that in the reduction process óf phosphate pellets with silica and carbon, the ... neutralises the phosphorus to copper phosphide and water cleans the gas stream ...
OPTIMISATION OF PELLET REDUCTION I N A PHOSPHORUS FURNACE C. Dresen', J. H.L. Voncken', W. Schipper^, R. de Ruiter^ and M. A. Reuter' 'Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Applied. Earth Science, Raw Materials Technology Group , Mijnbouwstraat 120, Delft 2628 RX The Netherlands ^Thermphos International BV, PO Box 406, Vlissingen 4380 AK The Netherlands
Abstract This paper discussed the reaction kinetics of the reduction of industrial phosphate pellets for P4 production. In the reduction process of various phosphate ores, the following parameters were investigated: •
Diffusion of the phosphorus oxide and other gaseous products from the reaction mixture after having been reduced on the reductant (coke) surface. • Influence of silica on reaction kinetics. " Temperature influence. o Influence of mineralogy and morphology (different ores). " • Particle size influence of silica and coke. Experiments were carried out in a vertical tube furnace with 2 ores, 2 types of silica and one type of coke. Selected experimental conditions simulated furnace conditions. The following conclusions could be made: o Reaction rate increases with the addition of Si02 into the pellet. o The reaction rate increases above 1350°C (melting temperature ofthe slag). • Mixing different ore types in the pellets leads to differences in reduction kinetics. Particle size of the coke particles influence reduction kinetics. The amount of slag formation is influenced by silica addition, coke size and temperature.
EPD Congress 2002 •Edited P.R.Taylor Fundamentals of Advanced Materials for Energy Conversion Edited by D. Chandra and R.G. Bautista TMS (The Minerals, Metals & Materials Society), 2002
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Introduction Tlie industrial production of white phosphorus is facilitated in submerged-arc furnaces. These furnaces are fed with a mixhire of raw materials: apatite (phosphate rock), silica (slag former) and coke (reducing agent). The phosphate ore is pellètised to ensure that the furnace bed remains stable and that permeability is maintained. Although the reaction mechanism of the P4 production is well documented, some unclarity exists with regard to the effect of mineralogy on the reduction mechanism. This paper discusses the influence of particle size mixing, temperature and ore source on thé reaction kinetics. Phosphate Reduction Mechanism The general reduction reaction of phosphate ore, can be summarised by the following reaction equation: Ca^,{PO,),F^ +\$C + 9zSiO^^
3P,(g)+9[CaO.&-03),] + C«F, +15C70
(1)
As illustrated by this equation, fluorapatite is reduced in the presence of silica and carbon to form calcium fluorite, calcium silicates, phosphorus gas and CO-gas. Thé reaction given above can be simplified to demonstrate the reaction mechanism using pure tricalciumphosphate. The first step is the liberation of the phosphorus from the óre as phosphoms oxide and at the same times thé formation of calcium silicates. This step is considered to be the rate-determining step, since phosphoms has to difflise from the tricalciumphosphate ore in a form of phosphoms oxide. Most of the P^Oy is assumed to be present as P2O5. Ca, {PO, )2 +
WaO • SiO, + Pp^ (g)
2PAy(g) -^ZyC -> xP,(g) + 2yC0(g)
(2) (3)
Finally the liberated phosphoms is transformed into its final form by cooling below 90Ö°C. 2P,(g)^P,(g)
(4)
During the different reactions various parameters seems to have a large influence of thé overall process, which include: o The diffiision ability, that describes thé ahility of the phosphoms oxide to diffuse from the apatite to the carbon as well as the removal of the gaseous products from the carbon surface after reduction of PxOy to P2. o The influence of silica oh the reaction kinetics. o Temperature influence. * The influence of mineralogy ofthe ores.
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I •
Particle size influence of silica, coke and phosphate ores. •
s According to Konevskii and Tyunni'^'^ one of the most important aspects in the reduction rate óf phosphate ore,is the removal of the gaseous reaction products. Laboratoi-y tests demonstrated that for both sedimentaiy ore as well as magmatic ore the reaction kinetics were higher when the gaseous products (phosphorus and CO) were removed with a N2 gas current. Mu et al.'^'^-' concluded from their investigation, that in the reduction process óf phosphate pellets with silica and carbon, the reaction rate contrdlling step is the gaseous removal bf reaction products. Silica is an additive, which promotes the reaction, not only by providing a thermodynamic driving force, but also by modifying the melting phenomena. Besides silica also alumina has a fluxing effect on thé reactión kinetics, biït less then silica. There are 2 types of silica; 1) silica already naturally present in the phosphate ore as feldspar and 2) silica added in the form óf sand, pebbles or quartz in addition to the phosphate ore, to accélerate the reaction kinetics and lower the initial reaction temperature. According to Dorn^'*-' the influence of an increasing basicity SiOa/CaO, ranging from 0.75-1.1 is described as follows: Ah increase in basicity from 0.75-0.90 resulted in a PaOs-siag content decreasing from l.ll%-0.39%. Further increase of the basicity to 1.0 leaves the P2O5 content on a level of 0.21%, but with this increase also a new reaction started to cornmence. Si02 is reduced by carbon to SiO (g), which affects the refractories ih the furnace and dilutes the phosphorus product. The viscosity of the slag is very impprtailt for the slag handling. In the binary CaO-Si02 diagrams an eutecticum can be found at'a SiOa/CaO ratio of 0.8-0.9. This explains the slagharidling optimurn. Jacob and Reynolds^^-' demonstrated that the reduction reaction starts at 1150°C. In a stable reaction environment the reduction reaction was completed in 1 hour at a temperature of 1325 "G and in 10 minutes at 1500 °C. Raw Material used in the Experiments Based On the conclusions of the previous investigation^^^ the experiments were earned out with a mixture of 2 ore types: o Magmatic phosphate oré. o Sedimenatary phosphate ore XRD analyses revealed that magmatic ores consist mainly of fluorapatite, calcite, and dolomite. Sedimentary ores consist of fluorapatite and quartz with sometimes calcite. Van der Pas'^^^, in a previous study conducted in co-operation with Thennphps International B. V , it was established that a mixture of magmatic/sedimentary ore of 70/30 led to an optimum in reduction kinetics. For the present investigation, the ores were mixed in a ratio of magmatic/sedimentary of60/40,70/30 aiid 80/20 to determine whether this optimum in reduction kinetics for a 70/30 mixture was accurate. To determine the influence of silica addition into the phosphate pellets on the reduction kinetics, quartz was used.
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After the experiments were carried cut to detennine the effect of pure silica in the phosphate pellets, the effect of sand in the pellets was also investigated. The sand was analysed and dried before it was mixed and ground with the phosphate ore for the pellet production. For reducing the PxOy formed in the pellets during the reduction process, Chinese coke was used. The coke was cioished and ground to a dgp size of 106.44 micron. The coke was dried ahd analysed. The pellets for this iilyestigation were produced at Thermphos International B.V. under normal pelletizing conditions and also tested for strength. Carbon cruipibles were used for the reduction experiments to ensure that optimal reducing conditions were present under the nitrogen atmosphere in the fiirnace. The graphite material of the crucible contains respectively 0.10 % and 0.60 % of ash and volatile matter.
! ll.-ln[i-,fLflj I -.
LL
, lJrtri»-/.rt
r- ^ lUllc-rÜkÜ
Il,.jil.-l1lfr.l vnhdiMt, llnr-llMfli/ ftl.-.U.Wdl
^ 1
: c-«-i,-nij: M
Dischqrga in fume cupboard
Fig. 1 - The experimental setup illustrating the thermal balance for the measurement of reduction kinetics of phosphorous. Experimental setup The reduction experiments were earned out in a vertical Carbolite furnace. During the reduction of phosphate ores the gaseous reaction products were passed through two cleaning bottles, containing copper sulphate ( C U S Ö 4 ) and water. Copper sulphate neutralises the phosphorus to copper phosphide and water cleans the gas stream fi'om the remaining phosphorus. During the reduction process it is essential that the gas removal system of the experimental set-up be under a slight negative pressiire, accomplished by using a water jet vacuum pump.
438
An electronic balance continuously measures the weight loss of the material in the crucible on a lifting tube. iSince the entire system has to be airtight, the balance was placed in a balance box, which is attached to the underside of the furnace. The Mettler Toledo balance is connected to a Computer, permitting the loss of . weight to be registered in an Excel spreadsheet. The balance box with balance and lifting tiibe can be lowered using an elevator system. Befoire each experiment, thefiamaceis heated to the required temperature and then the elevator system is lowered to place or replace the crucible with sample. The entire system is elevated again and secured to the furnace with bolts. Thefiirnacetube is made of AlSint (dense sintered AI2O3), while the crucible is placed on a lifting tube of silimanite. The length of this is such that the crucible is placed in the centre ofthe hotspot ofthe furnace. The top and bottom flanges are cooled with coding water, which is circulated through the system. During the reduction experiments, the furnace is kept under a N2 atmosJ)here, by flushing the system with an N2flow of0.151/min. Aiialvtical Techniques After the experiment the crucibles were weighed to determine the loss of carbon (from both crucible and coke) and the loss of phosphate pellet. The samples from the reduction experiments were analysed with different analytical techniques: ' • X-ray' diffraction (XRD) for mineralogy determination. • X-rayfluorescence(XRF) for semi-quantitative elements content. t> Some specific important samples were analysed with electron microprobe analysis (EMF) for pfoduciiig quantitative spot analyses, scans of pplished surfaces from sample cuts and photos of mineralogical and morphological structures, o Resin Was poured in the crucibles after which they were Crosscut to demonstrate the reaction profile, From these crosscut a polished sample was prepared for microprobe analysis Experimental results The pellets produced for these experiments were also carefiilly examined with respect to pellet strength and pellet porosity. As the paper focuses on the reactioïi kinetics, these results are not mentioned in this paper. Reduction Experiments To compare the reduction kinetics of the different pellets the assumption was made, that the loss of weight ofthe niixtures measured during the reaction With the balance installation, represented the amount pf P2(gas) and CO(gas) leaving the fiirnace. The calculation of the amPuiit of P2O5 reduction was made by assuming that the reduction reaction took place according the equatiotis 2 to 4. Using this equation the data obtained with the online weight loss measurements could be transformed to graphs demonstrating the amount of P2Ö5 reduced versus the time.
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The amount of phosphate reduction, calculated according to the reaction equations, did not equal the exact amount of phosphate reduced, shown by XRF analyses of the reaction products. The calculations made above hayé to be corrected with a number of con-ections, most notably for extra burn off of carbon, reduction of other metals (impurities) in the ore, such as FeaQa, ZnO, MnO, SO3, IC2O, etc. All data presented have been conected for these effects. Influence of silica This paragrajDh discusses the mfluence of silica in the form of quartz (SiOa) powder on the reduction kinetics of phosphate pellets. The experimental results will be demonstrated with P2O5 reduction versus time graphs and tables. The reduction experiments were canied out at 1350°C and 14ÖÖ°C. From Figure 2 it can be observed that ovër a period of 300 minutes the reduction kinetics are higher with a 60/40 20% SiOz mixture then with a 60/40 30% SiOi mixture. From the microprobe photos (bottom) can be obseived that the amount slag formation is much higher with 20% SiOa addition (bottom left) than with 30% Si02 addition (bottom right), in which the pellet struchire is still visible. In case ofthe 30% Si02 addition the pellet tends to form a slag layer at the outside of the pellet, obstructing the reaction process. From the shape of the curves can be noticed that different reaction mechanisms take place. The course of the 20% SiOa addition curve can be explained by the formation of polycalium silicates instead of monocalcium 'silicates due to the lack of silica, which require a higher temperature and more energy. The reaction temperature should be increased to continue the reaction. To investigate the behaviour of the pellet reduction process at a higher temperature, the same batch of experiments was carried put at 1400°C. The experiments were canied out at I400°C first for 90 minutes and later for 300 minutes. At 1400°C, 300 minutes experiments, the addition of quaxtz in the pellet leads to an increasing amount of P2O5 reduction. In this case the 60/40 30% $i02 mixture has been reduced almost completely, while in thé 60/40 20% SiOz mixhires the amount Pf P2O5 reduced for 75%. Backscattered electron images (not shown) Of the reaction products demonstrated that for (60/40 20% Si02) the slag formed at the bottom of the crucible is not unifoiin, but still contains phosphate grains and intermediate reaction products. Whereas for the other (60/40 30% SiOz) the slag is completely uniform, with no traces of phosphate grains. AlthPugh initially the reiductlon rate for the 20 % SiOi mixture is higher, is slows down after a While. The earlier decreasing reduction rate of the 60/40 20% Si02 mixture can be explained by the lack of Si02 present to form monocalcium silicates, so polycalciufn silicates havé to be fpnned.
440
Fig. 2 - Results of reduction experirnents at 1350°C. Shorter curves represent 90 minute experiments/long curves represent 300 minutes experiments. Images shown at the right are photographs of crosscut ci-ucibles, irnmCrsed in resin. White squares indicate areas, which were selected for investigation with the electron microprobe, after preparation of special polished sections from them. Photographs at the bottom are Backscattered Electron Images taken &om the polished sections.
t|mi (min)
Fig. 3 - Pellet reduction over a period of 300 minutes at 1400°C. !
AM
1
Influence of Coke Particle Size In the experiments can'ied out in the previous paragraphs the coke (reducing agent) was placed into the crucible in powder form. The dso of the powder was 31.74 micron. In the plant operations coke is added together with the pellet in the form of large fragments, ranging form 10-50 mm. To determine the influence of the coke particle size, three experiments were caitied out with a 60/40 30% Si02 mixture: o no extra coke, only the carbon cmcible as reduction agent, e coke fragments, screened at 10 mm. This size was practical for the laboratory experiments o coke powder as used in the previous experiments. The experiments were cairied out at 1400°C with silica inside the pellet. The results are demonstrated in Figure 4, including to the right crosscut photos of the crucibles after the reaction.
Fig, 4 - Reduction results of coke particle size experiments From the Figure 4 can be noticed that the reduction reaction will have a maximum P2O5 reduction When cöke powder is used. When no extra coke is added in the crucible the reduction will depend fully on the carbon cmcible and reduction will be low. When coke is added next to the pellet the reaction rate is increased together with the slag formation. When the coke is added in powder form next to the cmcible, the reduction kinetics increases more due to the further increasing amount of available reaction surface of the coke.
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Different ore mixtures Based on preceding investigations (Van der Pas^'^) experiments were earned witli three types of oremixWres; 60/40, 70/30 and 80/20 magmatic/sedimentaiy. The objective is to detennine an optimum in ore mixture composition. The amount of silica added was such that the ratio SiO2/CaO=0.88 (called "maximum SiOa"). It was observed that the phosphate pellets completely feacted to form a calcium silicate melt. The experimental reaction products were weighed to determine the loss of carbort and phosphate ore, m this case the remaining slag. This slag was analysed with XRP analysis to determme the remaining content of P2O5. With these measurements and analyses calculation could be can-led out to determine thè amount of reduced P2O5. These results are demonstrated in Table I. It appears that the reduction kinetics are the highest for the 60/40 mixture. Table I -P^O-;, Reduction results ore mixtures Tmaximum Si02l Peircentage p^Q^ ^j^g^^ption T = 1350°C_ 60/40 " " ' " 2 8 : 4 % " 70/30 23.8% 80/20 19.7% Ore Mixture Magmatic/Sedimentary
of Bercehtage of P2O5 Reduction T-140Ó°C _ Time =120 min .. 90^ 81 % 82 % _
Plant Simulation Experiments i n the previous paragraph the influence of quartz (silica) addition and the mixtures of different ores in the pellet on the reduction reaction have been investigated_ and described. These experiments were canied out with quartz addition, which is an idea pure form of siUca but which for practical/econpinic reasons can not be used m plant operations. In the real plant operations the Si02/CaO ratio ofthe feed tp thefomaceis controlled by the addition of silica pebbles. To investigate the real influence of silica in the pellet instead of next to the pellet, a set experiments were canied out simulating the process in the plant. Pellets were produced with addition of sand, in addition to sand placed beside the pellets in the cmcible. It appeared that the reduction Percentage of P2O5 increases from 11% with the 60/40 ref 3 mixture (all the silica is added outside the pellet in form sand) to 90% with the 60/40 30% sand addition mixture With no sand addition the pehet stmcture stays visible with a homogeneous phosphoms distribution. When sand is added the pellet dissolves Completely m a liquidus slag. Discussion of the Reduction Mechanism To determine the reaction mechanism of the pellet reduction, first a distinction is made between changing the reaction parameters inside or outside the pellet. Through observing and describing the effects of the pellet reduction separately a better understanding can be obtained of the overall mechanism. The changmg parameters inside the pellets are the addition of quartz or sand and the mixtures of different ores. Parameters outside the pellets are the size of coke particles and the removal of the gaseous reaction products. The temperafore parameter exerts mfluence both inside and
443
outside the pellet. A higher temperahrre favours the formation of mono- pr , polycalcium silicates, The PxOy gas has to escape from the pellet fu-st before it can be reduced by carbon at the coke surface. Before being removed, the PxOy gas has to be liberated first froih the apatite. First a closer look will be taken to the process of liberation and diffiision frpm the pellet of the P^Oy gas. After the PxOy has been removed from the pellets the mechanism ofthe reduction reaction at the carbon particles can be considered. Influence of parameters changes inside thepellet o
o
e
• o
The percentage P2O5 reduction increases rapidly when a considerable amount of calcium silicate slag is formed. When the pellets start to react with the slag, the reaction will progress at a higher rate ,. An increasing amount of silica addifiPn leads to an increasing amount pf calcium silicate slag fonnation and thus to an increasing amount of reduced P2O5. (The reaction has progressed fiirther.), The amount of slag formation at 14O0''C is obviously considerable more than at 1350°C, (The reaction has progressed fiirthei?). The range between 1350°C and 1400°C seems to be a transifion area, in which the liquidus temperature of the calcium silicate slag fprmation is exceeded. " The calcium silicate front is moving fi-om the edges of the pellet towards the centre: (silica present in the pellet), In the reduction experiments with different ore mixtures With maximum SiOi addition, it can be Pbserved that the 60/40 mixture has the highest values for the amount of recjuced P2O5. An explanation for this is that the mixture 60/40 could benefit more frptn the qualities of both ores, as established in a previpus study [6]. (According to this sfiidy, magmatic ore has a lower required reaction heat, but sedimentary ore has a higher reduction rate).
Influence of coke outside the pellet The effect of coke is clearly observed in Figure 4. When the coke size decreases inore reaction surface will be available, resulting in a higher PxOy reduction rate, which will favour the liberation of PxOy fi-om the apatite and finally the fonnation of calcium silicate slag due to the liberation of CaO. Comparison of sand and silica addition A comparison was made between the experiments with 30% quartz or sand addifipn into^ the pellet. From the results it is clear that the reduction kinetics for sand addition is higher than for quartz addition. With sand addition, in the first stage ofthe reaction, the reduction rate is faster than with the quartz powder. This can be explained with the presence of feldspar, which due to the presence of alkali's will melt at a lower temperature and react with the calcium phosphate. This initial islag formation may fijncfion as an accelerator. With this early formation of a liquidus slag the PxOy can diffuse more easily.
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Possible reactión mechanism i n the previous sections different effects have been discussed concerning the effect of the parameters on the increase of P2Ö5 reduction fi-Oni the pellets. From all the listed facts the conclusion can be made that the reduction reaction is increasing more rapidly when calcium silicate'slag is formed. When the SiOa hgs reacted with the calcium phosphate, the P^Oy can diffusefi-omthe apatite and the remaining GaO can react to form a calcium silicate slag. In this process also the silica rich slag reacting with the GaO could be thé driving force of liberating the PxOy.
In summary, the mechanism may be described as follows. When silica is added in the pellets at a SiOi/GaO ratio of 0.88, the rèaction will be much faster than with silica outside the pellet. In this reaction the temperature has to be at least 1400°C and sufficient cokes have to be added (Stage 1). The reaction front will be moving in the form of a melt front from the edges of the pellet towards the centre (Stage 2). In this melt fi-ont the SiOa in the pellet will react first with the melt followed by the GaO. This effect was observed with measuring SiOz/GaO ratios (electronmicroprobe) in the slag in the melting front, At this reaction front the PxOy is liberated from the apatite, It is not clear whether all the PxOy is liberated through the reaction of GaO with the slag or that some GaO is first formed, which then reacts with the slag after the P^Oy is liberated. Due to the generated PxOy gas a melt is formed of a foaming structure (Stage 3). The PxOy gas is forced Out of the slag structure via gas bubbles. Thé PxOy gas moyes towards the coke where it is reduced to P2 and CO (gas). When the amount of PxOy in thepellet/slag is decreased to a low level, the pellet will collapse and fonn a liquid slag (Stage 4 and 5). The rate-limiting step in this process can be either the liberation of PxOy or the removal ofthe PxOy from the pellet towards thé coke. Arguments in favour of the parameter of liberation of the PxOy are the following. The excess of Si02 in the slag (SiOz seems to dissolve first) favours the liberation of the PxOy through reacting with the GaO ofthe apatite, In the binary diagram (Figure 5) can be seen that starting out with silica, with an increasing amount of GaO the liquidus temperature of thé slag is decreasing. The slag also becomes less viscous. This might be the thermodyhamic driving force. The actual liquidus temperature, however, is about 50 degrees lower then in the binary diagram, because ofthe presence of K and Na in the firmace feed, and because of the presence of fluorine (F) liberated from the apatife-ore (Ca5(P04)3F), All three elements (Na, K, F) are known to decrease the liquidus temperahire of silicate melts considerably.
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Mole fraction SiOj
Moss 7, Sip,
Fig. 5 - Tiie system CaO-Si02. The effect of PxOy removal as rate controlling element can be explained assuming the viscosity of the slag decreases with more CaO addition. Conclusions From the study the following conclusions can be drawn: p
The reduction rate will increase with the addition of fine silica into the pellets, instead of in the fomi of sand outside the pellet. o The effect of silica addition in the pellet is considerable larger at 1400°C than at 1350''C.
