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Yaş yöntem fosforik asitin mono ve dikalsiyum fosfat kristalizasyonları ile saflaştırılması

Purification of wet-process phosphoric acid by crystallizations of mono and di-calcium phosphates

  1. Tez No: 14408
  2. Yazar: HALE GÜRBÜZ
  3. Danışmanlar: DOÇ.DR. A. NUSRET BULUTÇU
  4. Tez Türü: Doktora
  5. Konular: Kimya Mühendisliği, Chemical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1991
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 149

Özet

ÖZET Bu çalışma, fosfat kayasından yaş yöntemle üretilen ve öncelikle fosfat kayasından kaynaklanan birçok safsızl.ığı içeren fosforik asitin, uygun bir yöntemle saflaştırılmasıyla gübre dışındaki endüstriyel fos fatların üretiminde kullanılan, daha saf fakat daha pahalı olan ısıl yöntem fosforik asitin yerine kullanılabilirliğinin araştırılması ama cıyla yapılmıştır. Bu amaçla seçilen yöntem, yaş yöntem fosforik asitin fosfat içe riğinin kalsiyum tuzları halinde seçimli olarak kristal izasyonuna da yanmaktadır.. Yöntem dört kademeden oluşmaktadır. Bunlar: 1) Fosforik asit içindeki sülfat iyonlarının kireç ilavesiyle CaS04.1/2 H2O, ağır metal iyonlarının ise Na£S ilavesiyle sülfürleri halinde çöktürülerek, filtrasyonla ayrıldığı ön saflaştırma kademesi 2) ün saflaştırması yapılmış olan asite yeniden kireç ilavesiyle, 25°C de Ca(H2P04)2.H2Û kristal izasyonu ve kristallerin yıkanması. 3) CaC^PC^Îp-HjpO kristal izasyonundan çıkan ve safsızlıkları taşıyan ana çözeltinin kireçle kısmen nötralize edilmesiyle safsızlıkların çöktürülerek ayrılması. 4) Safsızlıkları giderilen çözeltide kalan P2O5 'in, yeniden kireç ila vesiyle CaHP04.2H20 halinde çöktürülmesidır. Prosesin ilk kademesinde ele geçen ve ağır metaller, sülfat ve çözünmeyen organik safsızlıkları taşıyan filtre keki, tuttuğu ana çözel tiden kaynaklanan yüksek P2O5 içeriği (%35-40 P2O5) nedeniyle gübre ü- retiminde kullanılabilecek, ikinci ve dördüncü.kademelerde elde edilen saf Ca(H2P04)2.H2Û ve CaHP04.2H2Û kristalleri ise sülfürik asitle bozun- durularak saf fosforik asite dönüştürülebilecektir. Yapılan çalışmada, prosesin her kademesi ayrı ayrı incelenerek, asit içindeki safsızlıkların davranışları ve mümkün olan en saf ürün lerin en yüksek verimle üretilebileceği optimum şartlar belirlenmiş tir. Sonuç olarak, elde edilen Ca(H2P04)2.H20 kristallerinin uygun bir şekilde yıkanarak ana çözeltiden kurtarılması sonucu elde edilen saf kristallerin fosforik asite dönüştürülmesiyle, yaş yöntem fosforik asitin solvent ekstraksiyonuyla saflaştırılmasıyla elde edilen temiz asitle, hatta ısıl yöntemle üretilen fosforik asitle kıyaslanabilecek kalitede bir asit elde edilebileceği belirlenmiştir. vi

Özet (Çeviri)

PURIFICATION OF WET-PROCESS PHOSPHORIC ACID BY CRYSTALLIZATIONS OF MONO AND DI-CALCIUM PHOSPHATES. SUMMARY Phosphoric acid which is an intermediate chemical for the produc tion of phosphate fertilizers and phosphatic salts is pruduced in two different methods. These are known as thermal process and wet process. Thermal process can be described shortly by the following equa tions: Ca1QF2(P04)6 + 15C + 6Si02+ 1.5P4+15C0 + 3(3Ca0.2Si02) + CaF2 P4 + 502 + 6H20 + 4H3P04 This process gives very pure acid, but its high production cost causes economic limitation in commercial uses and thermal grade phosphoric acid is only used in food, dentrifrice and detergent building applica tions. Wet-process phosphoric acid is produced by the reaction of sul furic acid and phosphate rock in aqueous media. The major reaction involved in wet acid manufacture is Ca1QF2(P04)6 + 10H2S04 + lOn H20 -»? 10CaS04.nH20 + 6H3P04 + 2HF The impurities commonly found in wet-process phosphoric acid include dissolved and suspended organic and inorganic materials. Sour ces of these contaminants are the phosphate rock itself, reagents that are adsorbed during benefication of the rock, sulfuric acid and pro cessing equipments that is physically and chemically attacked during manufacturing steps. All the dissolved and suspended impurities adversly affect the grade of acid, impart undesired color or turbidity, and change physical properties such as viscosity and density. The most widely experienced problem results from the instability of dissolved impuri ties and their pronounced tendency toward precipitation. The precipi tation of dissolved impurities occurs mainly during the evaporation step and continues for days or weeks during transportation and storage. The precipitated sludge increases the cost of transportation and cleaning of the storage. The first attemps to produce cleaner acid were directed toward viiremoving these problems. The methods used for producing cleaner acid can be classified in three groups except the settling of the acid at the manufacturers plant for a sufficient time. These are: 1) Volatilization of fluorine 2) Precipitation of insoluble fluoride compounds. 3) Chemical or physical methods of inducing or retarding postprecipita- tion. Considerable amounts of phosphate salts are used in the food, feed, dentrifrice, dye and detergent applications both all over the world and Turkey. In order to be able to use wet process phosphoric acid which is produced largely for fertilizer industry in these indus tries, it must be purified. The purification methods of wet process phosphoric acid can be classified in three main groups. These are: 1) Physical methods such as crystallization, solvent extraction, ad- sorbtion 2) Electrochemical methods 3) Chemical methods. In this work, the purification of impure wet process phosphoric acid by the chemical method has been searched. The chosen method depends on the modification of the method developed by Sae.man 1 20 0 [ for production reasonably pure phosphoric acid and dicalcium phosphate from wet process phosphoric acid. Following equations describes the basis of the method suggested by Saeman. Production of monocalcium phosphate by partial neutralization of phosphoric acid: CaO + 2H3P04 -*. Ca(H2P04)2 + H20 CaHP04 + H3P04 ?»? Ca(H2P04)2 Ca3(P04)2 + 4H3P04^ 3Ca(H2P04)2 Regenaration of pure monocalcium phosphate to pure phosphoric Ca(H2P04)2 + H2S04 -»? CaS04 + 2H3P04 Thermal dissociation of monocalcium phosphate: Ca(H2P04)2.*? CaHP04 + H3P04 viii acid:Various methods can be suggested for the production of monocal- cium phosphate monohydrate and di calcium phosphate di hydrate by the use of solubility isotherms in the system Ca0-P205-H20« These are: 1) Preparation of a solution saturated with Ca(H2P04)2«H20 and CaHP04 solid phases at high temperatures (110-11 5°C) by addition of lime to wet process phosphoric acid and crystallization of CaC^PO^.^O from this solution by cooling to room temperature under turbulent suspension conditions (Saeman Process). In this method, Ca(H2P04)2.H20 can be obtained as coarse crystals and these are. separated from the precipitated fine impurities in the solution by hydraulic elutriation. 2) Separation of the precipitated impurities from the saturated solu tion prepared by similar manner mentioned above by filtration at elevated temperatures and crystallization of pure Ca(H2PQ4)2.H2Û from the clear solution by cooling. 3) Partial purification of wet process phosphoric acid by adding lime and after separation of the precipitated impurities by filtration, crystallization of the Ca-(H2P04)2-H20 from the clear solution by adding lime at room or somewhat high temperature by reaction crys tallization. In these methods, the remained P2O5 in the solution from the Ca(H2P04)2-H20 crystallization can be recovered as CaHP04.2H20 by adding lime. The sludge which contains precipitated impurities and reasonable amount P2O5 can be.used in the fertilizer production. Pure Ca^PC^)?.^ crystals will be able to convert to the pure phosphoric acid by the reaction with H9SO4 or to the pure dicalcium phosphate and a solution which contains phosphoric acid by thermal decomposition. The main objects of this study are: a) To investigate the behaviours of the impurities which is found in the wet process phosphoric acid during the first neutralization (pre-purification) step and to determine the best conditions of this step for the production of the purest Ca^PCty^.^O crystals. b) To investigate the effect of temperature on the yield of the Ca(H2P04)2.H20 crystallization and determine the best temperature in order to obtain maximum yield. c) To investigate the behaviours of the impurities in the Ca(H2P04)2.H2Û crystallization step. d) For the production of the pure CaHP04.2H20 from the solution remain ed in the Ca(H2P04)2«H2Û crystallization, -to investigate the suitab le conditions of the pre-purification step before the precipitation of CaHP04.2H20 and to decrease the P2O5 losses in this step. e) To produce trie pure CaHP04.2H20. f) To propose a process for the production of pure Ca(H2P04)2«H2° and CaHPCty. 2^0 from the impure wet process phosphoric acid according to the results obtained in the experimental study. Industrially, the production of the phosphoric acid from the cal cium phosphates is well known in details. For this reason, this step hasn't been researched in this study. ixThe experiments which have been carried out according to the above mentioned objects consist of the following steps. 1) First neutralization step: Pre-purification of the black coloured wet process phosphoric acid. 2) Second neutralization step: Crystallization of the Ca^PO^.^O from the pre-purified clear phosphoric acid, 3) Third neutralization step: Removing of the impurities remained in the solution from the CaC^PO^.^O crystallization by partial neutralization of the solution with lime. 4) Fourth neutralization step: Production of the pure CaHPC>4.2H20 from the solution which has been separated from the impurities. The results obtained from the experiments are as follows. 1) The best conditions for the first neutralization step has been ob tained by adding 200 kg H2O and 50 kg Ca(0H)2 per 1000 kg wet pro cess phosphoric acid (52,54% P2Û5)and carrying out this reaction at 75-80°C for two hours. In this step, heavy metal ions in the phosphoric acid have been precipitated by adding of 2 kg NagS to the reaction mixture after the reaction with Ca(0H)2« 2) The chemical analysis and X-ray difraction patterns of the precipi tated sample from the first neutralization step showed that this precipitate consisted of CaS04. 1/21^0. The composition of the pre- purified clear solution obtained at these conditions is reasonably close to the composition of the solution saturated with the Ca(H2P04)2.H20 at 25°C. The P2O5 and CaO concentrations of this solution are 41-46% and 1.1-1.5% respectively. 3) In the first neutralization step, the suspended organic impurities in the wet process phosphoric acid have been adsorbed on the filter cake during the filtration of precipitated CaS04. 1/21^0 and a green coloured clear solution has been obtained. 4) The results obtained from the experiments which have been carried out by adding the required total lime as increasing portions to the fixed amount phosphoric acid showed that all of the Cr, V, Al, Fe, F impurities have remained in the clear solution after the separa tion of the precipitate. 5) In the production of Ca (^04)2^20 by addition of lime to the clear pre-purified solution, the P2O5 recovery increases as the reaction temperature decreases when trie final crystallization temperature is the same as 25°C. The recoveries which have been obtained at 75°C and 25°C are 54.“.7 and 84% respectively. At 25°C, the theoretical magma density is about 40%. This great value of the magma density shows a problem because of the difficulty of discharging the suspen sion from the crystal lizer. This problem has been eliminated by adjusting the value of magma density to 20% by adding the mother liquor of CaH2P04.2H20 crystallization at 25°C. In the continuous process this can be made by recycling of the mother liquor. 6) As a result of the experiments of Ca(H2P04)2.H20 crystallization which have been carried out by the addition of required total lime as increasing portions» it was found that all of the Cr', V, Al, Fe and F impurities have remained in the mother liquor and the green color of the Ca(H2P04)2.H20 crystals has sourced from the adhering mother liquor. The mother liquor retained by the crystals can beremoved by a suitable washing step. Because of the washing with. water causes to decomposition of the Ca^PO^o.F^O crystals, this step must be made by the use of the most dilute phosphoric acid solution which doesn't cause decomposition. The composition of this solution has been determined as 12.5,%P2Û5 from the solu bility isotherms in the CaO.P2O5.H2O system. 7) It has been thought that the adding order of the reagents in the Ca(H2P04)2-H20 crystallization can affect the dimension and the purity of the crystals. Filtration experiments showed that the best conditions have been obtained by diluting the pre-purified phosphoric acid with the mother liquor of the CaCHoPO^-H^O crystallization at first, then by adding the Ca(0H)2 to this mix ture for the crystallization of Ca(H2P04)2.H20. 8) When the total Ca(0H)2 required for the Cad^PO^.I^O crystal-. lization has been added at once, the temperature of the reaction mixture has risen to a value more than 50°C. At this temperature the composition of the mixture is in the region in which the Ca(H2P04)2sH20 and CaHP04 solid phases exist together in the equi librium with the saturated solution. In order to prevent of the formation of CaHP04 during the crystallization of Ca(H2P04)2>.H20, 3/5 of the required total Ca(0H)2 must be added at first, then the temperature of the mixture which is about 44°C, must be decreased by cooling' to 25°c before the addition of the remained Ca(0H)2. 9) As a result of the experiments made for determining the effect of the reaction time on the Ca(H2P04)2.H20 crystallization, it has been found that the required reaction time is one hour, preferably two hours.”If the reaction time is shorter than one our, the free Ca(0H)2 remained without reacting causes the reaction mixture to become sticky. The reaction time which is longer than two hours practically unuseful. 1ft) The mother liquor remained from the 03(^04)2. H2O crystallization contains 25-30% P2O5, 4.5-5.0% CaO and all of the Cr, V, Fe, Al and F impurities. The precipitation of these impurities during the production of the CaHP04.2H£0 from this solution by the addi tion of lime has caused to obtain a impure product. In order to eliminate these impurities before the production of the CaHP04.2H20, they have been precipitated together with a small amount P2O5 by partial neutralization of the solution. 11) From the experimental results it has been found that 95% of Cr and Fe, 80% of Al and F and 30% of V could have been removed from the solution by the addition of 1/5 of the required Ca(0H)2- In this stop the loss of P2O5 was about 25-30 %. 12) The addition of NaCl to the solution in the third neutralization step has increased the removing of vanadium and decreased the loss of P2Û5. The best conditions for the pre-purif i cation step before the pure CaHP04.2H20 production have been obtained by ad dition of NaCl to the reaction mixture in the concentration of 4.4% NaCl as weight and the precipitation of 20-23% of P2O5. This has been obtained by the use of 28% of the required Ca(0H)2. In these conditions, the 75% of Cr, 98% of Fe, 85% of Al and F and 71% of V could have been removed. 13) The use of NaCl in the pre-purification step before the CaHP04.2H2Û production is especially useful for the wet process phosphoric acid contains important amount of V. The NaCl added to the reaction mixture has remained in the solution except the loss xisourced from the mother liquor retained in the precipitate. The solution from the CaHPÛ4.2H20 precipitation can be returned to the process. 14) After the precipitation of the impurirites, the P2O5 which has remained in the solution can be precipitated as the pure (feed- grade) CaHP04.'2H20. 15) The quantitative flow sheet of the process is shown in the concessions part of this study. xn

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