JACOBS NEW PROCESS FOR REMOVING IRON FROM PHOSPHORIC ACID FINAL REPORT Stephen W. Hilakos Process Engineer 3149 Winter Lake Road, Lakeland, FL 33803 P.O. Box 2008, Lakeland, FL 33806-2008 Prepared for FLORIDA INSTITUTE OF PHOSPHATE RESEARCH 1855 West Main Street Bartow, Florida 33830 USA Presented at 39 th Annual Clearwater Conference June 5-6, 2015 Stephen.Hilakos@jacobs.com
ABSTRACT Jacobs has developed a new technique for removing iron from phosphoric acid. Details and results are expected to be available for presentation at the conference. With the new process, Jacobs has successfully treated high iron acids and reduced minor element ratios (MERs) by over 50% with only a minor loss of P 2 O 5 content. The new process should be attractive in regions with phosphate deposits that contain high concentrations of iron, such as those located in Australia, Canada, Northern Africa and areas of the Middle East. 2
SUMMARY Jacobs Engineering has discovered that iron can be removed from phosphoric acid by the addition of oxalic acid. Experimental results have demonstrated that an iron compound, FeC 2 O 4 *2H 2 O, can be precipitated from phosphoric acid without the co-precipitation of P 2 O 5. Most of the iron removal techniques found in the literature have high P 2 O 5 losses because some amount of the P 2 O 5 content is co-precipitated along with the iron component. As multiple phosphate mining projects around the world suffer from reduced economics due to the high iron content of the phosphoric acids produced, Jacobs believes this is a significant discovery and intends to offer a proprietary treatment process. INTRODUCTION Jacobs was investigating iron removal techniques in association with a project on the Abu Tartur phosphate deposit in Egypt. The Abu Tartur deposit has been noted in the literature as a particularly challenging project as multiple companies have failed to satisfactorily solve the problems associated with the high iron content. The ore resists typical beneficiation techniques because most of the iron is embedded in the concentrate as small grains that are locked in the larger apatite particles. Only a fraction of the iron, less than 50%, is attached to the surface of the apatite particles at size fractions that liberate over 95% of the apatite. Attrition scrubbing was somewhat successful at removing a portion of the surface material while maintaining reasonable P 2 O 5 recoveries. As a part of the investigation into iron removal techniques, it was suggested that surface iron be removed from the ore by an oxalic acid wash. Oxalic acid is used commercially to remove iron stains from ceramic fixtures and is used industrially to remove iron from silica for fiber optics and clay used to produce ceramics. This was only partially successful as P 2 O 5 was also removed and calcium oxalate was deposited on the ore s surface. Additional research revealed that oxalic acid had also been used to remove iron from the organic phase during uranium extraction and an older US patent mentioned that iron oxalate has very limited solubility in water. As a proof of concept, several tests were performed with 28% P 2 O 5 filter acid with iron levels that were artificially increased by dissolving elemental iron and/or iron sulfate powder (FeSO 4 *7H 2 O) in the acid. These initial tests were successful; however, an action plan had already been presented to the client prior to this discovery. BACKGROUND A literature search had previously been completed on iron removal techniques for the Abu Tartur project. The search revealed multiple methods for the removal of iron from phosphoric acid including solvent extraction, ion exchange, and the precipitation of iron from phosphoric acid. The solvent extraction and ion exchange techniques required additional processing steps 3
and more complex flow sheets. The precipitation methods were simpler, but had high losses for P 2 O 5 content and some required extended aging periods. The precipitation methods suggested to the client are summarized on the following page: De-Sulfating and Partial Ammoniation to Precipitate Iron Phosphate Sludge 1 Small amounts of NH 3 are added either at 26% P 2 O 5 or 44% P 2 O 5 to precipitate a mixture of the iron phosphate compounds Fe 3 NH 4 H 8 (PO 4 ) 6 *6H 2 O and Fe 3 NH 4 H 14 (PO 4 ) 8 *4H 2 O. The method requires an extended aging period of 4 to 7 days to precipitate the sludge. The procedure claims the solubility of the iron compounds can be further decreased by reducing the excess sulfate content prior to the ammonia addition. Excess sulfate content can be reduced by adding phosphate rock. Phosphate losses will be high, the estimated ratio of P 2 O 5 to Fe 2 O 3 in these compounds ranges from 1.78 to 2.34, unless a portion of the sludge can be reclaimed as a citrate soluble compound in a granular product. Incremental Precipitation of Ralstonite, Silica Fluorides, and Iron Potassium Phosphates 2 The method claims to incrementally precipitate the various products as sludge within hours. The Ralstonite and Silica Fluoride compounds can be discarded as they do not contain nutrient value. The Iron Potassium Phosphate produced is claimed to be citrate soluble by one of the methods. Ralstonite precipitations, however, can be influenced by other impurities that reduce the precipitation and filtration rates. Selective Precipitation of Iron with Chelating Agents 3 The method claims to selectively remove as much as 60% of the iron in filter acid within 1 hour at room temperature. The reagent, N(CH 2 PO 3 H 2 ) 3, combines in a 1 to 1 mole ratio with Fe 3+ to form a precipitate. The precipitate dissolves when the ph is raised to ph 5.0 and a new precipitate forms when the ph is raised to 8.0 using NH 4 OH. The solid formed at ph 8.0 is a waste product containing 3 moles of Fe and 1 mole of the reagent. The ammonium form of the regenerated reagent can be recycled and releases ammonia into the acid when it forms the next Fe complex. The ammonia would be recovered as a nutrient in granulation; however, about 30% of the chelating agent is lost during the regeneration. The cost of replacing lost reagent may make the overall method uneconomical. The MSDS data on the reagent implies there may also be some risk for exposure in concentrated forms. 1 FIPR & Baumann, Jacobs PN 28-G674-00 1995 2 TVA & Frazier US Patent 4,435,372, Michalski US Patent 4,639,359, additional patents exist 3 FIPR 01-151-171 & El-Shall 4
Precipitation of Iron Compounds with Na 2 CO 3, KOH, NaOH or K 4 Fe (CN) 6 These methods were compared in the September 1999 Indian Journal of Chemical Technology. For a 50% reduction in Iron content, the ranking according to P 2 O 5 losses were: 8.2% with K 4 Fe(CN) 6, 10% with KOH, 15% with NaOH, and 18% with Na 2 CO 3. Aging requirements were not reported. The most effective reagent K 4 Fe(CN) 6 was reported to be quite hazardous. The results were from a series of bench scale tests performed with 28.5% P 2 O 5 acid that contained 3.01% Fe 2 O 3. Test Work with Synthetic Acid Iron oxalate (FeC 2 O 4 *2H 2 O) is a bright yellow iron compound that is insoluble in water. Prior to the kickoff meeting for the acidulation portion of the project, the beneficiation test work had been completed and the need for iron removal after acidulation was obvious. The list of iron removal techniques to be discussed had been reduced to precipitation methods that would add nutrient value such as ammonia or potassium to the acid. The techniques to be suggested were the ammonia and potassium additions followed by aging and clarification and the chelating method developed by FIPR and El Shall. Additional oxalic acid testing with synthetic acids enhanced by a combination of elemental iron and iron sulfate powder (FeSO 4 *7H 2 O) was still being performed in the lab. As elemental iron was hard to dissolve in large quantities and the iron sulfate increased sulfate levels, the iron concentrations were elevated by a combination of additions with elemental iron providing the initial boost followed by the iron sulfate. The acids were targeting a 3.5% Fe 2 O 3 concentration. A corporate disclosure of a novel discovery had been filed. A patent attorney had conducted a literature search to determine the patentability of the process and the opinion was favorable. A portion of the initial test work with the synthetic acids is shown below: Test 1 To test the concept, a large excess of oxalic acid crystal (H 2 C 2 O 4 *2H 2 O) was added to a synthetic acid with a high Fe 2 O 3 content. The mixture was stirred for several minutes at room temperature and allowed to stand overnight. The mixture was filtered using a 40 micron glass fret filter the next day and the filtrate was analyzed for P 2 O 5 and metal content. The analyses are shown in Table 1. Table 1 %P 2 O 5 %Fe 2 O 3 %Al 2 O 3 %MgO %CaO ppm Cd ppm Co ppm Cu ppm Mn ppm Ni ppm Pb ppm Zn Before 26.21 3.55 0.31 0.14 0.14 7.2 1.6 19.8 76.7 49 3.7 284 After 25.58 0.54 0.25 0.13 0.12 3.2 nd 0.7 55.7 nd 1.2 nd 5
Test 2 A small amount of oxalic acid crystal (H 2 C 2 O 4 *2H 2 O) was added to a synthetic acid with a high Fe 2 O 3 content. The amount was calculated to remove 1.5% Fe 2 O 3 based on the stoichiometric dose required to produce FeC 2 O 4 *2H 2 O on a 100% efficient basis. The mixture was stirred for 10 minutes at room temperature and allowed to stand for 5 minutes before filtering on a 40 micron glass fret filter. The filtrate was analyzed for P 2 O 5 and metal content. The analyses are shown in Table 2. The precipitation was not complete at the time of filtration and the filtrate had produced additional solids by the next morning. Table 2 %P 2 O 5 %Fe 2 O 3 %Al 2 O 3 %MgO %CaO ppm Cd ppm Co ppm Cu ppm Mn ppm Ni ppm Pb ppm Zn Before 25.80 3.68 0.32 0.14 0.12 7.1 1.6 21.6 76.9 49.2 1.9 270 After 23.49 2.69 0.29 0.13 0.13 5.9 0.9 2.1 68.3 24.1 1.4 67.4 Test 3 A concentration profile was performed with various amounts of oxalic acid crystal (H 2 C 2 O 4 *2H 2 O) being added to a synthetic acid with a high Fe 2 O 3 content. A small portion, the last addition, was performed using oxalic acid dissolved in water. The liquid addition was substituted with water on a blank and the lower dosages so that acid dilution would be consistent for all the samples. The mixtures were stirred for about one minute at room temperature and allowed to stand for two hours before filtering the solutions on 40 micron glass fret filters. The filtrate was analyzed for P 2 O 5 and metal content. The analyses are shown in Table 3. The precipitation was not complete at the time of filtration and the filtrates produced additional precipitate within the next three hours. Table 3 g/100g %P 2 O 5 %Fe 2 O 3 %Al 2 O 3 %MgO %CaO ppm Cd ppm Co ppm Cu ppm Mn ppm Ni ppm Pb ppm Zn 0 23.73 3.36 0.27 0.12 0.17 6.6 2.8 11.4 84.5 163 1.8 253 0.3 23.77 3.44 0.27 0.13 0.17 6.6 3.0 11.6 86 165 1.0 255 0.6 23.77 3.45 0.27 0.13 0.16 6.6 2.9 11.6 86.6 166 2.3 258 1.15 23.46 3.41 0.27 0.13 0.16 6.7 3.1 10.7 86.1 163 2.1 249 1.7 23.67 3.28 0.27 0.12 0.16 6.5 2.8 6.4 84.6 151 1.2 218 2.7 24.01 2.80 0.26 0.13 0.16 5.8 1.8 3.1 82.7 102 1.7 109 4.2 23.44 2.49 0.26 0.12 0.15 5.5 1.6 3.2 79.6 81.5 0.9 75.6 5.7 23.51 2.36 0.26 0.12 0.15 5.3 1.4 3.7 79.2 71.9 1.0 68.8 7.2 24.21 2.21 0.25 0.12 0.15 5.1 1.1 4.2 76.1 64.3 1.6 69.5 8.6 23.79 2.12 0.25 0.13 0.14 5.0 1.0 3.7 77.6 56.7 1.3 48.2 10.2 23.53 1.80 0.25 0.12 0.14 4.5 0.7 4.4 74.7 38 0.8 35.6 Test 4 Another concentration profile was performed with various amounts of oxalic acid crystal (H 2 C 2 O 4 *2H 2 O) being added to a synthetic acid with a high Fe 2 O 3 content. A small portion, the last addition, was performed using oxalic acid dissolved in water. The liquid addition was substituted with water on a blank and the lower dosages so that acid dilution would be 6
consistent for all the samples. The mixtures were stirred for about one minute at room temperature and allowed to stand for 90 hours before filtering the solutions on 40 micron glass fret filters. The filtrate was analyzed for P 2 O 5 and metal content. The analyses are shown in Table 4. The precipitation was complete for these tests and no additional precipitate was produced by the filtrates. Table 4 g/100g %P 2 O 5 %Fe 2 O 3 %Al 2 O 3 %MgO %CaO ppm Cd ppm Co ppm Cu ppm Mn ppm Ni ppm Pb ppm Zn 0 24.40 3.50 0.28 0.13 0.15 7.1 2.9 8.7 85.6 170 0.9 266 0.6 24.49 3.51 0.28 0.13 0.15 7.0 2.8 7.9 85.7 169 1.4 263 1.7 24.55 2.92 0.27 0.13 0.16 6.4 1.8 0.4 84.3 79 0.6 115 4.2 24.40 2.01 0.26 0.13 0.15 5.2 0.9 nd 77.5 34 0.8 48 7.2 24.52 1.64 0.25 0.13 0.15 4.8 0.6 nd 74.4 22 0.9 32 A Recovery Process Iron oxalate (FeC 2 O 4 *2H 2 O) is a bright yellow iron compound that is insoluble in water. The solubility limits are 0.008g /100g H 2 O cold and 0.097g /100g H 2 O hot. The initial results provided convincing evidence that a better solution, or precipitate, could be presented at the kick off meeting. But there were concerns about the cost of treatment. Depending on the dosage rate, the precipitate could be too expensive to simply discard (H 2 C 2 O 4 *2H 2 O is $600/tonne). So the search for a recycling process was begun. Enough bright yellow powder had been generated to at least start the recycling tests. CaC 2 O 4 *H 2 O water solubility = 6.7 x 10-4 g /100g H 2 O This looked like the answer: calcium oxalate monohydrate. An old US Patent suggested a purification method that precipitates calcium oxalate by lime addition, followed by acidification with sulfuric acid that produces oxalic acid and gypsum. Sulfuric acid and gypsum were already a part of the process, and the lab had some experience with oxalic acid solubility in water. Four and five molar solutions of heated oxalic acid could easily be recrystallized above room temperature. The lime addition, however, was only performed once. Iron hydroxide was not the answer. Oxalic acid techniques are used in silica purification, uranium extraction and other metal purifications such as cobalt. Another old US Patent suggested recovery of oxalic acid from soluble iron oxalate solutions using CaCl 2. The mixture is heated and agitated for 2 hours. A solid calcium oxalate is recovered and can be converted to oxalic acid di-hydrate. This became part of the reclaiming process. 7
Oxalate Recycling Process To reduce the amount of oxalic required and the cost of treatment, Jacobs has developed a recovery/recycle process to reclaim and reuse the oxalic acid. Figure 1 depicts the unit operations for the oxalate recycling system associated with the Jacobs Iron Removal Process. The FeC 2 O 4 *2H 2 O is converted to calcium oxalate (CaC 2 O 4 *H 2 O) by reacting the iron oxalate solids with a solution of CaCl 2 under slightly acidic conditions. During the reaction, the iron associated with the oxalate is replaced by calcium and a new precipitate, calcium oxalate monohydrate, is formed. The reaction is shown below: FeC 2 O 4 + CaCl 2 FeCl 2 + CaC 2 O 4 * H 2 O The calcium oxalate is filtered and washed before being reacted with a hot (80 o C) solution of 50% sulfuric acid to regenerate the oxalic acid and form gypsum. The general reaction is shown below: CaC 2 O 4 * H 2 O + H 2 SO 4 + 3 H 2 O H 2 C 2 O 4 + 2 H 2 O + CaSO 4 * 2H 2 O After being agitated for about 1 hour, the gypsum (CaSO 4 *2H 2 O) is filtered from the hot solution (80 o C) and washed with boiling water to remove the residual acids (oxalic and sulfuric). The gypsum produced during the reaction has very low levels of impurities and is expected to be wall board quality. Additional sulfuric acid is added to the exit stream from the oxalic acid crystallizer along with a portion of the wash water from the gypsum filter and is used to react the next batch of calcium oxalate. Oxalic acid (H 2 C 2 O 4 * 2 H 2 O) is re-crystallized from the first filtrate by quenching the solution at about 20 o C. The solubility of oxalic acid in water is 84g/100g at 80 o C and only 10g/100g at 20 o C. The oxalic acid di-hydrate crystals are filtered from the chilled solution. Recovered oxalic acid has been tested and was found to have a similar performance as the virgin chemical when used to remove iron from 28% filter acid. It is important to recrystallize the oxalic acid to reduce the phosphoric acid dilution during treatment. The water content of the re-crystallized H 2 C 2 O 4 * 2 H 2 O contains about 20% water. The recovery process has only been performed to demonstrate the proof of concept. The overall recovery of the oxalic acid was low (~30%), reagent usage was high and the process still requires refinement. The recovery of oxalic acid by this process is not unique. The problem was solved. The verbiage for this section came right from the final report. All that remained to be done was clean up the parts and pieces, improve the recovery, and size 8
the equipment. Unfortunately, the current market prices for CaCl 2 and the amount of CaCl 2 required made the process economics unfavorable for this process at this time. So does Jacobs have a viable recycling process? Yes, but it s not the one presented here and in the final report. Based on additional lab work, the OPEX cost for the next version of the recycling process is about half the cost of virgin oxalate. The recovery and the quality are good, but the new flow sheet is still confidential. Figure 1: A Recovery Process for Oxalic Acid using CaCl 2 to Produce CaC 2 O 4 *H 2 O 9
The Real Acid (Adding Iron to Remove Iron) Iron oxalate (FeC 2 O 4 *2H 2 O) is a bright yellow iron compound that is insoluble in water. Fe 2 (C 2 O 4 ) 3 *6H 2 O is light green in color and highly water soluble. Oxalic acid is often used commercially as a stain and rust remover. At the acidulation kick off meeting, the three previously mentioned precipitation techniques were presented as treatment options. Although the details of the Jacobs process could not be disclosed, a portion of the preliminary data was presented as a fourth treatment option. After a brief private discussion, the client decided to test only the Jacobs process with the condition that if the treatment failed, the bulk of the acid would be processed using one of the other techniques. As soon as enough real acid was produced, small quantities of weak recycle acid were treated with oxalic acid, and nothing happened. No precipitate was formed, not a trace. So, the dosage was increased and still no precipitate. The REDOX potential was wrong; the iron was in the +3 state. This had been suspected, but not to this extent. When elemental iron was added to the mixture, a bright yellow precipitate began to appear. Process Description for Iron Removal The Jacobs iron removal process is conducted in two stages. A reduction in the oxidation state of the phosphoric acid solution is followed by an addition of oxalic acid to precipitate an iron oxalate compound. Although the reduction in oxidation state and the oxalic acid addition can be performed in any order or together, it is Jacobs opinion that the reduction should be performed prior to the oxalate addition. The reducing step is critical to the overall efficiency of the process as the oxidation state of the dissolved iron determines the solubility of the iron oxalate compound formed. Specifically, Fe 2 (C 2 O 4 ) 3 *6H 2 O, the iron oxalate complex formed when dissolved iron is in the Fe +3 state, is very soluble in phosphoric acid while FeC 2 O 4 *2H 2 O, the iron oxalate complex formed with iron in the Fe +2 state, is only slightly soluble in phosphoric acid. In addition to this difference in solubility, it is Jacobs experience that the formation of the soluble Fe 2 (C 2 O 4 ) 3 *6H 2 O, complex occurs preferentially to the insoluble FeC 2 O 4 *2H 2 O. Therefore in a phosphoric acid solution that contains a mixture of iron in both the +2 and +3 states, the first additions of oxalic acid will form soluble iron compounds and no iron precipitate will be formed. Table 5 illustrates this effect and shows that the efficiency of the iron oxalate precipitation increases as the EMF or oxidation state of the acid solution is reduced. The oxalic acid 10
treatment continues to improve in efficiency with lower EMFs until almost all of the iron in solution is in the +2 state at EMF values less than 300 mv. As expected, the amount of Fe 2 O 3 removed is also dependent on the quantity of oxalic acid added. The combined effect of dosage rate and REDOX potential are illustrated in Figure 2. Although the formation of the iron oxalate precipitate begins shortly after the addition of oxalic acid, a thorough mixing stage is recommended during the reagent addition, followed by a 24 hour aging period prior to clarification and filtration. The oxalate solids after filtration should contain only very small amounts of P 2 O 5. 11
Table 5: Efficiency of Fe 2 O 3 Removal at Multiple REDOX Potentials (EMF) Figure 2: Combined Effect of Oxalate Addition and REDOX Potential on Fe 2 O 3 Removal 12
Overall Plant Flow Sheet with Jacobs Iron Removal Process Figure 3 depicts a typical phosphoric acid plant with the Jacobs Iron Removal Process equipment inserted in the clouded area. The Iron Removal process is expected to treat only a portion of the acid from the 28% acid storage tanks prior to evaporation. Depending on the final product to be produced, the percentage of low iron acid would vary from 0% to 60% of the acid feed to evaporation. Blend percentages for the Abu Tartur filter acids were expected to be on the order of 40/60 to 60/40 treated to untreated filter acid for DAP production. Lower percentage blend requirements were expected for TSP and MAP production. The general plant items depicted in Figure 3 are: Phosphoric Acid Reactor Phosphoric Acid Filter 28% Acid Clarifier 28% Acid Storage Evaporation MGA Clarifier MGA Storage Jacobs Iron Removal The process is shown as treating a portion (stream 1) of 28% filter acid after 28% clarification. Treated acid (stream 2) is blended with untreated acid from 28% acid storage to obtain a 28% filter acid feed to evaporation of the desired quality (i.e., a target MER value). Inputs to the basic process are a reducing agent, oxalic acid and water. For the basic process, the exiting streams would be a waste stream consisting primarily of iron oxalate solids and a dilute stream (~10% P 2 O 5 ) of phosphoric acid. A diagram of the basic iron removal process is shown in Figure 4. If the process employs an oxalate recycling option, the iron oxalate solids would enter the recycling process and re-crystallized oxalic acid would be recycled to the initial stages of the process. The exit stream would be an aqueous iron waste stream. 13
Figure 3: Typical Phosphoric Acid Plant with the Jacobs Iron Removal Process Equipment 14
Figure 4: Jacobs Iron Removal Process CONCLUSION Jacobs has a better way to decrease the iron content in phosphoric acid. Currently, the costs are high and need to be improved, but the basic process is simple to implement. Oxalate has the ability to precipitate many of the elements in the center of the periodic table, especially the +2 valence ions. The list of elements that form low solubility oxalate compounds contains As, Cd, Co, Cu, Hg, Ni, Pb, Sb, Zn and all of the rare earths. The path forward: 1) Electrolysis Jacobs electrolysis experts at Chemetics are working to perform the REDOX step with electricity. This could reduce the oxalate usage rates and costs by 30% verses the current iron addition. 2) Recycling The Recycling process has the potential to supply a portion of the oxalate at 50% of the cost of virgin material. The real question is what fraction of the oxalate input can be reasonably reclaimed and the value of the waste stream. 3) Recovering Valuable By-Products Although most of the rare earths were lost to the gypsum during acidulation, a portion of them are being precipitated with the iron oxalate. Recovering rare earths or converting the iron waste stream into a viable product, such as FeSO 4 *7H 2 O, may provide another cost reduction. 15