Improvement of the production of Rhenania phosphate fertilizer by means of investigations in a laboratory rotary kiln reactor

Powder Technology, 23 (1979) 1 - 14 0 Elsevier Sequoia S-A., Lausanne - Printed in the Netherlands

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Improvement of the Production of Rhenania Phosphate Fertilizer by Means of Investigations in a Laboratory Rotaq Kiln Reactor

H. JANTZEN, Instilut

K. SCHUGERL

fiir Technische

(Received

Chemie

and H. HELMRICH der Technischen

Universitiit.

Hannover

(F.R.G.)

April 26, 1978)

SUMMARY

The reaction of the ‘raw mixture’ of soda, quartz sand and fluorine apatite into the wellknown Rhenania phosphate fertilizer with an ammonium citrate (Petermann) solubility of about 98% was investigated in a laboratory rotary kiln oven in a temperature range of up to 1300 “C. The concentration of the products in the outlet gas, the properties (rheology, ammonium citrate solubility, DTA, X-ray structural analysis) of the solids and the dust formation and composition as well as the sublimation of fluorine compounds during the heat treatment were investigated as a function of the reaction time for different programmed heating-up rates, rotational speeds and properties of the ‘raw mixtures’. During the hear;treatment the reacting solid undergoes two sintering states in the temperature ranges 250” - 500 “C and 1050” - 1200 “C due to two consecutive reactions. This sintering causes a fusing of the solids to the wall in the first range and/or fusing together to large granulates in the second range making it difficult to handle the sohd mixture. It also reduces the reaction rate and the product quaIity deteriorates. By means of the analysis of the process, the formal kinetics of the consecutive reactions which lead to the destruction of the apatite crystahine structure (and to the ammonium citrate solubihty) and the rate-determining steps were determined; furthermore the fluerine recycling was proved and the processes which cause the sintering were evaluated. As a result of these findings the following improvements were achieved: (1) Dust formation (30% loss of solids) and the occurrence of the first sintering state of

the reaction mixture were avoided by applying raw material in form of granules with medium to high strength and mean diameter larger than a critical value. (2) By applying these granules the occurrence of the second sintering state was also avoided by limiting the reaction temperature to 1000 “C without reducing the ammonium citrate solubility and increasing the necessary reaction time. (3) By omitting these sintering states the solid load in the oven, the throughput and the productivity were increased by 100% at a constant mean residence time of the solid mixture together with an improvement of the product quality.

INTRODUCTION

One of the most important reactors for high temperature solid state reactions is the rotary kiln. This type of reactor has a series of advantages which explain its wide use in industry: simple construction and great flexibility; utilization of the heat content of the flue gas by counter-current operation; simpIe control of residence time of the reaction mixture in the kiln; the kiln can be fitted easily to the actuaI operation conditions by varying its length, diameter, inclination, speed of revolution and by applying built-in constructions; kilns can be built to very high throughputs. Rotary kilns 5 m in diameter and 220 m long are in operation; processes with very corrosive reaction mixtures can be carried out. The layer of ‘raw mixture’ and product protect the wall of the kiln from the aggressive melt;

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processes at very high temperatures can be carried out. The Iayer of ‘raw mixture’ and product insulate the wail of the kiln fIom the high temperature of the melt; sticky reaction mixtures can also be handled; the process does not depend on the particle size. One can apply mixtures consisting of lumpy particles together with powders. The most important disadvantages of rotary kiIns are: low productivity with regard to the reactor voIume; Iow efficiency in energy utilization; difficult control of temperature and composition of the reaction mixture along the kiln at large distances from its two ends. At the present time the rotary kiln is the only reactor to carry out several types of high temperature reactions in metaIIurgicaI, inorganic-chemical and construction-materiaI manufacturing industries_ In spite of the importance of rotary kilns in industrial production, our basic knowledge is modest in this field. With the exception of one paper on ca.Icination [1] and two papers on metaIIurgicaI application [2,3], no paper exists which deals with the chemical engineering probIems of very corrosive high temperature reactions in rotary kilns. The aim of the present paper is to investigate such a reaction in a high temperature rotary drum batch reactor to evaluate data for the optimal construction and operation of the industrial unit. As model reaction the production of a phosphate fertilizer from fIuorapatite, soda and quartz sand was chosen.

EXPERIMENTAL

Reaction According to Rothe and Brenek [4 3, during the anneabng process of the mixture of fIuorapatite, soda and quartz sand an aIkaIi-dicaIcium phosphate is formed and the caIcium is displaced by alkali, and the excess Iime is bound by the silica as orthosihcate: 2Ca,(PO&

+ SiOs + 2NazCOs

2(NazO - 2CaO. P,O,)

=

+ 2Ca0 - SiOa + 2COz (1)

2CaCOs + SiOz = 2CaO- SiOs + 2COs

(2)

Since in practice fluorapatites are applied, eqns. (1) and (2) are to be modified to Ca10(P04)sF2 6CaNaP04

+ 3NazCOs

f 2CazSi04

Ca,0(P04)sFs

+ 3CO2 + 2HF

f 4NasCOs

GCaNaPO, + 2Ca2Si04

+ 2SiOa + Hz0 = (3)

+ 2SiOz =

+ 4COs + 2NaF

(4)

From eqn. (3) the development of gaseous hydrogen fluoride due to water vapour and from eqn. (4) the extra soda consumption due to the binding of Ruorine as sodium fluoride can be recognized. The destruction of the lattice of apatite due to the removal of the fluorine from the raw phosphate (fluorapatite) is of the utmost importance for the success of the annealing process_ The aim of the annealing process is to destroy the apatite lattice by the removaI of the fIuorine and thus to make the phosphate utilizable for the plants. The fluorine removed from the Iattice remains in the final product (phosphate fertihzer) in a form which has not yet been elucidated. The mechanism of the fluorine removal and the after-reaction of the fluorine in the appIied phosphate mixture is not known. Presumably the fluorine removal begins at a temperature of 800 “C and is accelerated with increasing temperature [5]. At 1100” 1200 “C the fluorine removal is complete. It is believed that the fluorine is removed not as hydrogen fluoride but as SiF, or HzSiF,, which reacts with the raw mixture. The removed fluorine, compounds partly volatilize (sublimate) and are transported by the counter-current gas to the preheating section of the kiln, where they desubhmate. However, as they are transported back into the reaction section they do not react to yield fluorine apatite, but they presumably form alkali fluorides, which solidify together with the silicates and phosphates. Since NaF can easiIy react wi+h phosphates and silicates in the melt, it can form monofluorinesihcate, whose structure corresponds to that of the AaX04 type due to the equivalence of oxygen and fluorine. Therefore it is possibIe that fbrorine is built into the mixed crystal formed during the annealing process [S] _ This assumption is supported by the fact that only one part of the fhrorine compounds remaining in the fertilizer is water-soluble.

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This ability of the components to exchange thus agrees with TrSmel’s statement that the compound ‘Rhenanit’ found on the X-ray diagram is isomorphic with the ternary compound found in the alkali-free system CaQ-PsOsSiOs: Ca1s(P0&(Si04)s. Based on microscopic and X-ray analysis, one can conclude that crystals of cu-CaNaP04 prevail in Ca orthosilicate and thus only at very low cooling-down rates is the /I modification formed. The crystals probably consist of a solid solution of 2CaO - NasO - P205 and 2CaO- SiOa, which probably contains small amounts of 3CaO-P,Oa and CaO. They form rounded-off grains which are embedded in a glassy mass. The latter is presumably a eutectic mixture of fluorides, phosphates and silicates formed during the decomposition of apatite.

Equipment

The following requirements were expected of the laboratory rotary kiln reactor: batch operation; operation temperature up to 1300 “C; the temperature time function should be programmable; the speed of rotation should be variable; the reactor tube should be easily exchangeable; sampling of solids should be possible at all times; continuous measurements of the temperatures of the gas and solids should be possible; gas composition should be measurable continuously;

it should be possible to take photographs and tine pictures of the reaction mixture during the reaction; the reactor tube should be resistant to the reaction melt. In accordance with these requirements the reactor consisted of a sintered carborundum tube with 99.6% Also,, impermeable to gases, 220 mm id., 240 mm 0-d. and 960 mm long. It was embedded in a layer of rock-wool 120 mm thick (Fig. 1). The insulated reactor was enclosed in a cage consisting of two stainless steel barrel rings 540 X 300 X 10 mm in outer and inner diameter and in width and of six screwed spindles, which kept the two barrel rings at a parallel distance of 900 mm. The two end covers of the oven were equipped with openings for the heating rod. The speed of rotation could be varied between 0.45 and 4.5 r.p.m. Continuously operated at 1250 “C in the oven, the temperature of the outer mantle remained below 60 “C. Through one of the end covers one could take samples from the reaction mixture in any section of the oven. This opening could also be used to take photographs or tine of the reaction mixtire at any time. The heating rod, 33 mm diam. and 1420 mm long, with a heating section 864 mm long, consisted of silicon carbide. Its two ends were coated with metal to diminish the resistance between the rod and its electrical connections_ Since the silicon carbide reacts with the volatile fluorine compounds, it was protected by a sintered corundum tube 45 X 39 mm in outer

Fig. l_ Laboratory rotary kiln reactor. 1 reaction volume, 2 sintered corundum reactor tube, 3 rock-wool insulation, 4 barrel ring, 5 spreader bar (tie rod) with tube adjusting device, 6 outer mantle, 7 silicon carbide heating rod with its protecting tube, 8 opening for sampling, 9 end covers with rock-wool insulation, 10 dust-protecting cover.

and inner diameter, 1200 mm long. The electrical circuit of the heating rod allowed a maximal Ioad of 100 A and 100 V (for details see C7J)The temperature was measured in the reactor by a Pt/PtRh 90/10 ‘Thermocoax’ thermocouple protected by a stainless steel tube and a gas-impermeable sir&red corundum tube. As soon as the temperature increased, the intensity of the radiation allowed the use of an optical pyrometer. Nitrogen gas saturated with water vapour was led through the reactor and a gas sample was taken from the centre of the reactor volume, and after the separation of water and dust from the gas CO= was continuously measured by an infrared analyser. Since the temperature has a large influence on the reaction, the longitudirral and transverse temperature profiles were measured in the reactor volume during its operation. In Fig. 2 typical longitudinal profiles are shown with and without insulation on the end covers_ The relative temperature with regard to the temperature at the centre is pIotted as a flunction of the longitudinal coordinate. In Fig. 3 a typical transverse temperature profile is shown between the heating rod and the reactor wall. The reIative transverse temperature with regard to the temperapare on the surface of the protecting tube of the heating rod is nearly constant_ One can recognize from Figs. 2 and 3 that in a section of the reactor volume about 70 cm long almost isothermal conditions prevailed. Because of this and since the intensity of transverse mixing is very high in comparison with the intensity of the longitudinal mixing [ 9,101, and since the samples were taken in the middle of the reactor, the applied equipment allowed isothermal measurements to be carried out if one converted a small amount i

d

_=--

-----=--

9 b

i

90 70 80 700

%

I 6o

I

z

1

Fig. 2. Typical longitudinal temperature profiles in the reactor. 2, longitudinal distance. - - -Without insulated end covers, -with insulated end covers. The dimensionless temperature is referred to the temperature in the middle of the reactor_

Fig. 3. Typical transverse temperature profde in the reactor. The dimensionIess temperature is referred to the temperature at the wall of the protecting tube of the heating rod, To ; r is the transverse distance.

of ‘raw mixture’ in the reactor. The transverse temperature distribution was furthermore measured in the gas phase and in the solid layer 2.6 1 in volume during the heating-up of the reactor at a rate of 6 “C/minute. As the temperature increases, the transverse temperature difference in the gas phase and the differences between the temperatures of the gas and the solids layer gradually diminish. However, also, at high temperatures (>800 “C) an almost constant difference between the gas and solid temperatures of about 20 “C remains. During the rotation of the reactor one of the thermocouples was periodically immersed in the solid layer. The periodical variation of the temperature of this thermocouple with a frequency corresponding to the speed of reactor rotatior, indicates the relatively low time-constant of the thermocouples. Therefore, if the thermocouple is immersed in the solids layer, it can also follow the temperature variation of this layer if the layer is heated up quickly. Experimental

condr’fions

The ‘raw mixture’ consisted of fluorine apatite, soda and quartz sand according to eqn. (4). The p&cIe size distribution of the raw mixture was wide. Soda and apatite had small size and quartz sand medium size. 76% of the ‘raw mixture’ had a particle diameter befxeen 0.4 and 0.1 mm; 6.5% was smaller than 0.063 mm. The bulk dens+ amounted to 1.2 g/ml. The ioad volume was varied between 1 and 6 I. The corresponding load ratios are given in Table 1. Below 11 load volume the concentrations of COP in the gas samples were too low; above 3 I the thermocouple could be destroyed by the reaction mass, which became sticky at given temperature ranges. Therefore most investigations were carried out with

TABLE 1 Loads of ‘raw mixture’ Load volume (1) 1 2 3 4 5 6

powder

Relative load (VOI.%) 3.0 6.1 9.1 12.1 15.2 18.2

(kg/m3 1 37.6 75.2 112.7 150.3 157.9 225.5

11(3 vol. 9%) ‘raw mixture’ (runs A) 2 l(6.1 vol. W) ‘raw mixture’ (runs B) 3 l(9.1 vol. %) ‘raw mixture’ (runs C) Based on the investigations of Akerman [lo, 111 as well as of Merz and Vogg [14] on industrial rotary kilns 50,75 and 45 m long producing the same fertilizer (Fig. 4), the heatup rate of the reaction mixture passing through the kiln was calculated from the measured longitudinal temperature profiles and passage times. By means of the maximum heating-up rate of 6 “Cimin of the laboratory reactor, the heating-up rates of the industrial reactor investigated by Akerman and Vogg could be approximated (Fig. 5). Therefore the investigations were carried out with a heating-up rate of 6 “C/min. Under these conditions the properties of the reaction mixture vary with the temperature as follows: At first, up to 1000 “C, a weak calcine is formed, consisting of a darkcoloured porous and bubble-containing basic material, which becomes glassy after cooling down. This basic material consists of Na-Ca phosphate of varying composition. The apatite lattice can be clearly recognized by X-ray measurements in &is material. In this mass several white ro.:+d grains are embedded, consisting of Na-Ca silicates with an approximate composition NaaSiaOs-CaaSiO.+ The density of this weak calcine was 1.26 g/cm3. The diameter of its grains depend on the load of the kiln. With 3,6 and/or 9 vol. % load the weak calcine has granulates 30 mm, 50 -70 mm and/or 90 mm in diameter. At a higher load the size of the grains is so large that they can easily destroy the thermocouple. In the second phase of the reaction, up to 1250 “c, the finaI product is formed. It is uni-

m

30

LO

50

Fig. 4. Longitudinal temperature profile T, = f(s) and passage time t = f(s) in an industrial rotary kiln during the production of the phosphate fertilizer according to Akerman [lo, 111.

form and corresponds to the industrial product and has an ammonium citrate solubility (Petermann solubility) of 99%, Le. 99% of the phosphates are available for the plants [7,12,13] _ This is due to the destruction of the apatite structure, as the X-ray investigations indicate. According to the measurements of Jantzen [7], there is a strong correlation between the amount of CO* appearing in the gas phase and the Petermann solubility. Therefore it is possible to follow the reaction by measuring the concentration of the COP in the gas phase of the reactor volume. The concentration of CO=, measured continuously by the inliared analyser, appears as a periodical function of the time. The Bequency of the fluctuations is determined by the speed of rotation of the kiln_ This indicates that the COs, formed by the decomposition of soda, is reIeased periodically from the solid layer. The greater the volume of the solid layer the greater are the amplitudes of the fluctuations. With increasing speed of rotation the amplitude gradually diminishes. Format

kinetic measurements

To evaluate the formal kinetics of the reaction, a 13 g sampIe of ‘raw mixture’ was added to the pre-heated kiln at once (within 1 set), uniformly distributed along the axis within the 420 mm long middle section of the kiln, and the concentration of the COs in the gas

Fig_ 5_ Comparison of the heat-up rate of an industrial rotary kiln 75 m long (0) with the maximum and real heat-up rate of the laboratory rotary kiln.

sample measured. Seventy l/h nitrogen gas, saturated with water, passed through the volume of the oven. The sampling rate was kept constant. ln this way the amount of COa formed by the decomposition of the soda could be estimated_ These measurements were carried out in the temperature range 200” 1250 “C under isothermal conditions at temperature intervaIs of 50 % (21 different temperatures). Since the non-isothermal and isothermal measurements as-well as the DTA (differential thermal analysis) indicate that two main reaction regions prevaiI, these two regions are discussed separately: (1) The reaction starts at about 250 “C with a maximum COa concentration at about 70 sec. With increasing temperature, up to 500 “C, this maximum shifts to shorter times, but the total amount of CO2 formed during this reaction remains constant. It was assumed that the increasing part of the concentration-time curve is due to the heating-up and to the induction period of the reaction. To make sure that only data which were evaluated under isothermal conditions were used, only the parts of the concentration-time curves after the inffexion point were used to evaluate the formal kinetics of the reaction. Since this part of the curve follows an exponential function, from its slope the rate constants of the reaction first order were calculated. (2) The second reaction range begins at 850 “c with a longer reaction time and larger amount of CO2 generation than range (1). The evaluation of the kinetic data for this

heat-up rate

range was similar to (1). Since the temperature interval of this reaction range was large enough, it was possible to plot the rate constant on the Arrhenius plot (Fig. 6) to evaluate the apparent activation energy of the reaction to 0.26 kcal/moI = 1.09 kJ/mol. This result indicates that a transport process is the ratedetermining step. Fluorine

recirculation

Based on some indirect evidence it is believed that a fluorine recirculation in the industrial rotary kiln exists. However, up to now it has not been possible to discover the fluorine compound which is transported upstream to the particle movement, and no direct evidence has been found forsuch a transport across the gas phase. To find direct evidence for the fluorine recirculation, the kiln was filled with 2 1 ‘raw mixture’ and rotated at 0.45 r.p.m. Seventy l/h nitrogen saturated by water vapour was fed through the oven. The oven was kept at a temperature of 800” - 850 “C for two days. During this time white deposit appeared on the cold surfaces at the reactor exit. Using pure, dry nitrogen, no deposit is formed. Deposit formation could only be observed with N2/Hz0 and N&Q as well as with Ns/COs/HsO. The Debye-Scherrer analysis indicated that the deposit consisted of chemically pure NaF. The analysis of the samples from the solid layer indicates that of the 3.2% fluorine originally present in the ‘raw mixture’, after this treatment approximately half of this amount (1.7 f O-17%) is still in the reaction mixture.

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Fig_ 6. Arrhenius

plot of the rate constants

of the second reaction phase in the temperature

range 950”

- 1190

%.

Another investigation with a higher fluorine content in the ‘raw mixture’, and those carried out under production conditions, indicate that a residue of about 1.6% fluorine cannot be removed from the mixture. One can therefore conclude that in the temperature range between 600” and 900 “C the reaction mixture is able to bind 1.6% fluorine in the lattice. The excess fluorine is in the form of NaF, volatilized and transported back by the gas flow and at low temperatures desubIimed. Therefore only this fluorine takes part in the fluorine recirculation.

‘weak bum-out’. This decomposition occurs formally according to a first-order reaction with a transport process as rate-determining step. (5) Second range of sintering between 1000” and 1250 “C. The ‘weak burn-out’ turns, under development of gas, into the final ammonium citrate-soluble product due to the destruction of the apatite lattice. This process is strongly endothermic_ The temperature temporarily decreases. The size of granules increases_ No sticking to the wall can be observed.

Non-&o

The main problem in improving production is to avoid the occurrence of the two sintering zones. To avoid this sintering, the taw mixture’ was diluted with different additives (SiOZ, A1203). However, either the sintering could not be avoided ( Si02) or the Petermann solubility was very low (A1203)_ Finally, by applying the ‘raw mixture’ as granules this problem couId be solved.

thermal reaction

The behaviour of the reaction mixture in the temperature range 25” - 1250 “C was estimated by means of photographic and tine pictures_ The following temperature ranges could be distinguished: (1) Preheating of the solids, which are in non-caking condition between 25” and 250 “C without any reaction_ (2) First range of sintering between 250” and 500 “C. One pa~i-tof the ‘raw mixture’ sinters to a sticky mass without melting. This mass fuses to the oven wall. (3) First range of granule formation between 500° and 850 “C. The mass fused to the wall is separated and forms granules 5 - 25 mm in diameter_ (4) The &id mixture reacts under strong degassing and sublimation of NaF between 850” and 1000 “C. During this reaction the granules survive. The ‘raw mixture’ turns into

APPLICATION

Qualitative

OF GRANULATES

considerations

By means of an eccentric press and using 2 tonnes pressure, tablets of ‘raw mixture’ 16 mm in diameter and 6 mm high were manufactured. By means of special equipment the abrasion was estimated: at 25 vol. % load and 2 hours, 10% abrasion for ‘raw mixture’; at 25 vol. %

8 TABLE 2 Comparison of the course of reaction with raw mixture as powder and as tablet. Operation conditions: 10 vol.% load, 1 r.p.m. speed of rotation, 6 “C/mm heating-up rate, 50 I/h N,/I&OCourse of the reaction Raw mixture as powder

Raw mixture as tablet

Up to 400 “C the atmosphere in the reactor volume is fuII of dust-

No dust in the oven

Between 400” and 500 “C the dust disappears due to the sintering of the ‘raw misture’. The sintered materiaI fuses to the wall.

No change in mobility and in mixing of the tablets. The reaction occurs within the tablets without interference between the tablets. The basic mass of tablets is dark in colour. White granuIes are embedded in this mass.

Between 500” to 850 “C formation of granuIes 1 - 30 mm in diameter. COa development up to 1000 “C.

No change in the rheologicai behaviour between 800” and 950 “C. COz development with constant rate. By increasing the rate of rotation the COz development can be accelerated.

Between 1100” and 1150 “C the granules begin to sinter again. ParaIIeI to this, COs development. Temperature decreases by 30 “C due to endothermal reaction.

At 2 100 “C the small white grains begin to disappear_ The tablets begin to stick together, but they are separated again and again.

As the temperature increases again the reaction is finished_

Temperature decreases due to endothermic reaction. As the temperature increases again the reaction is finished.

load and after 2 days degassing, 3% abrasion was measured. In order to compare the behaviours of the ‘raw mixtures’ applied as original powder and as tablets, parallel investigations were carried out (Table 2). One can recognize from Table 2 that characteristic dif?erences between the behaviour of the reaction mixtures applied as powder and as tablets prevaiI. In the foIIowing these differences wiII be discussed in detail. The first sintering occurs due to the Na-Ca phosphates which stick to the waII. White solid grains consisting of Na-Ca sihcates are embedded in the sticky mass of the phosphates. Since the surface of the tablets is covere 1 by these grains, which do not have a sticky property, the tablets cannot fuse to the waJ.I.No sintering and fusing occur.

The second sintering occurs due to the NaCa silicates, which become sticky at this high temperature (>l 000 “C). Therefore the sintering cannot be hindered by them in the case of the application of tablets either. Only if the tablets have a size larger than 8 mm in diameter, can this adhesion be partly overcome due to the weight of the tablets. The result of this observation is that one should try to avoid the high temperatures at which the Na-Ca silicates become sticky. Thus the occurrence of the second sintering could be omitted. Therefore the course of the reaction as a function of the temperature was carefully investigated. A comparison of the course of the reaction pIotted as per cent COs in the gas as a function of the temperature for raw mixtures as powder and as tablets (Pig. 7) indicates that the

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TABLE

3

Loads of ‘raw mixture’

10 9

Load volume (1)

B 7 6

1 2 3 4 5 6

5 ‘ 3 2

granules and powder

Relative load powder

granules

37.6 75-2 112.7 150.3 187.9

36.5 73.1 109.6 146.2 182.7 219.3

225.5

1

12

3

L

5

G

7

8

9

10

11

72

z iOL)T

Fig_ 7. Course of the reaction plotted as CO2(‘%) in the gas phase as a function of the temperature_ ----ct‘raw mixture’applied as powder, -A‘raw mixture’ applied as tablets.

reactions in both regions are quicker if tablets are applied, than those measured with powder. Therefore the reaction temperature can be reduced if tablets are used. The temperature difference between the peaks of the concentration-temperature curves measured for tablets and powder amounts to 120” - 150 “C (e.g. Fig. 7). This is probably due to the increase of the rate of the transport processes in the case of tablets. To investigate the possibility of diminishing the reaction temperature, the behaviour of the tablets during the reaction was investigated_ Changesof thepmpertiesof the tabletsduring the reaction A series of investigations was carried out using tablets 10.0 mm in diameter and 8.3 mm high. The applied loads are given in Table 3. In Fig. 8 the variations of the density, mass and volume of an average tablet are given as a function of the reaction temperature. One can recognize five reaction ranges: (1) swelling of the tablet due to COz development, (2) gas deveIopment and voIume contraction; (3) volume contraction due to sintering. No pores. Dark-coloured ground mass with white grams. Start of Petermann solubihty; (4) second swelling of the tablet due to COz development; (5) uniform sintering and degassing with permanent vohume contraction.

Fig. 8. Variations of the density, mass and volume of an average tablet as a function of the reaction temperature.

This course of mass variation with the course of the DTA.

agrees well

Investigations with ‘raw mtiture’ tablets Tablets 10 mm in diameter and 8 mm high were used, which were produced by a cylinder press with a pressure of 2 tons. The rotation speeds, the heatiig-up rates and the loads were varied. For alI investigations the concentration of COz in the gas phase as well as the Petermann solubility of the reaction mixture were measured_ In general there is good agree-

Trial 2810

100’

7.

75

-

50

-

25

-

Petermann = f (time1

0

Fig. 9. Comparison of the Petermann soIubZity as a function of the (G) and co2 loss (-=--) temperature_ Load 3 1, heating-up rate 8 “C/min_

100 %

600

CO2 = f [Heating rate1

9w

12ooYT

Fig. 10. Relative degassing (partial snm of the developed COz) as a tunction of the temperature for different heating-up rates: 3,6 and 8 “C/min.

ment between the changes in degas-sing rate (COa loss) and Petermann solubility (Fig. 9) with temperature_ The half-width of the CO= peak is also characteristic of the rate of the degassing, Le. of the reaction rate. This halfwidth is smaller, i-e_ the reaction rate is higher with tablets than with powder of the ‘raw mixture’. Furthermore, this haIf-width is independent of the speed of rotation (for powders as well as for tablets> and depends only

2

6

h

Fig. 11. Petermann soiubility as a function of the residence time for different heating-up rates.

slightly on the load in the reactor in the range between 1 and 6 litres. Thedegassing (COa loss) and the Petermann soIubility as a function of the temperature depend on the heating-up rate, as can be seen from Fig. 10. The higher the heating rate the later the reaction starts, but then the quicker the reaction proceeds and finally the earlier the reaction is finished (Fig. 10). With the knowledge of the longitudinal temperature profile in the kiln, it is possible to calculate the residence time of the reaction mixture for a given heating-up rate. In Fig. 11 the Petermann solubility is plotted as a function of the residence time at different heatingup rates (the heating-up rate 12 “C/min was evaluated by extrapolation). The heating-up rates 8 - 12 *C/min correspond to industrial practice. Estimation ture

of the upper limit of the tempera-

Because of the occurrence of the second sintering in the temperature range above 1000 “C, it was of practical importance to investigate if it is possible to carry out the reaction at 1000 “C, before this sinteringrange is reached, instead of raising the temperature to 1250 “C as is usually done in practice. Therefore a series of measurements was carried out with the following temperature programmes: heating-up rate 6 “C/mm, then keeping the reaction mass at a constant tem-

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perature, namely 800”, 850”) 900” and 1000 “C. The runs with the limiting temperature of 800”, 850” and/or 900 “C gave products with low Petermann solubilities. The runs with the terminal temperature of 1000 “C yielded products with 99% Petermann solubility (Fig. 12). Comparison of press granulates with roll granulates with regard to the operation of the rotary kiln To make it possible to transfer the results of the investigations carried out on a laboratory reactor to an industrial kiln, it was necessary to apply granules instead of tablets, since for industrial production only the former are suitable. Two different types of granules were applied: (1) The press granule (P), manufactured by means of a roll pressure briquette process, is 56 X 15 X 9 mm long, wide and deep. (2) The roll granule (R), manufactured on a granulation plate by means of 30% water. The granules (R) were screened and dried_ The product was separated into three fractions: 40% coarse granule (d > 5.5 mm), 20% intermediates (5.5 > d > 1 mm) and 40% fines (d < 1 mm). 9fl

100

‘A

Q

Pefemmnn

*co* 75

50

_.h

Fig. 12. (a) Degassing (sum of developed CO,) and Petermann solubility as a function of the reaction time. (b) Temperature as a function of the reaction time.

TABLE

4

Properties of applied granules Particle size (mm)

Bulk density (g/cm3 )

Press granules

5 - 10 4-5 2-4 l-2 0.5 - 1 0.25 - 0.5

2.21 1.08 1.08 1.08 1.10 1.19

Roll granules

5 - 10 4-5 2-4 1-2 0.5 - 1 0.25 - 0.5

0.77 0.84 0.72 0.86 1.01 1.09

The coarse granules were hollow and burst during the drying into two half-spheres_ In TabIe 4 the bulk densities of R granules and P granules are shown for different particle sizes. By applying granules only, the first sinter range can be avoided if the temperature is raised above 1000 “C. Therefore it was investigated whether the reaction course and the quality of products depend on the terminal temperature. No significant differences prevail between the two runs with 1000 “C and 1300 “C terminal (maximum) temperatures with regard to the Petermann solubility and the degassing conversion. However, during the runs with a terminal temperature of 1000 “C the reaction mixture remained non-caking and could be cooled down in a fluidized bed, in contrast to the runs with a terminal temperature of 1300 “C. The solids in the latter case sintered together and formed only three large granules, 63,68 and 91 mm in diameter_ The same is true of roll granules, as can be seen from Fig. 13. However, from Fig. 13 it can also be recognized that there is a significant difference between R and P granules. The larger porosity of the roll granules promotes the degassing and by this the reaction. This is a clear advantage of roll granules in comparison with press granules. However, they have also disadvantages- The surface of the roll granules is relatively soft, therefore the size h&ions 2 - 4 mm axe sir&red at 700 “C due to their own fines (abrasives). In the case of press granules this occurs only with the size fraction

12 Petermann

60

120

=

f IfI

180

240

Min

Fig. 13. Petermann solubility as a function of the reaction time for different granulates_ No. 812

1012 1112 1312 1412 1512 1612

Type

d (mm)

T maxW)

P R P R P P P

5 -10 5 -10 4-5 4-5 12-50 2-4 2-4

1000 1000 1000 1000 1000 1000 1300

1 - 2 mm. The expense of the manufacture of the press granules is lower than that of the roll granules. Furthermore, at the same Ioad volume the throughput is higher with press granmes than with roll granules due to the higher bulk density of the press granules.

Isothermal reaction w&h gmnuIes One can see from Fig. 13 that the reaction actually starts at 120 min and finishes at 240 min. This ‘induction time’ is due to the low heating-up rate of 6 “C/mm_ An increase of the heating-up rate was not possible because of the sensitivity of the reactor tube (sintered corundum) to quick temperature changes. Therefore investigations were carried out with the oven preheated to 1000 “C. At the time t = 0 the oven was stopped and 5 kg granules were put into the oven at once (within 80 set), uniformly distributed along the oven axis. The oven was started at the maximum speed of rotation (5 r.p_m.) and was heated by the max.!nmm electrical output until the temperature of 1000 “C was again reached. This took about 4 min. After that the temperature was kept constant at 1000 “C. The first sample was taken at t = 5 min and after that at time inteivaIs of every 5 min. The ‘raw mixture’ granules l-2 mm and 2 - 4 mm in diameterwere fused to the reactor

wall immediately after they were put into the oven. The ‘raw mixture’ granules 5 - 10 mm in diameter also fused to the wall, but kept their original granule shape. In Fig. 14 the degassing is plotted as a function of the reaction time for these granules under isothermal conditions at 1000 “C. One can recognize that no ‘induction period’ exists_ This indicates that this ‘induction period on Fig. 13 is due to the lower heating-up rate. The reaction occurs under these conditions within 60 min. However, these runs are unrealistic, because such high heating-up rates cannot

be realized in an industrial

rotary kiln

Soda reaction under isothermal conditions, degessing et T,OOO“C No. 700 %

- _ i _ -__

75

50

-.-__-

25

F

OO 1

20

._-

D

LO :x?

Sbrnl 3-f.

D

am

I-I

_-.-

40

Mm

60I

Fig. 14. Degassing (sum of CO2 developed) as function of the reaction time under isothermal conditions at 1000 SC. Press granulates: 0 2 - 4 uun &am.. 0 I2mmdiam.

13

reactor; furthermore, at these high heating rates the "raw mixture" granules are also fused to the wall.

P R A C T I C A L C O N C L U S I O N S F R O M T H E INVESTIGATIONS From the investigation of the annealing reaction of fluorine apatite, soda and quartz sand to manufacture phosphate fertilizer the main reaction ranges were discovered and assigned to definite chemical, rheological and morphological states of the reaction mixture. Great attention was paid especially to two reaction ranges in which the mixture becomes sticky, fuses to the wall and/or sinters together into large lumps and therefore becomes difficult to handle. The formation of these ranges can be avoided by applying granules of the 'raw mixture'. The investigation of the behaviour of these granules shows several advantages with regard to the handling of the reaction mixture in comparison with the "raw mixture" powder. It was possible to identify the fluorine compound which is transported upstream to the solid flow across the gas phase and which desublimes in the cooler regions in the kiln a n d is t r a n s p o r t e d b a c k i n t o t h e h o t r e a c t i o n r a n g e b y t h e s o l i d m i x t u r e . A b o u t 1.6~o o f t h e f l u o r i n e is b o u n d t o t h e g r a t i n g o f a p a t i t e . Only the fluorine in excess o£ this 1.6% takes part in this recirculation. The following practical conclusions can be drawn from the present investigations: (l) By applying Yaw mixture' granules, the dust formation which causes 30% loss of the solids can be avoided and one can save the investment for large dust separators necessa/T for such factories. (2) By applying the "raw mixture" as granules, especially press granules with high strength and 20 mm in diameter, the sintering of the reaction mixture and its fusing to the w a l l b e t w e e n 2 5 0 ° a n d 5 0 0 °C c a n b e a v o i d e d . ( 3 ) B y a p p l y i n g g r a n u l e s i t is p o s s i b l e t o limit the reaction temperature t o 1 0 0 0 °C and thus avoid the sintering of the reaction mixture that occurs between 1000 ° and 1 2 5 0 °C. (4) By preventingthe reaction mixture from sintering, the solid load and the throughput as well as the productivity of the kiln can be

increased by 100% at a constant mean residence time of the solid mixture. (5) Under these conditions the product has a uniform size, which is smaller than the average size of the non-granulated product. The latter has strongly non-uniform size distribution. Because the cooling-down rate of the c o a r s e p a r t i c l e s is m u c h l o w e r t h a n t h a t o f small particles, the Petermann solubility can be diminished during cooling due to the backreaction. The product of granulated "raw mixture' consists of small granules of uniform size, therefore no back-reaction occurs due to the high cooling-down rate. The Petermann s o l u b i l i t y is h i g h e r a n d t h e p r o d u c t q u a l i t y is better. (6) By means of the close correlation between degassing (CO2 loss} and the Petermann s o l u b i l i t y , i t is p o s s i b l e t o u s e a q u i c k a n a l y s i s for the reaction mixture and the product based on the estimation of the remaining CO2, instead of applying the tedious estimation of the Peterrnann solubility.

ACKNOWLEDGMENTS T h e authors a c k n o w l e d g e the financial support of the German Federal Ministry of Research and Technology and the Kali Chemie AG Hannover.

REFERENCES

1

A. Manitius, E. K u r e y n s z and W. Karwecki, Mathematical m o d e l o f t h e a l u m i n i u m o x i d e r o t a r y kiln, Ind. Eng. Chem. Process Des. Dev., 13 ( 1 9 7 4 ) 1 3 2 - 1 4 2 .

2

G. Reuter, Das T r a n s p o r t - u n d Mischverhalten y o n D r e h r o h r o f e n m b l l e r bei d e r Erzeugung y o n Eisenschwamm, Diss., T. H. Aachen, 1975. 3(a) F. Lucke, H. S e r b e n t and G. Meyer, Versuche z u r R e d u k t i o n y o n Eisenerzen inn D r e h r o h r o f e n , Stahl Eisen, 8 2 ( 1 9 6 2 ) 1 8 , 1 2 2 2 - 1 2 3 2 . ( b ) H . S e r b e n t and H. Krainer, U n t e r s u c h u n g e n z u r E r z e u g u n g y o n E i s e n s c h w a m m im Drehr o h r o f e n , Tech. Mitt. K r u p p , 23 (1965) 1 - 12. 4 F. Rothe and H. Brenek, DRP (Ger. Pat.) 429 310, 447 665, 4 8 1 1 7 7 , 4 8 5 0 7 0 , 4 8 7 9 5 6 , 498 662. 5 F . M . Lea, O. E. Bessey and A. C. R i d d l e , T h e p r o d u c t i o n o f a l k ~ i p h o s p h a t e fertilizers b y high t e m p e r a t u r e processes, I - IV, Ministry o f S u p p l y M o n o g r a p h 11.108, H.M.S.O., L o n d o n , 1951. 6 G. Tr~mel, U n t e r s u c h u n g e n i i b e r die Bildung eines h a l o g e n f r e i e n A p a t i t s a u s b a s i s c h e n C a l c i umphosphaten,

Z. Phys. Chem., 158 (1932)

42~

14 7 8

9

10

H. Jantzen, Diss., TU Hannover, 1977. J. Lehmbertz M. Hebi and K. Scbiigeri. Transverse mixing and heat transfer in ho&or&xi rotary dreactors, Powder Technol., 18 (1977) 149 - 163. M. Hehl, H. KrGger, H. Helmrich and K. Schiigeri, Longitudinai mixing in horizontal rotary drum reactors, Powder Technol., 20 (1978) 29 - 37. K. Akerman, Anwendung von Leitisotopen in der chemischen Industrie und Hiittenindustrie, Chem. Ing. Tech., 43 (1971) 1204_

11

12 13

14

K. Akerman, Anwendung radioaktiver Indikatoren aur Untersuchung der Materialbewegung in Drehofen, in Eurisotop. 80 Serie. Monographien 26 (1973). W. Werner, Die Rhenania-Diinger, Verlag Schaper, Hannover, 1967. G. Triimel, Die chemischen und tech&when Grundiagen der Herstellung von PhosphatDiingemitteln, Tellus-Verlag, Essen, 1952. A. Men and H. Vogg, Fortschritte der verfahrenstechnischen Forschung durch die Radionuklidtecbnik, Chem. Ing. Tech., 50 (1978) 108 - 113.