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Chlorine washing of calcined rock phosphate of the Red Sea region
The Chemical
Engineering
Journal,
28 (1984)
49 - 52
49
Chlorine Washing of Calcined Rock Phosphate of the Red Sea Region T. A. BIBAWY* Central
Metallurgical
Research
and Development
Institute,
National
Research
Centre,
Dokki,
Cairo (Egypt)
C. C. DELL Mining
and Mineral
Engineering
Department,
The
University
of Leeds,
Leeds
(Gt. Britain)
(Received September 27, 1982; in final form November 7, 1983)
ABSTRACT
In the manufacture of fertilizers from phosphate ores, corrosion problems arise unless the chlorine content is less than 0.05%. This problem is acute with the Red Sea phosphate ores which are first calcined and then quenched and washed with sea water. Desalinated water must be used to wash the salt from the calcine, and of course desalinated water is expensive. This investigation seems to be an ideal application of a new apparatus called the countercurrent washing column, which has been under development in Leeds for washing chlorine from the calcined phosphate. The various parameters affecting the performance of the apparatus such as the rate of solids throughput, the water inflow rate, the density of balls in each barrier and the number of washing stages were investigated. By applying the optimum conditions selected, a washed phosphate product could be obtained with a chlorine content reduced to l/234 of its value in the original feed.
performs this operation in a single vessel, separated into chambers by horizontal barriers. These barriers allow the solid particles to pass through and thus can be washed several times with minimum intermixing of the liquid through the chambers. This study is an evaluation of the best performance of the apparatus and its utilization for the washing of calcined rock phosphate of the Red Sea region.
2. EXPERIMENTAL
DETAILS
A calcined rock phosphate sample supplied by Misr Phosphate Company (Red Sea) was used in this investigation. The calcined sample which had been washed with sea water contained 30.10 wt.% PZOs, 47.72 wt.% CaO, 3.67 wt.% MgO, 1.66 wt.% FezOs, 1.05 wt.% AlzOs, 2.80 wt.% SiOZ, 1.02 wt.% CO2 and 0.68 wt.% CIZ. The size distribution of the particles was found to be in the range 0.3 - 5.0 mm; the solid suspension was therefore pumped to the top of the column without floccuiant addition.
1. INTRODUCTION
The partial removal of a solution from fine solid particles may be carried out by washing processes, which must be repeated several times to make the separation more complete. Conventionally, separate thickeners are arranged so that the solids are treated in a countercurrent to the liquid flow. The multistage washing column under investigation *Present address: International Fertilizer Development Center, Muscle Shoals, AL 35660, U.S.A. 0300-9467/84/$3.00
3. THE COUNTERCURRENT
WASHING COLUMN
The countercurrent washing column consists of a tall tank divided into a number of chambers by horizontal barriers each of which typically consists of an array of slightly buoyant balls pressing up into seatings like non-return valves. When the solids settle on the balls, the balls drop down, turn over and discharge their load of solids into the compartment below. The solids then become 0 Elsevier Sequoia/Printed in The Netherlands
50
suspended for a short time before settling onto the next barrier. This is repeated in each chamber until the solids reach the bottom chamber where they are pumped out as a thick suspension. Each barrier consists of buoyant rubber balls which press up and seal circular holes in a flat stainless steel plate. One of the aims in the design is to obtain the highest possible concentration of solids in the material discharging through the barriers. After passing down through all the barriers, the solids collect in the bottom compartment from which they are pumped out. At the same time, water is fed into the bottom compartment from which it escapes upwards through the barriers, eventually diluting the solution overflow at the top. It is important that a net flow is maintained in order to preserve the concentration gradient from the bottom to the top of the column. In the designing of the column an attempt has been made to overcome the disadvantages of the high cost, excessive mechanical complexity, chemical and mechanical abrasion, low underflow densities and low washing efficiencies which occur in many of the washing systems at present in use [l - 41. Details of the design and dynamics of the apparatus have been published elsewhere [4-91. In a single operation in which the solids are initially suspended in a salt solution and need to be discharged as a suspension in water a certain amount of water is fed into the bottom compartment. If the number of stages is large, the countercurrent water flow needs only to be little more than is required to replace the liquid initially in the pores of the settled solids. However, the cost of extra stages must always be balanced by extra performance. Figure 1 shows a schematic diagram of the 1 ft’ pilot-scale countercurrent washing column used in this study with three washing stages. The average throughput rate of phosphate ore suspension pumped to the top of the column was 5 1 min-’ ft-*. The volume of solids in the suspension was 0.92 1 min-’ ftm2, equivalent to 2.75 kg ore min-’ ft-*. As the column needed continuous feeding and hence large amounts of solids, the thick suspension that arrived at the bottom compartment was pumped back to the top of the column with the same amount of sea water (equivalent to
Fig. 1. Schematic washing column.
diagram
of the countercurrent
that of the original feed) to keep the column in continuous operation for at least 9 h. Samples were taken every 5 min from the various chambers and the chloride ion concentration was measured. At steady state, when the chloride ion concentration ceased to change (usually after 2 h), the concentration ratio at each barrier was calculated and the concentration ratio for the whole column was obtained.
4. RESULTS
AND DISCUSSION
4.1. Water inflow rate To study the effect of the water inflow rate on the performance of the apparatus, all the other variables were kept constant, each at its optimum level, while the water inflow rate was varied. Six runs were made with water rates of 3, 4, 5, 6, 7 and 8 1 min-‘. Samples were taken at intervals of 5 min from the various chambers until the salt concentrations ceased to change, indicating that a steady state had been reached. Chlorine was then analysed in these samples using a specific chloride electrode. The results of the tests are presented in Fig. 2, which indicates that the ratio of chloride ion concentrations between adjacent stages is substantially constant. The chloride ion concentrations obtained from Fig. 2 coupled with the water inflow rate were then used to calculate the amount of water accompanying the solids passing down through the barrier. It was observed that the downflow of water associated with the solids was 30%. Such a value is obviously conducive to ex-
51
I
I
I
0.1
0.2
0.1
I 0 ctl,or,ne
tonccntratlan
, mOle,Lltrc~
Fig. 2. Effect of the water inflow rate on the chlorine concentration.
cellent performance, especially in view of the fact that a packed bed of the same solids contains 24 vol.% of water. 4.2. Relationship
0
between the number of washing stages and the water requirements
Let us assume that the reduction in chloride ion concentration from that of sea water to that of the underflow is lOO:l, so that the chloride ion concentration in the underflow can be neglected: for overall concentration ratios of 5O:l and lOO:l, the chloride ion concentrations are 0.04% and 0.02% respectively
.
If we consider that the chloride ion concentrations in the various stages are c 1, c2, c3 and c4, then the concentration ratio c per stage is given by c = “2 = c_1 = “2 c2
c3
c4
(I)
(2)
where n is the number of washing stages (barriers). If p is the downflow of water and associated solids passing the barrier and q is the net upflow of water, then ClP =
c=
CAP + a)
(3)
p+q P
P
4 (Litrrr
6
I Kg SDlidr
8
1
Fig. 3. The relationship between the number of washing stages and the loss of water in the overflow.
Thus q =p(c---1) = P(C
1/n -
1)
(5)
From eqn. (5) it is possible to calculate the number n of stages and the net upflow q of water. This relationship is depicted in Fig. 3 which indicates that, as the number of stages increases, the net water upflow rate markedly decreases. Solids were pumped to the top of the column at various throughput rates. Figure 4 illustrates that, as the solids throughput rate per square foot increases, there is a gradual decrease in the concentration ratio of chlorine per stage. This is mainly due to the possibility of increasing the intermixing of salt through the various stages at high rates of solids throughput. As our objective was to decrease the chloride ion concentration by lOO:l, 0.92 1 min-’ ftW2was selected as an optimum. 4.4. Density
z1.4
of Water
4.3. Rate of solids throughput
The concentration ratio C for the whole column is given by c = c”
.?
Net Upflow
(4)
of balls
The density of balls in each washing stage (barrier) plays an important role in the performance of the apparatus. Figure 5
0.9 to 0.7 g cme3. It can be safely concluded that a density of balls equal to 0.8 g cmm-3is necessary to give an overall concentration ratio of 1OO:l. The scale-up of these results is possible at the optimum conditions obtained provided that the concentration ratio c remains constant. Large-scale tests are essential to confirm process scalability before its application commercially. 0
0.1
02
toncrntrst,on
thiorlnc
c
0.3
mole,LItreI
Fig. 4. Effect of the rate of solids throughput chlorine concentration.
on the
ACKNOWLEDGMENT
The authors would like to express their deep sense of gratitude to Professor P. A. Young, Department of Mining and Mineral Engineering, The University of Leeds, for his encouragement and the help that he offered to make this work possible.
REFERENCES
I
I 0
0.1 Chlorine
Fig. 5. Effect concentration.
Concentr2t,on
of the density
0.2 ,mo,e,!_,trc
03 1
of balls on the chlorine
presents the effect of varying the density of balls on the concentration of chloride ion in the various washing stages. Remarkably, the concentration decreases as the density of balls decreases. A calculation of the overall concentration ratio C for three washing stages shows that its value increases from 25:l to 234:l as the density of balls decreases from
D. Bradely, The Hydrocyclone, Pergamon, Oxford, 1965. J. E. Colman, Countercurrent calculations, Chem. Eng. (London), 27 (1963) 93 - 98. S. T. Corben, Min. Miner. Eng., 6 (1970) 42 - 48. J. H. Perry, Chemical Engineering Handbook, McGraw-Hill, New York, 1958. T. A. Bibawy and C. C. Dell, Tech. Rep., 1980 (Mining Department, The University of Leeds). C. C. Dell, Tech. Rep., 1976 (Mining Department, The University of Leeds). C. C. Dell and P. E. Preece, Inst. Chem. Eng. Symp. Ser. 59 (1977) 14 - 23. D. I. Hughes, M.Sc. Thesis, Mining Department, The University of Leeds, 1977. M. Raza, MSc. Thesis, Department of Chemical Engineering, The University of Leeds, 1978.
Engineering
Journal,
28 (1984)
49 - 52
49
Chlorine Washing of Calcined Rock Phosphate of the Red Sea Region T. A. BIBAWY* Central
Metallurgical
Research
and Development
Institute,
National
Research
Centre,
Dokki,
Cairo (Egypt)
C. C. DELL Mining
and Mineral
Engineering
Department,
The
University
of Leeds,
Leeds
(Gt. Britain)
(Received September 27, 1982; in final form November 7, 1983)
ABSTRACT
In the manufacture of fertilizers from phosphate ores, corrosion problems arise unless the chlorine content is less than 0.05%. This problem is acute with the Red Sea phosphate ores which are first calcined and then quenched and washed with sea water. Desalinated water must be used to wash the salt from the calcine, and of course desalinated water is expensive. This investigation seems to be an ideal application of a new apparatus called the countercurrent washing column, which has been under development in Leeds for washing chlorine from the calcined phosphate. The various parameters affecting the performance of the apparatus such as the rate of solids throughput, the water inflow rate, the density of balls in each barrier and the number of washing stages were investigated. By applying the optimum conditions selected, a washed phosphate product could be obtained with a chlorine content reduced to l/234 of its value in the original feed.
performs this operation in a single vessel, separated into chambers by horizontal barriers. These barriers allow the solid particles to pass through and thus can be washed several times with minimum intermixing of the liquid through the chambers. This study is an evaluation of the best performance of the apparatus and its utilization for the washing of calcined rock phosphate of the Red Sea region.
2. EXPERIMENTAL
DETAILS
A calcined rock phosphate sample supplied by Misr Phosphate Company (Red Sea) was used in this investigation. The calcined sample which had been washed with sea water contained 30.10 wt.% PZOs, 47.72 wt.% CaO, 3.67 wt.% MgO, 1.66 wt.% FezOs, 1.05 wt.% AlzOs, 2.80 wt.% SiOZ, 1.02 wt.% CO2 and 0.68 wt.% CIZ. The size distribution of the particles was found to be in the range 0.3 - 5.0 mm; the solid suspension was therefore pumped to the top of the column without floccuiant addition.
1. INTRODUCTION
The partial removal of a solution from fine solid particles may be carried out by washing processes, which must be repeated several times to make the separation more complete. Conventionally, separate thickeners are arranged so that the solids are treated in a countercurrent to the liquid flow. The multistage washing column under investigation *Present address: International Fertilizer Development Center, Muscle Shoals, AL 35660, U.S.A. 0300-9467/84/$3.00
3. THE COUNTERCURRENT
WASHING COLUMN
The countercurrent washing column consists of a tall tank divided into a number of chambers by horizontal barriers each of which typically consists of an array of slightly buoyant balls pressing up into seatings like non-return valves. When the solids settle on the balls, the balls drop down, turn over and discharge their load of solids into the compartment below. The solids then become 0 Elsevier Sequoia/Printed in The Netherlands
50
suspended for a short time before settling onto the next barrier. This is repeated in each chamber until the solids reach the bottom chamber where they are pumped out as a thick suspension. Each barrier consists of buoyant rubber balls which press up and seal circular holes in a flat stainless steel plate. One of the aims in the design is to obtain the highest possible concentration of solids in the material discharging through the barriers. After passing down through all the barriers, the solids collect in the bottom compartment from which they are pumped out. At the same time, water is fed into the bottom compartment from which it escapes upwards through the barriers, eventually diluting the solution overflow at the top. It is important that a net flow is maintained in order to preserve the concentration gradient from the bottom to the top of the column. In the designing of the column an attempt has been made to overcome the disadvantages of the high cost, excessive mechanical complexity, chemical and mechanical abrasion, low underflow densities and low washing efficiencies which occur in many of the washing systems at present in use [l - 41. Details of the design and dynamics of the apparatus have been published elsewhere [4-91. In a single operation in which the solids are initially suspended in a salt solution and need to be discharged as a suspension in water a certain amount of water is fed into the bottom compartment. If the number of stages is large, the countercurrent water flow needs only to be little more than is required to replace the liquid initially in the pores of the settled solids. However, the cost of extra stages must always be balanced by extra performance. Figure 1 shows a schematic diagram of the 1 ft’ pilot-scale countercurrent washing column used in this study with three washing stages. The average throughput rate of phosphate ore suspension pumped to the top of the column was 5 1 min-’ ft-*. The volume of solids in the suspension was 0.92 1 min-’ ftm2, equivalent to 2.75 kg ore min-’ ft-*. As the column needed continuous feeding and hence large amounts of solids, the thick suspension that arrived at the bottom compartment was pumped back to the top of the column with the same amount of sea water (equivalent to
Fig. 1. Schematic washing column.
diagram
of the countercurrent
that of the original feed) to keep the column in continuous operation for at least 9 h. Samples were taken every 5 min from the various chambers and the chloride ion concentration was measured. At steady state, when the chloride ion concentration ceased to change (usually after 2 h), the concentration ratio at each barrier was calculated and the concentration ratio for the whole column was obtained.
4. RESULTS
AND DISCUSSION
4.1. Water inflow rate To study the effect of the water inflow rate on the performance of the apparatus, all the other variables were kept constant, each at its optimum level, while the water inflow rate was varied. Six runs were made with water rates of 3, 4, 5, 6, 7 and 8 1 min-‘. Samples were taken at intervals of 5 min from the various chambers until the salt concentrations ceased to change, indicating that a steady state had been reached. Chlorine was then analysed in these samples using a specific chloride electrode. The results of the tests are presented in Fig. 2, which indicates that the ratio of chloride ion concentrations between adjacent stages is substantially constant. The chloride ion concentrations obtained from Fig. 2 coupled with the water inflow rate were then used to calculate the amount of water accompanying the solids passing down through the barrier. It was observed that the downflow of water associated with the solids was 30%. Such a value is obviously conducive to ex-
51
I
I
I
0.1
0.2
0.1
I 0 ctl,or,ne
tonccntratlan
, mOle,Lltrc~
Fig. 2. Effect of the water inflow rate on the chlorine concentration.
cellent performance, especially in view of the fact that a packed bed of the same solids contains 24 vol.% of water. 4.2. Relationship
0
between the number of washing stages and the water requirements
Let us assume that the reduction in chloride ion concentration from that of sea water to that of the underflow is lOO:l, so that the chloride ion concentration in the underflow can be neglected: for overall concentration ratios of 5O:l and lOO:l, the chloride ion concentrations are 0.04% and 0.02% respectively
.
If we consider that the chloride ion concentrations in the various stages are c 1, c2, c3 and c4, then the concentration ratio c per stage is given by c = “2 = c_1 = “2 c2
c3
c4
(I)
(2)
where n is the number of washing stages (barriers). If p is the downflow of water and associated solids passing the barrier and q is the net upflow of water, then ClP =
c=
CAP + a)
(3)
p+q P
P
4 (Litrrr
6
I Kg SDlidr
8
1
Fig. 3. The relationship between the number of washing stages and the loss of water in the overflow.
Thus q =p(c---1) = P(C
1/n -
1)
(5)
From eqn. (5) it is possible to calculate the number n of stages and the net upflow q of water. This relationship is depicted in Fig. 3 which indicates that, as the number of stages increases, the net water upflow rate markedly decreases. Solids were pumped to the top of the column at various throughput rates. Figure 4 illustrates that, as the solids throughput rate per square foot increases, there is a gradual decrease in the concentration ratio of chlorine per stage. This is mainly due to the possibility of increasing the intermixing of salt through the various stages at high rates of solids throughput. As our objective was to decrease the chloride ion concentration by lOO:l, 0.92 1 min-’ ftW2was selected as an optimum. 4.4. Density
z1.4
of Water
4.3. Rate of solids throughput
The concentration ratio C for the whole column is given by c = c”
.?
Net Upflow
(4)
of balls
The density of balls in each washing stage (barrier) plays an important role in the performance of the apparatus. Figure 5
0.9 to 0.7 g cme3. It can be safely concluded that a density of balls equal to 0.8 g cmm-3is necessary to give an overall concentration ratio of 1OO:l. The scale-up of these results is possible at the optimum conditions obtained provided that the concentration ratio c remains constant. Large-scale tests are essential to confirm process scalability before its application commercially. 0
0.1
02
toncrntrst,on
thiorlnc
c
0.3
mole,LItreI
Fig. 4. Effect of the rate of solids throughput chlorine concentration.
on the
ACKNOWLEDGMENT
The authors would like to express their deep sense of gratitude to Professor P. A. Young, Department of Mining and Mineral Engineering, The University of Leeds, for his encouragement and the help that he offered to make this work possible.
REFERENCES
I
I 0
0.1 Chlorine
Fig. 5. Effect concentration.
Concentr2t,on
of the density
0.2 ,mo,e,!_,trc
03 1
of balls on the chlorine
presents the effect of varying the density of balls on the concentration of chloride ion in the various washing stages. Remarkably, the concentration decreases as the density of balls decreases. A calculation of the overall concentration ratio C for three washing stages shows that its value increases from 25:l to 234:l as the density of balls decreases from
D. Bradely, The Hydrocyclone, Pergamon, Oxford, 1965. J. E. Colman, Countercurrent calculations, Chem. Eng. (London), 27 (1963) 93 - 98. S. T. Corben, Min. Miner. Eng., 6 (1970) 42 - 48. J. H. Perry, Chemical Engineering Handbook, McGraw-Hill, New York, 1958. T. A. Bibawy and C. C. Dell, Tech. Rep., 1980 (Mining Department, The University of Leeds). C. C. Dell, Tech. Rep., 1976 (Mining Department, The University of Leeds). C. C. Dell and P. E. Preece, Inst. Chem. Eng. Symp. Ser. 59 (1977) 14 - 23. D. I. Hughes, M.Sc. Thesis, Mining Department, The University of Leeds, 1977. M. Raza, MSc. Thesis, Department of Chemical Engineering, The University of Leeds, 1978.