The thermal process produces a pure acid with huge energy consumption. The wet-process involves reaction of phosphate rock with an acid (mainly sulfuric acid). This process is economic and practiced everywhere in the world (3, 4). Phosphoric acid produced by Wet-Process contains small amounts of uranium together with many kinds of impurities, such as calcium sulfate disposal, fluorine, cadmium and uranium as well as organic impurities (5). These impurities vary according to the origin of the minerals. The presence of such impurities in phosphoric acid adversely affects the process performance as well as the quality of produced acid (6). The most important environmental concern is related to specific elements present in the composition of the phosphate rock such as uranium and lead. These elements are found in small amounts in phosphate fertilizers. In the phosphoric acid wet process based on sulfuric acid dissolution, about % of uranium passes into solution and the rest precipitates in the phosphogypsum (2). Uranium was removed from phosphoric acid by several techniques such as precipitation, liquid membranes, solvent extraction and ion-exchange (7-14). Precipitation technique has many advantages than other techniques where it can apply for high phosphoric acid concentration (52 % ) as well as low phosphoric acid concentration (30 % ) and it do not require any pre-treatment for the phosphoric acid before application. In addition, precipitation technique is preferred when concentration of uranium in phosphoric acid is small where the uranium production from phosphoric acid is not economic (15). The present work aims to study the different factors as precipitation time, precipitation temperature, phosphoric acid concentration, ammonium fluoride/ phosphoric acid mass ratio, acetone/ phosphoric acid mass ratio and free sulfate concentration that affecting the precipitation of uranium 38

2 from crude phosphoric acid by iron powder and ammonium fluoride in order to suggest the maximum uranium precipitation efficiency in relation to the lowest precipitation efficiency. EXPERIMENTAL PROCEDURE Raw Materials: Ammonium fluoride 99.9 % NH 4F (MERCK, Germany), acetone (ADWIC, Egypt), barium chloride BaCl 2 (ADWIC, Egypt) and citric acid C 6H 8O 7 (MERCK, Germany) were chemical reagent grade. Iron powder was obtained from El Goumhoria Trade Pharmaceuticals & Chemicals, Egypt. Crude phosphoric acid under study was produced from Abu Zaabal Company for Fertilizer and Chemical Materials, its chemical composition is given in Table (1). Table (1): Chemical analysis of crude phosphoric acid produced from Abu Zaabal for Fertilizer and Chemical Materials Company Procedure: Component Concentration 45.0 % Ca 0.44 % -- SO % SiO % F 1.20 % Fe 2.40 % Cd 6 ppm Co Pb U 4 ppm 50 ppm 60 ppm The reaction was carried out in cylindrical beaker of 1 L volume and 10 cm diameter. It was fitted with stirrer and placed in thermostatically controlled water bath. The impeller tip speed was adjusted at 300 rpm. Filtration was performed using buchner type filter of 4.6 in. diameter. Polypropylene filter cloth of 80 mesh aperture size was used. A vacuum pump was used for filtration. Experimental procedure: Unless otherwise stated, the phosphoric acid sample (100 g of 45 % ) was added into the beaker with 0.15 g of iron powder for 30 min (16). after reduction process finishes a proper amount of acetone was added then immediately another proper amount of ammonium fluoride added into the reactor. After the desired reaction time, the leach slurry was immediately separated by filtration. The remaining solids were dried and weighed. In the first filtrate evaporation process was done to remove the remaining of acetone in solution. The content and uranium content were determined by a colorimetric method (spectrophotometer type Shimadzu UV Ammonium molybdate and ammonium metavanadate were used for analysis and the procedure reported by Gorecka (17) was used for uranium analysis. RESULTS AND DISCUSSION The following is a discussion for the results obtained from the uranium removal from crude phosphoric acid by precipitation technique using iron powder and ammonium fluoride. Here, the effects of precipitation time, ammonium fluoride / phosphoric acid, g/ g, mass ratio, acetone / 39

3 phosphoric acid, g/ g, mass ratio, precipitation temperature, effect of free sulfate and phosphoric acid concentration on the uranium precipitation process have been investigated. Effect of Precipitation Time: To study the effect of reaction time for the uranium and precipitation from 45 % crude phosphoric acid several experiments were carried at different times ranges from 1.0 to 30.0 min. at a reaction temperature of 25 C, ammonium fluoride/ phosphoric acid mass ratio; 3 g/ 100 g, acetone/ phosphoric acid mass ratio; 50g/100g. The experimental results are given in Figure (1) as a relation between uranium and precipitation efficiency % and Time. From the figure it is clear that, as the time increases from 1.0 to 10.0 min., the precipitation efficiency % of uranium and increased from 6.2 to 50.8 % for uranium and from 15.2 to 32.5 % for which mean that the reaction time is fast, where after 10 min the time has slight effect on the precipitation of both uranium and. Therefore, 10 min represents the preferred time to maximize the uranium and precipitation from crude phosphoric acid. Precipitation efficiency, % Fig. (1): Effect of reaction time for uranium and precipitation from 45 % crude phosphoric acid (ammonium fluoride / phosphoric acid mass ratio; 3 g/ 100 g, acetone/ phosphoric acid mass ratio; 50 g/ 100 g at room temperature). Effect of Ammonium Fluoride / Phosphoric Acid Mass Ratio, (g/ g): The effect of ammonium fluoride/ phosphoric acid mass ratio, g/ g, ranging from 1/ 100 to 7/ 100 on the precipitation efficiency of uranium and was studied using phosphoric acid concentration 45 % for 10.0 min reaction time at a temperature of 25 o C with acetone/ phosphoric acid mass ratio; 50 g/ 100 g. The results obtained in Figure (2) as a relation between uranium and precipitation efficiency % and ammonium fluoride/ phosphoric acid mass ratio indicate that, as the ammonium fluoride/ phosphoric acid mass ratio, g/ g, increases from 1/ 100 to 5/ 100 the recovery percent of the uranium increased from 4.6 to 84.1 and for, from 14.1 to 35.5 %. This means that the increase of ammonium fluoride/ phosphoric acid mass ratio has greater effect on the uranium precipitation rather than the precipitation. Accordingly the ammonium fluoride/ phosphoric acid mass ratio of 5 g to 100 g is the choice ratio for the precipitation process of uranium from crude phosphoric acid by ammonium fluoride. 40

4 Precipitation efficiency, % Fig. (2): Effect of ammonium fluoride/ phosphoric acid mass ratio on uranium and precipitation efficiency from 45 % crude phosphoric acid (time 10 min, acetone/ phosphoric acid mass ratio; 50 g/ 100 g at room temperature). Effect of Acetone / Phosphoric Acid Mass Ratio, (g/ g): The effect of acetone/ phosphoric acid mass ratio, g/ g, ranging from 10/ 100 to 60/ 100 on the uranium and precipitation efficiency from 45 % phosphoric acid was investigated using ammonium fluoride/ phosphoric acid mass ratio; 50 g/ 100 g for 10.0 min reaction time at a temperature of 25 o C. The results shown in Figure (3) as a relation between uranium and precipitation efficiency % and acetone/ phosphoric acid mass ratio clear that, the acetone/ phosphoric acid mass ratio, g/ g, increases from 10/ 100 to 60/ 100 the precipitation efficiency % of the uranium decreased from 89.2 to 82.7 and for, from to 24.9 %, which indicate that the increase of acetone/ phosphoric acid mass ratio has a slight effect on the uranium precipitation while it has great effect on the precipitation. Therefore, acetone/ phosphoric acid mass ratio of 50 g to 100 g is the choice ratio for the precipitation process of uranium from crude phosphoric acid by ammonium fluoride for economic aspects. Precipitation efficiency, % Fig. (3): Effect of acetone/ phosphoric acid mass ratio on uranium and precipitation efficiency from 45 % crude phosphoric acid (time 10 min, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g at room temperature). 41

5 Effect of Precipitation Temperature: The effect of reaction temperature on the precipitation process was investigated for the temperatures of 20, 30, 40 and 50 o C at a reaction time of 10 min, phosphoric acid concentration of 45 %, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g and acetone/ phosphoric acid mass ratio; 50 g/ 100 g. Figure (4) shows the relation between uranium and precipitation efficiency % and temperature. The results obtained indicated that, by increase reaction temperature from 20 to 50 o C the uranium and precipitation efficiency % changed from 83.8 to 83.3 and from 27.8 to 24.9 respectively. This means that the change in temperature has a slight effect on the uranium and precipitation process. Therefore, room temperature is the preferred reaction temperature for the precipitation process. Precipitation efficiency, % Fig. (4): Effect of temperature on uranium and precipitation efficiency from 45 % crude phosphoric acid (time 10 min, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g, acetone/ phosphoric acid mass ratio; 50 g/ 100 g). Effect of Concentration of Free Sulfate: Several precipitation experiments were performed using of 45 % phosphoric acid with different free sulfate concentration ranging from 1.4 to 5.4 % and reaction time of 10 min, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g, acetone/ phosphoric acid mass ratio; 50 g/ 100 g at room temperature to study the effect of free sulfate concentration on the uranium and precipitation process. The experimental results are given in Figure (5) as a relation between uranium and precipitation efficiency % and free sulfate concentration. As can be seen from the Figure, by increase free sulfate concentration from 1.4 to 5.4 % the uranium and precipitation efficiency % increased slightly from 84.6 to 86.9 for uranium and from 27.9 to 28.5 for. This means that the concentration of free sulfate has no significant effect on the precipitation of uranium and. Accordingly, all experiments were carried out without free sulfate pretreatment process. 42

6 Precipitation efficiency, % Fig. (5): Effect of free sulfate concentration on uranium and precipitation efficiency from 45 % crude phosphoric acid (time 10 min, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g, acetone/ phosphoric acid mass ratio; 50 g/ 100 g at room temperature). Effect of Phosphoric Acid Concentration: The effect of phosphoric acid concentration on the uranium and precipitation process were studied at different phosphoric acid concentration from 25 to 45 % however, the other parameters were fixed at a precipitation time of 10 min, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g, acetone/ phosphoric acid mass ratio; 50 g/ 100 g and temperature of 25 o C. The experimental results given in Figure (4) as a relation between uranium and precipitation efficiency % and phosphoric acid concentration clear that, as phosphoric acid concentration increased from 25 to 45 %, the uranium precipitation % increased from 83.1 to 87.1 and precipitation % increase from 1.1 to This means that the increase of phosphoric acid concentration nearly has no effect on the uranium precipitation efficiency, while it has a great effect on the precipitation. Therefore this process is recommended for the concentrated phosphoric acid and the dilute phosphoric acid, but it preferred to the dilute phosphoric acid. Precipitation efficiency, % Fig. (6): Effect of phosphoric acid concentration on uranium and precipitation efficiency from phosphoric acid (time 10 min, ammonium fluoride/ phosphoric acid mass ratio; 5 g/ 100 g, acetone/ phosphoric acid mass ratio; 50 g/ 100 g at room temperature). 43

7 Precipitation Process Investigation: From the aforementioned investigations, a precipitation experiment was carried out by adding 1.5 g of iron powder to 1.0 Kg of 45 % phosphoric acid and stirring for 30 min, then filtrate the phosphoric acid. After that 500 g of acetone was added to the filtrate phosphoric acid followed by adding 50 gm of ammonium fluoride and stirring for 10 min at room temperature. After filtration, the obtained precipitate was analyzed and the results obtained are given in Table (2). Table (2): The chemical analysis of the produced precipitate (1.5 g iron powder, 1.0 Kg of 45 % phosphoric acid, 500 g acetone, 50 g ammonium fluoride and mixing for 10 min at room temperature) Component Concentration 31.5 % Fe 2O % Na 2O 7.1 % CaO 0.13 % Cd As Pb U < 3.0 ppm < 3.0 ppm < 3.0 ppm 410 ppm Citric acid as complexing/ chelating agent was used for the dissolution of uranium from phosphates according to the following equations; C 6H 8O 7 (C 6H 5O 7) H + (1) (C 6H 5O 7) UO 2 2+ (UO 2) 3(C 6H 5O 7) 2 (2) A maximum uranium solubilization (96 %) was obtained by using 20 % of citric acid solution within 10 minutes of the dissolution time (18). The collected precipitate was dried in an oven at 40 o C for 5 h. The produced precipitate was analyzed by X-ray diffraction technique (Philips PW-3710/31), before the uranium dissolution using citric acid and after the dissolution process, to identity the produced precipitate (Figures 7 and 8). 44

8 Fig. (7): XRD for the produced phosphate precipitate before the dissolution of uranium by citric acid. Fig. (8): XRD for the produced phosphate precipitate after the dissolution of uranium by citric acid. Figures 7 and 8 clear that, the phosphate precipitate in the two figures is the same ammonium iron phosphate hydrate. This means that the type of the phosphate precipitate is not affected by the uranium dissolution process. A proposed flow sheet for processing 1 ton of phosphoric acid produces 139 Kg ammonium iron phosphate hydrate precipitate is given in Figure (9). 45

9 Fig. (9): Flow sheet for 139 Kg of ammonium iron phosphate hydrate from 1.0 Ton of 45 % crude phosphoric acid CONCLUSION Uranium can be removed from crude phosphoric acid by precipitation technique using iron powder and ammonium fluoride successfully with 87.8 % efficiency. The precipitation reaction was fast and uranium & recovery % was increased by increase the amount of ammonium fluoride. The amount of acetone, temperature and free sulfate concentration have a slight effect on uranium and precipitation %. The precipitation process is applicable for the concentrated phosphoric acid and the dilute phosphoric acid but it preferred to the dilute phosphoric acid. The preferred precipitation conditions were time 10 min, temperature at 25 o C, ammonium fluoride/ phosphoric acid mass ratio, g/ g, 1/ 20 and acetone/ phosphoric acid mass ratio, g/ g, 1/ 2. The uranium dissolution from the produced ammonium iron phosphate hydrate was achieved with efficiency 96 %. 46

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