Ind. Eng. Chem. Res. 2003, 42, 6697-6704
6697
Production of Molten Defluorinated Phosphates in a Submerged Combustion Melter L. S. Pioro,† A. M. Osnach,† and I. L. Pioro*,‡ The Gas Institute, National Academy of Sciences of Ukraine, Degtyaryovskaya Street 39, Kyiv, 03113 Ukraine, and Institute of Engineering Thermophysics, National Academy of Sciences of Ukraine, 2A Zhelyabov Street, Kyiv, 03057 Ukraine
A new effective one-stage method for production of molten defluorinated phosphates in the submerged combustion melter (SCM) for feed and fertilization in agriculture was developed and tested. The method involves melting raw phosphate ores (phosphate rock, apatite, etc.) in a SCM with certain additives (phosphoric acid, sand, etc.) with consecutive melt draining and granulation. Specific to the melting process is the use of a gas-air mixture with direct combustion inside a melt. Located inside the melt are high-temperature zones with increased reactivity of the gas phase, the existence of a developed interface surface, and intensive mixing, leading to intensification of the defluorination process. The high specific heat of the molten materials and their intensive mixing help the fast melting of coarse-grade and even-lump-size material loaded in the melt. The ability to use granulated charge allows effective reprocessing of the raw phosphate materials and a significant decrease in dust entrainment compared with rotating kilns and cyclone furnaces. The experimental data for different aspects of the proposed method are presented. The viscosities of the melts and their impact on the melting temperature are also discussed. The granulated molten defluorinated phosphate can be used as an effective fertilizer and for feed in agriculture. 1. Introduction Thermal treatment methods for phosphates have been known for a long time and in many cases are more effective than acid methods with their complicated multistage technologies.1,2 This explains the keen interest in these methods in many countries worldwide.3-12 In the former USSR, studies on the production of phosphate fertilizers by means of thermal treatment were started in 1935 at the Scientific-Research Institute of Fertilizers and Insecto-Fungicides (NIUIF; Moscow, Russia) by Vol’fkovich.5 In 1950, similar studies were started in the Kazakh Academy of Sciences by Bekturov.10 The main thermal facilities in which natural phosphates are reprocessed are rotary drum-type furnaces and electric-arc furnaces. The major disadvantage of these furnaces is the relatively low specific productivity, because the speed of reaction between fluorine and hydrogen is limited by diffusion. With the objective to improve the process, Elmore et al.3 performed experiments and developed theoretical prerequisites of the process for phosphate defluorination in a melt. Their work was further developed in experiments conducted by Hignett and Hubbuch.4 These experiments were performed in a shaft furnace in which a process of defluorination took place in a melt. Oil was used as * To whom correspondence should be addressed. Present address: Station 87, Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada. Tel.: (613) 584-8811 ext. 4805. Fax: (613) 584-8213. E-mail:
[email protected]. † The Gas Institute, National Academy of Sciences of Ukraine. ‡ Institute of Engineering Thermophysics, National Academy of Sciences of Ukraine.
a fuel, and air was used as the oxidizer. Combustion products were blown through the melt, and at low water vapor concentration in the combustion products, a final product was obtained with a F2 content of 0.5%. After the water vapor content in the combustion products up to 12% was increased, the fluorine content decreased to 0.2%. The melting temperature of the charge (the charge contains 20-25% of silicon dioxide) was about 1450 °C at an oil consumption of 190 L and an energy consumption of 130 kW‚h/t of final product. The final product content was as follows: P2O5, 2630%; CaO, 38-42%; SiO2, 20-25%; (Al; Fe)2O3, 7-12%. Hydrothermal methods of decomposition of raw phosphorus materials have some advantages compared to chemical and electrochemical methods. These advantages are a one-stage process, low consumption of energy, and acid-free processing. In the former USSR a hydrothermal method was developed at NIUIF and implemented at a chemical plant in Sumy (Ukraine). This method consists of reprocessing fluorine apatites with steam in rotating kilns at a temperature above 1450 °C. The main point of this process is the hydrothermal decomposition of the fluorine apatite molecule:
2Ca5F(PO4)3 + 2H2O f 2Ca5(OH)‚(PO4)3 + 2HF 2Ca5(OH)‚(PO4)3 f 2Ca3(PO4)2 + Ca4P2O4 + H2O In the presence of silica, the first stage of the process can be rewritten as
6Ca5F(PO4)3 + 4H2O + SiO2 a Ca5(OH)‚(PO4)3 + 2HF + SiF4 The product obtained has a high concentration of P2O5 and can be used as a fertilizer because it has better
10.1021/ie030527v CCC: $25.00 © 2003 American Chemical Society Published on Web 11/08/2003
6698 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
physical properties than superphosphate. However, rotating kilns are huge furnaces with a length of about 100 m and a low production capacity of 4-6 t/h. Therefore, it is impossible to construct a facility with a large unit capacity. Later researchers from NIUIF and MEI (Moscow Power Engineering Institute, Russia) developed a hydrothermal method of reprocessing phosphorus materials by melting them inside a cyclone furnace.6-9 Operation of the melting cyclones at the Dzhambul superphosphate plant (Kazakhstan) for reprocessing phosphorites into defluorinated phosphates showed that at a nominal production capacity the degree of defluorination is 80-85% and the fluorine content in the final product is 0.4-0.6%, 2 times higher than the permitted limit. An increased degree of defluorization can be achieved with a decrease in production at the same heat flux. The latter study points out that the time the melt stays inside the cyclone is less than the required time for the complete defluorization reaction. Despite this, the use of cyclone furnaces permits a highly productive process. However, this process is intended only for production of quality defluorinated phosphate (without using oxygen) from fluorine materials with low melting temperatures. Therefore, an objective of the current work is to develop a large-capacity and high-thermal-efficiency chemical reactor-melting converter with submerged combustion for production of molten defluorinated phosphates of high quality and low fluorine content.
Table 1. Operational Characteristics of the Research and Pilot SCMs for Production of Molten Defluorinated Phosphates SCMs parameter reaction zone volume, m3 capacity during continuous operation, t/h melt temperature, °C natural gas consumption, m3/h combustion air flow rate, m3/h air preheating temperature, °C specific heat density, kW/m3 of molten materials specific heat consumption, kJ/kg of product specific natural gas consumption, m3/t of product content in product, % fluorine P2O5 (total) P2O5 (salt-soluble)
Research pilot Sumy Gas Institute chemical plant 0.36 0.28
0.84 0.75
1390 76 675 25 2128
1540 280 2400 400 3373
9797
13649
270
370
0.17 27.3 20.5
0.15 39.8 39.0
2. Submerged Combustion Melter (SCM) 2.1. Main Concept. The Gas Institute of the National Academy of Sciences of Ukraine, together with the chemical plant in Sumy (Ukraine), proposed a method of producing molten defluorinated phosphates in the SCM13,14 using their previous experience with submerged combustion.15-23 The technological process is as follows. A natural gas-air mixture is injected through the lower part of a melting bath underneath the melt. (Such a combustion process is called submerged combustion.) Combustion products bubble through the melt and therefore mix it. Charge is continuously supplied into the melting zone, and melting occurs as a result of interaction with the melt and combustion products. Molten and defluorinated material is drained from the melting bath and undergoes intensive cooling in water (i.e., quenching). The charge inlet and melt outlet are located at opposite ends of the melting zone. Submerged combustion relies on such properties of the combustion products as their increased reactivity during their production, which is due to the formation of monatomic hydrogen as a result of breaking up of the molecules of hydrocarbon fuel during combustion. The location inside the melt of high-temperature zones with increased reactivity of the gas phase, the existence of a developed interphase surface, and intensive mixing lead to intensification of the defluorination process. The large specific heat of the molten materials and their intensive mixing help fast melting of coarse-grade and even-lumpsize material loaded into the melt. The ability to use lump-size charge allows reprocessing of raw phosphate materials and significantly decreases dust entrainment compared with rotating kilns and cyclone furnaces. 2.2. Preliminary Test Results. With the objective to verify this concept, i.e., production of defluorinated
Figure 1. Photograph of the pilot SCM in Sumy. Melt draining.
phosphates inside the SCM, tests were performed in a two-chamber laboratory melter with a reaction chamber volume of 0.36 m3 built at The Gas Institute24 (Table 1). Preliminary results showed a possibility of reprocessing raw phosphate materials by melting them inside a SCM and producing defluorinated magnesium phosphate with a satisfactory content of assimilated forms of P2O5 and sufficiently complete extraction of fluorine. To verify the proposed method with charge similar to the chemical content of reprocessed materials inside rotating kilns, a pilot SCM was built at the chemical plant in Sumy (Figure 1 and Table 1). The main differences between the research SCM at The Gas Institute and the pilot SCM in Sumy are air preheating and an enlarged volume of the melting chamber in the pilot SCM (see Table 1). During operation, the prepared charge of the apatite concentrate with 2% sand as an additive was sprayed with phosphoric acid and loaded with a feed screw conveyer into the upper part of the first melting chamber. Melt from the first melting chamber was supplied through the opening underneath the separating water-cooled partition into the second melting chamber. In the second melting chamber, the final stage of defluorination took place and the melt was drained through the tap for quenching and granulation. Enlargement of the Sumy pilot SCM in size compared to that of The Gas Institute, and air preheating to 400 °C, allowed melt temperatures beyond 1500 °C. There-
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6699 Table 2. Content of the Major Components in Samples of Molten Defluorinated Phosphates from the Pilot SCM in Sumy time of content of P2O5 (%) sample melt pickup temp saltinsoluble (h + min) (°C) total soluble fluorine CaO residue 22 + 10 23 + 0 0+7 1 + 31 2 + 25 3 + 30 4 + 30
1535 1545 1535 1560 1560
39.5 38.7 39.9 40.1 39.6 39.5 40.7
39.1 38.5 39.9 39.84 38.7 39.5 40.6
0.130 0.070 0.074 0.070 0.200 0.110 0.057
11.3 12.3 10.2 10.2 12.4 11.0 10.2
49.7 49.0 49.1 49.1
Table 4. Chemical Content of the Molten Phosphates after the Viscosity Determination (%) P 2 O5 sample no.
SiO2
CaO
total
saltsoluble
1 2 3 4 5 6 7 8
3.62 3.48 3.84 3.20 6.48 9.50 13.15 15.85
51.3 50.1 48.5 47.1 50.2 48.0 46.5 50.2
39.46 41.08 42.36 44.28 38.38 36.75 34.29 37.50
39.24 39.39 42.30 42.20 33.31 36.11 34.29 33.90
calculated
F2
39.5 41.3 42.8 44.5 38.4 26.9 35.5
0.14 0.12 0.09 0.09 0.12 0.10 0.09 0.05
Table 3. Component Content of the Charge for Determination of Viscosity phosphoric acid defluorinated ground recalculated sample phosphates quartz sand on P2O5 strength 70% no. (weight parts) (weight parts) (weight parts) (volume parts) 1 2 3 4 5 6 7 8 a
100 100 100 100 100 100 100 100a
3.1 6.2 9.3
4 8 12
3 7 11
Defluorinated phosphate from an experimental converter.
fore, defluorinated phosphates were obtained with a higher P2O5 content compared with the laboratory SCM at The Gas Institute. The results of these tests are listed in Table 2. The final product was characterized by high coefficients of transformation of nonassimilated forms of P2O5 into the assimilated forms. High coefficients of transformation in these samples, together with the low content of fluorine, supported the possibility of obtaining a quality product in the continuous operational regime. These results were used to construct an industrial pilot SCM with a production capacity of 4 t/h at the chemical plant in Sumy. 2.3. Determination of the Molten Phosphates Viscosity. To correctly choose the thermal regime for reprocessing raw phosphate materials by melting them in a SCM, the physicochemical properties of the phosphates have to be known. However, the data for molten phosphates viscosity are very limited. Therefore, tests were performed to determine the viscosity of molten defluorinated phosphates from the apatite concentrate with additives of ground sand and phosphoric acid. These tests helped obtain viscosity data of real phosphate melts and determine the effect of the above-mentioned additives on the fluidity of the melt. Melt viscosity25 was determined using a spindle viscometer within the range of 0.1-20 Pa‚s and over the temperature range 1300-1650 °C. A test sample was melted in a molybdenum crucible, and the melt was kept in an overheated state up to 30 min. After that, a molybdenum spindle was inserted into the melt. The depth of insertion was controlled with a micrometer device. The viscosity was determined as the melt temperature decreased. Component contents of charge for which the viscosity was determined are listed in Table 3. The main component of these charges is defluorinated phosphate, a product of the chemical plant in Sumy.
Figure 2. Effect of temperature on the viscosity of defluorinated phosphates (charged with H3PO4). The polytherm number corresponds to the sample number in Table 3.
Using defluorinated phosphate instead of the apatite concentrate was done to exclude the effect of the fluorine content in the melt on viscosity properties of the melt during the test. Results of the chemical analysis of the defluorinated phosphates after the viscosity tests are listed in Table 4. The results of viscosity determination of phosphates with phosphoric acid as the additive (sample nos. 2-4) are shown as the viscosity polytherms in Figure 2. Phosphates with sand as the additive (sample nos. 5-7) and molten defluorinated phosphate from the SCM (sample no. 8) are shown in Figure 3. The viscosity polytherm of the primary defluorinated phosphate without additives (sample no. 1) is shown in both figures (Figures 2 and 3) for comparison. Figure 2 shows that the solidification temperature of the melts with phosphoric acid as the additive is the temperature corresponding to a viscosity of 1 Pa‚s. For the melts with sand as the additive (Figure 3), the temperature corresponded to 0.5 Pa‚s. Such a boundary is conditional but allows an approximate determination of the temperature interval between regions of fluidity and solidification. In this interval the most significant changes in viscosity behavior take place. This interval is about 70 °C for sample nos. 2-4 and 20-80 °C for sample nos. 5-7. All tested contents belong to the so-called “short” melts with a small temperature range between regions of fluidity and the beginning of solidification. This
6700 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003
Figure 5. Effect of the silica content in molten phosphates (phosphates from apatite concentrate) on the fluidity of the melt.
Figure 3. Effect of temperature on the viscosity of defluorinated phosphates (charges with ground quartz sand). The polytherm number corresponds to the sample number in Table 3.
Figure 4. Effect of P2O5 (as additive) on the melt fluidity of molten defluorinated phosphates from apatite concentrate.
applies especially to the phosphate melts with sand as the additive (sample nos. 5-7, Figure 3). The experimental data (Figure 2) show that for melt fluidity it is necessary to have the lower limit of temperature for both groups of phosphates equal to the temperature corresponding to a viscosity of 0.5 Pa‚s. Using these polytherms, which correspond to a viscosity of 0.5 Pa‚s, a plot was produced for the effect of temperature versus the amount of phosphoric acid (H3PO4) as the additive to the phosphate, based on the fluidity of the melt (Figure 4). A similar plot was produced for the effect of temperature and silica content in the melt (Figure 5). In Figure 4, beside the isocom (curve of equal viscosity) for 0.5 Pa‚s for comparison, there is an isocom for 0.2 Pa‚s, which corresponds to the droplet-liquid state of the molten phosphates. On the basis of the results of viscosity determination of the molten phosphates, the following conclusions can be made. Adding phosphoric acid to the primary phosphate base decreases the melting temperature as well as the temperature of the fluidity region of the melts. Thus, the melt fluidity region of the defluorinated phosphate without any additives (sample no. 1) corresponds to a temperature of 1600 °C and higher (Figures 2 and 3).
At the same time, when just to 8 parts of P2O5 (weight parts equal to 10 volumetric parts of the 70% H3PO4) are added, approximately 100 weight parts of the defluorinated phosphate decreases the temperature boundary of the fluidity region to about 200 °C (Figure 4). With a larger amount of acid added to the phosphate, the melt fluidity region temperature boundary can be decreased further. This leads to two positive results: (1) energy consumption decreases and (2) the percent of assimilated P2O5 increases. With an increase of the silica content up to 10-12%, the melting temperature and temperature boundary of the fluidity region decrease (Figure 5). Further increasing the SiO2 content in the melt leads to a further melting temperature increase and a raising of the temperature boundary of the fluidity region. With a SiO2 content in the melt of about 10-12%, the fluidity region was recorded at a temperature of 1380 °C. At the same time, with 6% or 16% content of SiO2 the melts of the investigated contents became fluidic only at temperatures higher than 1500 °C (Figure 5). Although sand is also a cheap fusing agent, in our opinion it is better to use phosphoric acid as a fusing agent (flux) during melting of high-melting phosphates because adding it to the phosphate base does not decrease the phosphate content (as is the case for sand addition) but increases the phosphate content in the final product (Tables 2-4). It can be concluded that the optimal addition of phosphoric acid as the fusing agent is 2-3 weight parts of P2O5 to 100 parts of primary phosphate base. These materials were used as the basis for the development of the technology for production of molten defluorinated phosphates from apatite concentrate with the SCM method at the chemical plant in Sumy. 2.4. SCM Method for Production of Molten Defluorinated Phosphates. 2.4.1. Industrial Pilot SCM. 2.4.1.1. Design. To develop the SCM method for production of molten defluorinated phosphates in the continuous regime, a production line was built on the industrial pilot SCM at Sumy chemical plant with a production capacity of 4 t/h. The production line26 (Figure 6) consisted of a SCM, an air heater, a melt granulation system, a turbocharger, and an instrumentation system. The SCM is a metal bath with water-cooled walls. The internal walls of the melting bath are covered with the melt lining from the molten material. The melt lining
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6701 Table 5. Content of Major Components in the Charge and Molten Product (%) molten product charge without H3PO4 P2O5 total
CaO
37.62 37.84 37.78 37.89 37.65
49.76 49.68 49.37 49.60 49.41
P2O5 charge with H3PO4
F2
insoluble residue
P2O5 total
F2
2.75 2.97 2.56 3.02 2.93
5.73 5.45 4.81 3.86 7.70
39.52 39.59 39.10 39.45 39.51
2.70 2.53 2.64 2.60 2.64
Figure 6. Technological scheme of industrial pilot facility: 1, stock bin; 2, batch box; 3, conveyer; 4, H3PO4 sprayer; 5, mixer; 6, feed screw conveyer; 7, SCM; 8, granulator; 9, steam outlet; 10, bucket elevator; 11, granule bin; 12, cooling water inlet and outlet; 13, air heater; 14, exhaust gases outlet; 15, natural gas inlet for burners; 16, pressurized air inlet for burners.
prevents walls from corroding and decreases heat losses (i.e., serves as thermal insulation). The melting bath consists of two chambers with melt flow underneath the partition. Two burners are installed in each bottom water jacket. The apatite concentrate is loaded into the space above the melt level in the loading chamber into the flow of hot gases flowing from the melt-draining chamber. The SCM vault is made in such a way that it directs the flow of gases in which charge is loaded onto the melt mirror. This feature decreases entrainment of the charge and improves the separation of discrete particles-melt ejections. Further improvements in the SCM design can be attained by changing from water cooling to evaporative cooling and including the use of two-phase thermosiphons in the most heat-loaded places (burners, etc.).27,28 Also, the heat from the evaporative-cooling system can be recovered later. The charge was apatite concentrates of 25 kg of P2O5 (in the form of phosphoric acid) in 1 t of charge. A piston feeder was used for loading the charge into the converter. The average capacity of a melt was about 2 t/h. 2.4.1.2. Test Results. During operation of the converter, it was found that bottom burners installed underneath the melt worked efficiently, providing routine operation of the converter. During the first days of operation, the melt was completely drained off from the lower tap in the bottom during full operational breaks. Later, during full operational breaks, the gas supply was interrupted but air still was supplied through the burners underneath the melt. The melt was cooled quickly, and air from the burners prevented blocking of the burner nozzles. Also, air created voids in the melt, so it was possible to restart the burners and to melt the solidified materials in the melting bath. The final product quality was good. The coefficient of transformation of P2O5 in soluble form in 0.4% HCl was high in all samples (about 97-
total
saltsoluble
citric acid soluble
CaO
F2
41.20 41.19 41.00 41.19 41.16
40.62 40.80 40.74 70.88 70.74
40.00 40.15 40.21 40.17 40.35
47.32 47.30 47.50 47.65 47.73
0.051 0.052 0.044 0.120 0.051
99%). The content of citric acid soluble P2O5 in the final product was higher than expected, which is characteristic of defluorinated phosphate as a fertilizer. The content of citric acid soluble P2O5 reached 38% in the molten defluorinated phosphate. (In general, the content of this form of P2O5 is about 26-29% in the final product obtained in rotating kilns.) Sample analysis showed that defluorination of the primary charge inside the SCM took place very intensively and completely. Therefore, the following test meltings were performed without a water-cooled partition between the melting chambers with the objective to decrease heat losses inside the bath and also to investigate the possibility of operating with a single chamber in the continuous regime. Experimental meltings of the second series were performed with charge consisting of apatite concentrate, silica (6-8%), and extraction of phosphoric acid up to 40 kg of P2O5 in 1 t of charge. Gas consumption was 1200 m3/h during loading of the 2.5 t/h charge. Without charge loading, the gas consumption was decreased to 800 m3/h. In this case the temperature regime was unchanged. The results of the chemical analysis of the charge samples and final product performed at NIUIF are listed in Table 5. The results show that during continuous loading of the charge and melt draining in the singlechamber melting bath the defluorination process is intensive, the fluorine content is much less than the permitted limit, and the phosphoric product from the primary raw materials was almost completely transferred into the assimilability form. To determine the time molten materials stay in the melting chamber, tests were performed using the method of “sample labeling”. Cobalt nitrite was used as the tracer. Changing the tracer content in the charge and melt with time is shown in Figure 7. The average time of material staying inside the melting bath is determined by the time in which the maximum tracer content inside the melt corresponds to that at the bath exit. This time is about 20-25 min, about 4-6 times longer than the time required to reach the desired level of defluorination. The dust content in the exhaust gases downstream of the recuperator was also determined during SCM operation. The average concentration was about 1 g/m3 according to the data of NIFKhI (Scientific-Research Physico-Chemical Institute by Karpov, Moscow, Russia). Analysis of aerosols showed that aerosols originating through condensation are 50 wt % with an average particle size of 0.5-0.7 µm. Dispersion aerosols have larger size particles and are about 30 wt %. The results for fluorine and P2O5 determination in the exhaust gases showed that 80% of fluorine substances are in the form of hydrogen fluoride and 20% are in the form of silicon tetrafluoride. The content of P2O5 was
6702 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 6. Economic Comparison for Production of Defluorinated Phosphates in Rotary Kiln, Fluidized-Bed Furnace, and SCM furnace production parameter output,a
t of product per year unit output, t/h no. of units capital investment, millions of monetary units specific capital investments, monetary unit/ t of product product cost, monetary unit/t of product a
rotary kiln
fluidized-bed furnace
SCM
300 000 4 6 21
300 000 5 4 20
300 000 20 1 13
135
146
92
59
64
53
Recalculated to content of P2O5 of 26%.
Figure 7. Changes in the time of indicator (cobalt nitrate) content in the charge and in the melt at the outlet.
not more than 0.12 g/m3. The appearance of P2O5 in the gas phase is explained by intensive evaporation of phosphoric acid, which was used for wetting the charge loaded into the converter. Meltings of the first and second series of tests showed that the process operates well with a high-melting charge. It was found that some melting converter systems required improvements. The first improvement relates to the charge-loading system and melt-draining system. It was noted that periodically a loading system piston seized during loading of a powderlike charge wetted with phosphoric acid and due to that disposed to grouting. Melt draining was difficult because of fast solidification of the melt at the outlet of the tap hole. This is explained by the small melt jet diameter and melt viscosity properties.25 Therefore, to improve the loading method, the charge was granulated in the third series of tests and was loaded through the feeder pipe installed into the melting bath through the vault panel. It was found that loading the granulated charge through the feeder pipe could be efficient if gases did not escape from the pipe. To avoid this, the feeder pipe should be 1.5 m from the surface of the melt inside the bath. In addition, it was proposed to install a ring burner (gas-air burner with preliminary mixing) on the end of the feeder pipe to apply “flame blow out”. The results of tests of melting charge with various silica contents showed that the best fluidity of the melt was with the charge containing 8% SiO2 (Figure 5) and 40-45 kg of P2O5 (in the form of phosphoric acid) as the additive per 1 t of charge (Figure 4). The final product had high quality, which supported the results of the chemical analysis: a total content of P2O5 of about 40.8%, which includes about 40.7% P2O5 soluble in 0.4% solution of hydrochloric acid (the so-called “feeding part”), about 39.8% P2O5 soluble in the 2% solution of citric acid (the so-called “fertilizing part”), and about 0.06% fluorine (data from NIUIF). Apatite concentrate with P2O5 up to 60 kg/t of charge was also melted. The analysis results (Sumy chemical plant) are as follows: P2O5, 41.91%; fluorine, 0.12%. These results were used in a technical-economical comparison of molten defluorinated phosphates produced in melting converters, in rotating kilns, and in fluidized-bed furnaces (Table 6). However, the results listed in Table 6 do not account for the effect of producing the final product on a large scale.
Figure 8. Scheme of industrial SCM for production of molten defluorinated phosphates: 1, tap hole; 2, SCM case; 3, tubes of water evaporative cooling; 4, melt lining; 5, submerged burners; 6, ring burner; 7, vibrator; 8, corrugated joint; 9, sand lock; 10, loading pipe; 11, steam pipe; 12, boiler tubes; 13, support; 14, boiler case; 15, charge preheater; 16, exhaust gases duct; 17, loading bunker.
2.4.2. Concept of Industrial SCM. The industrial SCM concept is shown in Figure 8 and its major calculated parameters are listed in Table 7. The industrial SCM for production of molten defluorinated phosphates operates similarly to the industrial pilot SCM built in Sumy. The major differences between the industrial SCM and previous SCM designs (Table 7) are (a) larger capacity (10 t/h), (b) higher thermal efficiency due to granulated charge heating, combustion air preheating, and heat utilization from the exhaust gases for steam production, and (c) the possibility of using natural gasoxygen burners. 3. Effect of Molten Defluorinated Phosphates Application in Agriculture In general, phosphates soluble in citric acid are better to use when their fast effect is not necessary and when the fertilizers can be used well ahead of plant growth.29 This timing is best for effective and economical use of fertilizers. Field tests showed that application of phos-
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6703 Table 7. Technical Characteristics of the Industrial SCM (Calculated Values) characteristic
unit
value
melting capacity natural gas consumption natural gas pressure combustion air consumption combustion air pressure amount of combustion products temperature of combustion products fluorine content in the combustion products
t/h m3/h MPa m3/h MPa m3/h °C g/m3
10 2700-3000 0.18 up to 30 000 0.16 up to 35 000 1600 3.5
Charge Preheater width height inlet combustion product temperature outlet combustion product temperature
m m °C °C
2.4 4.6 1100 800
Melter inlet charge temperature outlet charge temperature combustion product pressure drop
°C 20 °C 900 MPa 0.12-0.15
Combustion Air Heater heat-transfer area inlet air temperature outlet air temperature air pressure drop combustion product pressure drop
m2 °C °C MPa MPa
400 50 400-450 0.01 0.02
Heat Recovery Boiler heat-transfer area inlet combustion product temperature outlet combustion product temperature combustion product pressure drop steam capacity
m2 °C °C MPa t/h
220 450 250 0.20-0.25 25
Melter
Melter steam pressure
MPa 1.2
phoric-potassium fertilizers once in 3 years can save up to 50-75% of the total costs. By using regular fertilizers for 4 years, there were 10, 80, and 50% of phosphorus in the upper soil layers. This phosphorus appears in the form of superphosphate, phosphate flower, and basic phosphate slag (in general, phosphate slag is similar to the molten defluorinated phosphates). The rest of the phosphorus was drained in the lower soil layers. By comparison, the molten phosphates are less subject to degradation. Therefore, application of the citric acid soluble fertilizers (molten defluorinated phosphate) allows an increase of 2 times the coefficient of phosphorus usage. In general, phosphates and potassium residues are fixed in the soil or in the form of easily movable compounds drained out from the soil beyond the plant roots, leading to saturation of the watering area with phosphorus and therefore leading to growth of bluegreen water plants.30 4. Conclusions On the basis of the results of a large volume of scientific, design, and development studies, it can be concluded that the technology developed for production of molten defluorinated phosphates in the SCM has the following advantages compared to current technologies: (a) There is a possibility of designing a large-capacity facility on a much smaller site and with significantly improved technical-economical and ecological parameters. (b) Molten defluorinated phosphate is produced in the form of granules, which are not subjected to bonding,
are hydrophilic, and do not require special packaging and storage. (c) Production of the molten phosphate does not require use of sulfuric acid. (d) Multiple field tests performed on various soils and with different agricultural plants showed that the effectiveness of the molten defluorinated phosphate is equal to superphosphate. (e) Application of the molten defluorinated phosphates as fertilizer prevents contamination of the phosphogypsum soils and water systems and therefore contributes to protection of the environment. (f) Granulated forms of the molten defluorinated phosphate do not create dust and therefore create more comfortable conditions for workers using the fertilizer. (The process actually decreases the number of pulmonary illnesses.) (g) Molten defluorinated phosphates can find wide application in rice production, because it makes the rice stems stronger and prevents the stems from lying flat. Literature Cited (1) Giulietti, M. Clean Process for the Production of Defluorinated Dicalcium Phosphate Using Phosphate Rock. J. Hazard. Mater. 1994, 37 (1), 83-89. (2) Aly, M. M.; Mohammed, N. A. Recovery of Lanthanides from Abu Tartur Phosphate Rock. Hydrometallurgy 1999, 52 (2), 199206. (3) Elmore, K. L.; Huffman, E. O.; Wolf, W. W. Defluorination of Phosphate Rock in the Molten State. Ind. Eng. Chem. 1942, 34 (1), 40-48. (4) Hignett, T. P.; Hubbuch, T. N. Fused Tricalcium Phosphate. Ind. Eng. Chem. 1946, 38 (12), 1208-1216. (5) Vol’fkovich, S. I. Progress in Chemistry and Chemical Technology of Phosphorous Fertilizers (in Russian). Adv. Chem. 1956, Vol. XXV, Issue 11. (6) Vol’fkovich, S. I.; Ionass, A. A.; Postnikov, N. N.; Remen, R. E.; Sidel’kovskiy, L. N.; Shuryigin, A. P.; Derevitskiy, P. F.; Yagodina, T. N. Hydrothermal Process of Natural Phosphates Defluorination in a Cyclone Furnace (in Russian). Chem. Ind. 1959, 11 (8), 28-34. (7) Vol’fkovich, S. I.; Ionass, A. A.; Mel’nikov, E. B.; Remen, R. E.; Sidel’kovskiy, L. N.; Troyankin, Yu. V.; Shuryigin, A. P.; Yagodina, T. N. Hydrothermal Reprocessing of Phosphates in a Cyclone Furnace (in Russian). Chem. Ind. 1961, 13 (6), 24-29. (8) Vol’fkovich, S. I. Cyclone-Melting Power-Technological Processes (in Russian); Metallurgizdat Publishing House: Moscow, Russia, 1963; p 84. (9) Vol’fkovich, S. I. Hydrothermal Reprocessing of Phosphates for Fertilizers and Forage (in Russian); Khimiya Publishing House: Moscow, Russia, 1964. (10) Bekturov, A. B.; Serazetdinov, D. Z.; Urikh, V. A. PhysicoChemical Basis for Production of Polyphosphate Fertilizers; Nauka Publishing House: Alma-Ata, Kazakhstan, 1979. (11) Wzorek, Z.; Kowalski, Z. Calcium Feed Phosphate Production Using the Low-Temperature Method. J. Loss Prevent. Process Ind. 2001, 14 (5), 365-369. (12) Bilali, L.; Aouad, A.; El Harfi, K.; Benchanaˆa, M.; Mokhlisse, A. Pyrolysis of the Moroccan (Youssoufia) Rock Phosphate. J. Anal. Appl. Pyrolysis 2002, 65 (2), 221-237. (13) Zakharikov, N. A.; Vilesov, G. I.; Gorbik, P. I.; Pioro, L. S.; Osnach, A. M.; Gol’derbiter, M. S.; Chechik, B. S.; Kilochitskiy, I. M. Production method for defluorinated phosphates. USSR Patent 285,934, 1964 (published in 1970). (14) Pioro, L. S.; Vol’fkovich, S. I.; Osnach, A. M.; Pirogov, V. I.; Kravchenko, A. I.; Chechik, B. S.; Karpovich, E. A.; Ayvazov, A. A.; Galina, V. M. Production method for phosphate fertilizers. USSR Patent 465,082, 1971. (15) Pioro, L. Betriebsintensivierung der Bestehenden und Entwicklung von Prinzipiell Neuen Schmelzaggregaten (in German). International Congress on Glass, Praha, C ˇ SSR, 1977; pp 408-417.
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the Apparatus of Converter Type (in Russian). Chem. Technol. 1974, (5), 8-10. (25) Osnach, A. M.; Pioro, L. S.; Bryantsev, B. A. Viscosity of Melts of Defluorinated Phosphates (in Russian). Chem. Ind. Ukraine 1970, (1). (26) Pioro, L. S.; Osnach, A. M.; Olabin, V. M.; Pirogov, V. I.; Vilesov, G. I.; Kravchenko, A. I.; Karpovich, E. A.; Sadovskiy, B. F. Production of Molten Defluorinated Phosphates in Apparatus of Converter Type (in Russian). Chem. Technol. 1975, (4), 3-5. (27) Pioro, I. L. Investigation of Closed Two-Phase Thermosyphons Used for Cooling of Melting Converters (in Russian). In Applied Problems of Heat Transfer and Hydrodynamics; Naukova Dumka Publishing House: Kiev, Ukraine, 1982; pp 76-79. (28) Pioro, L. S.; Pioro, I. L. Industrial Two-Phase Thermosyphons; Begell House: New York, 1997. (29) Kuk, D. U. Fertilization System for Obtaining Maximum Crops (in Russian); Kolos Publishing House: Moscow, Russia, 1975. (30) Demolon, A. Croissance des Ve´ ge´ taux Cultive´ s; Dunod: Paris, France, 1956.
Received for review June 30, 2003 Revised manuscript received September 17, 2003 Accepted September 21, 2003 IE030527V
