A CLEAN TECHNOLOGY
PHOSPHORIC ACID
PROCESS
S. van der Sluis
V» T T'
A C L E A NTECHNOLOGY
* ' PHOSPHORIC ACID
fiy & ^ PROCES
■Tl
A CLEAN TECHNOLOGY
PHOSPHORIC ACID
PROCESS
Proefschrift ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft, op gezag van de Rector Magnificus,
Prof.dr. J . M . Dirken, in het openbaar te verdedigen ten overstaan van een commissie aangewezen door het College van Dekanen
op 11 juni 1987 te 14.00 uur
door
Sierd van der Sluis
scheikundig ingenieur geboren te Oudemirdum
Delft University Press/1987
Dit proefschrift is goedgekeurd door de promotoren: Prof .dr.ir. G.M. van Rosmalen
Prof.ir. J . A . Wesselingh
Copyright ©1987 by S. van der Sluis
No part of this book may be reproduced in any form by print, photoprint, microfilm or any other means without written permission from the publisher: Delft University Press, Delft, The Netherlands.
VOORWOORD
Dit proefschrift is mede tot stand gekomen door de inzet van veel personeel, materieel en geld door de overheid en DSM, waarvoor ik hen hierbij bedank. De onmisbare bijdragen van vele afstudeerders, stagiairs, bijvakkers en omscholers, vaste medewerkers en collega's zijn in een aantal gevallen al terug te vinden in een co-auteurschap van een artikel of in woorden van dank, die daarbij vermeld staan. Z i j , die om wat voor reden dan ook, daar niet bij genoemd zijn, wil ik hierbij alsnog bedanken voor hun inzet en bijdragen.
Een speciaal woord van dank is op zijn plaats voor mijn collega Tjien T. Tjioe, want zonder zijn adviezen en hulp was dit proefschrift zeker nu nog niet tot stand gekomen.
Tevens past mij een woord van dank voor de ondersteuning, die de werknemers en de diensten van de Technische Universiteit Delft, van DSM Meststoffen B.V. en van DSM Research B.V. mij gegeven hebben.
Bovenal wil ik de grote groep medewerkers en medewerksters bedanken die, onbezoldigd, dag en nacht klaar stonden om, in continu dienst, proeven met het fosforzuur fabriekje op laboratorium schaal uit te voeren en hen, die daarbij weliswaar thuis bleven, maar dag en nacht oproepbaar waren.
Tenslotte wil ik die mensen bedanken, die het op welke manier dan ook, mogelijk gemaakt hebben om bij verschillende fosforzuur fabrieken over de hele wereld rond te kijken en via het houden van lezingen op binnen- en buitenlandse congressen en symposia, enige internationale ervaring op te doen.
CONTENTS
1. INTRODUCTION 7 1.1. General aspects of phosphoric acid 7
1.2. The phosphate ore 7 1.3. Phosphoric acid production processes 11
1.3.1. Introduction of process routes 11
1.3.2. The dry processes 11 1.3.3. The wet processes 12 1.3.3.1. General procedure 12 1.3.3.2. Acidulation with hydrochloric acid 13
1.3.3.3. Acidulation with nitric acid 14 I.3.3.I. Acidulation with sulphuric acid 16 1.1) Impurities in relation to the applications of the calcium
sulphate byproduct 19 1.5 Aim of this study 21
1.6 Literature 21
2. SCOPE OF THIS INVESTIGATION 24
3. OVERVIEW OF THE CTPA PROCESS 28
3.1. Summary 28 3.2. Introduction 29 3.3. Objective of the study 32
3.4. Description of the CTPA process 32 3.4.1. Introduction to the process 32
3.4.2. The digestion stage 33 3.4.3. The crystallisation stages 37
3.4.4. The filtration stages 41 3.4.5. The fluoride removal 46 3.5. Simplified process flowsheet and mass balance 50
3.6. Conclusions 52 3.7. Literature 53
14. THE DIGESTION OF PHOSPHATE ORE IN PHOSPHORIC ACID 56
4.1. Summary 56 4 . 2 . I n t r o d u c t i o n 56 4 . 3 . The d i g e s t i o n s t a g e of t h e CTPA process 57
4 . 4 . Experimental 59 4 . 5 . R e s u l t s 60 4 . 6 . D i s c u s s i o n 62 4 . 6 . 1 . I n f l u e n c e of t h e phosphoric a c i d c o n c e n t r a t i o n 62 4 . 6 . 2 . I n f l u e n c e of the t e m p e r a t u r e 63 4 . 6 . 3 . Conclusion 63 4 . 7 . A k i n e t i c model of the d i g e s t i o n p r o c e s s 63 4 . 8 . D e t e r m i n a t i o n of the m a s s t r a n s f e r c o e f f i c i e n t s 66 4 . 9 . Conclusive remarks 68 4 . 1 0 . Nomenclature 68 4 . 1 1 . L i t e r a t u r e 69
5 . CRYSTALLISATION OF CALCIUM SULPHATE HEMIHYDRATE 70
5 . 1 . Summary 70 5.2. Introduction - 70
5.3. Experimental 72 5.4. Results and discussion 71
5.4.1. The hemihydrate crystals 74 5.4.2. Incorporation of phosphate ions 77 5.4.3. Incorporation of cadmium ions 80
5.5. Conclusions 84 5.6. Literature 84
6. THE FILTRATION OF CALCIUM SULPHATE HEMIHYDRATE 86
6.1. Introduction 86 6.2. Theory of filtration 87 6.3. Experimental 90 6.3.1. Equipment 90 6.3-2. Chemicals 91 6.3.3. Procedure 91 6.4. Results and discussion 91
6.5. Nomenclature 97 6.6. Literature 97
FLUORIDE DISTRIBUTION COEFFICIENTS (G/L) IN WET
PHOSPHORIC ACID PROCESSES 98
Summary 98 Introduction 98 Literature survey 100 Model development 102 General remarks 102 Calculation of the activity coefficients 103
Calculation of the hydrogen ion concentration in
phosphoric acid 105 Determination of an expression for the fluoride
distribution coefficients 107 Experimental 111 Chemicals 111 Equipment 111 Procedure 11 2 Analyses 113 Results and discussion 114
Conclusions 120 Nomenclature 121 Literature 122 Appendix: Conversion of w$ into molalities 124
MASS AND HEAT BALANCES OF THE CTPA PROCESS 125
Introduction 125 Process description 126
General approach 126 Digestion of phosphate ore 127
Cadmium removal 129 Crystallisation of calcium sulphate hemihydrate (HH) 130
Recrystallisation of HH to gypsum 131
Solid-Liquid separation 131
Fluoride removal 132 Combined mass and heat balances for each stage 132
Discussion and conclusion 147
Literature 149 Appendices 151 The solubility of HH and DH 151
8.6.2. Incorporation in HH and DH 151
8.6.2.1 Incorporation in calcium sulphate hemihydrate 151
8.6.2.2 Incorporation in calcium sulphate dihydrate 152
8.6.3. Vapour pressure of the PpO^-H^O system 152
8.6.1. Fluoride distribution coefficients 153
8.6.1.1. Liquid-gas 153
8.6.1.2. Liquid-solid 151
8.6.5. Density correlations 155
8.6.5.1. Phosphoric acid 155
8.6.5.2. Sulphuric acid 155
8.6.5.3. CDHP solutions 155
8.6.5.1. *V
01
-^2
S0H~
H2°
m i x t u r e s 156
8.6.5.5. Calcium sulphate hemihydrate (HH) 156
8.6.5.6. Calcium sulphate dihydrate (DH) 156
8.6.5.7. The phosphate ore 156
8.6.5.8. Water 156
8.6.6. Heat capacities and heat contents 157
8.6.6.1. Phosphoric acid 157
8.6.6.2. Sulphuric acid 157
8.6.6.3. CDHP solutions 157
8.6.6.1. H POjj-H SO^-H 0 mixtures 158
8.6.6.5. Calcium sulphate hemihydrate (HH) 158
8.6.6.6. Calcium sulphate dihydrate (DH) 158
8.6.6.7. The Phosphate ore 159
8.6.6.8. Water 159
8.6.6.9. Water vapour 159
8.6.6.10. Carbon dioxide 159
8.6.6.11. Other compounds 160
8.6.7. Enthalpies of reaction 160
8.6.7.1. Digestion of phosphate ore 160
8.6.7.2. Crystallisation of calcium sulphate hemihydrate 160
8.6.7.3. Recrystallisation of HH into gypsum 160
8.6.8. Heat of mixing in the H P0.-H SO.-H 0 system 161
8.6.8.1. General procedure 161
8.6.8.2. Binary heat of mixing in the H^O^-H-O sytem 163
8.6.8.3. Binary heat of mixing in the H ^ O ^ - H ^ sytem 163
8.6.8.1. Binary heat of mixing in the H^PO^-H SO. system 163
8.6.9. Total mass and heat balances of the CTPA process 161
9. 9.1. 9.2. 9.2.1 9.2.2. 9.2.3. 9.2.1. 9.2.4.1. 9.2.1.2. 9.2.1.3. 9.2.1.1. 9.2.5. 9.3. 9.3.1. 9.3.2. 9.3.3. 9.3.1. 9.3.5. 9.1. 9.5. 9.6. 9.6.1. 9.6.2. 9.6.2.1. 9.6.2.2. 9.6.2.3. 9.6.3.
THE BENCH-SCALE PLANT Introduction Experimental Process conditions Chemicals Equipment Procedures Safety Startup procedure Working procedure Stop procedure Analyses
Results and discussion
The performance of the bench-scale run The cadmium incorporation in HH
The phosphate incorporation in HH The permeability of the filter cakes Additional results
Conclusions Literature Appendices Service round
Pressure filter procedures The precoat procedure The filter procedure The filter change procedure The HH washing procedure
175 175 176 176 177 177 180 180 181 183 183 181 185 185 187 188 189 191 193 193 191 191 195 195 195 195 196 SUMMARY 198 SAMENVATTING 200 PUBLICATIONS 202
1. INTRODUCTION
1.1. General aspects of phosphoric acid
Phosphoric acid is an important intermediate chemical product. It is mainly used for the manufacturing of fertilisers, as demonstrated in table 1.
» 1000 tons P_0C percentage Fertilisers
Detergents Animal feed
Food and beverages Surface treatment Water treatment
Dentistry, tooth pastes Fire extinguisher Others Total 32 1 1 ,000 ,590 , 180 . 240 230 90 80 40 110 35,600 90 4.5 3.3 0.7 0.6 0.25 0.22 0.11 0.3 100
Table 1: World phosphate consumption and application in 1980 [ 1 ] .
To meet the food needs of the increasing world population, there is a steadily growing demand for phosphate fertilisers. The phosphoric acid production is directly linked to the phosphate fertiliser consumption, which is expected to rise from 30.7 million tons P-O,- in 1982, to about 41.7 million tons
<!■ 5
P-0,- in 1990. This represents an increase of the phosphoric acid production from about 19.7 million tons P_0C in 1982 to 27.8 million tons P.O.- in 1990 [3].
£ 0 2 5
1.2. The phosphate ore
P2°5'
According to Becker [ 1 ] , about 200 known minerals contain more than 1 w$ The most important minerals for the phosphoric acid industry are those belonging to the apatite group, with the general formula Ca. 0(P°i|)6X2' i n w n i c n
X can be fluoride, chloride or hydroxide [12]. Because the fluoride ion is the smallest of the three, fluoroapatite is commonly believed to be the most stable.
The phosphate rock normally not only contains the apatite mineral, but also several other minerals, such as calcite, dolomite, pyrite, kaolinite, quartz, feldspar and fluorite [14]. Phosphate rock deposits are located in abundance in nearly all parts of the world. The phosphate rocks can be divided in two groups, depending on their origin:
- sedimentary phosphate rock - igneous phosphate rock
Igneous phosphate rock is crystallised from magma and thus found in areas with vulcanic activity. Its availability is therefore limited. The igneous phosphate rock mainly consists of fluoroapatite, in which calcium is partly replaced by barium and strontium and chloride partly substitutes fluoride.
Sedimentary phosphate rock deposits were formed by precipitation of dissolved phosphates from prehistoric seas [32]. During this process the phosphate rock was subjected to an interaction with water of varying temperature and impurity content. A large number of these impurities became included in the rock by either coprecipitation or by incorporation in the apatite crystal lattice. In this way phosphate rocks with a large variety in impurity composition and concentration were created. Most sedimentary phosphate rocks are found to be more than a million years old [27]. The sedimentary phosphate rock mainly consists of fluoroapatite, in which part of the phosphate ions are replaced by fluoride and carbonate ions. This fluoroapatite is commonly refered to as francolite.
In 1981 more than 85 % of the world's raw phosphate came from sedimentary
deposits [1]. The four major raw phosphate producers in 1981, were the USA, the USSR, Morocco and China [1]. The raw phosphate, produced by open mining, contains several other minerals next to the apatite mineral. Only the apatite mineral is needed for the production of phosphoric acid. The other minerals have to be removed prior to the phosphoric acid production. The raw phosphate is normally first washed with water to remove the water soluble minerals. Thereafter several flotation steps are applied to remove the larger part of the other minerals and to obtain the phosphate ore. This process to obtain phosphate ore from raw phosphate is often called beneficiation. The removal of in particular the siliceous and carbonate gangue is necessary to avoid serious problems during the phosphoric acid production. The siliceous gangue consists of small particles, which can cause problems during filtration. Carbonate gangue removal is a prerequisite prior to the digestion of the phosphate rock by an inorganic acid. Otherwise the development of carbon dioxide in combination with the release of organic material from the rock gives rise to excessive foaming.
The current techniques for beneficiation of phosphate rock are reviewed by Lawver e.a. [21]. The procedure selected for a specific phosphate rock is determined by its carbonate content as well as by its origin. Sedimentary and igneous deposits are treated differently. One of the most important processes for the beneficiation of phosphate rock, which is applied on all rocks, is flotation. An overview of the most frequently used flotation techniques is given by Houot [16]. Flotation is used mainly to remove the siliceous gangue, but in some cases also the carbonate gangue can be removed by this technique as described by Dufour [13] for Florida phosphate rock and by Vaman RaO [39] for Mussorie phosphate rock.
The phosphate ores obtained after beneficiation of sedimentary phosphate rocks, consist mainly of francolite. McClellan and Lehr [23] derived a formula for the mean composition of francolite by averaging the composition of 110 commercially available sedimentary phosphate ores:
Ca
iO-aVVV
PV6-x
(C03
)x
F2
+0.<,
Xwith x = 0 - 1.5 and a = 0.6 * x.
The 2H sedimentary phosphate ores described by Gremillion e.a [1t] conformed
very closely to this mean francolite composition.
The chemical composition of phosphate ores varies widely with their mining location [11]. McClellan and Lehr [23] found the weight ratio of Ca0/P205 to
vary from 1.32 in pure fluoroapatlte, to about 1.62 in highly substituted francolite, while according to Caro e.a. [7] the weight ratio of F/P-O- varies from 0.09 to 0.15 respectively. The influence of the weight ratio FVP2°5 o n t n e
reactivity of the ore with acid is unclear. The reactivity of the ore with acid was found to be higher at increasing C0./P„0c weight ratio in the phosphate ore.
2 2 5
This observation was confirmed by Chien [10,11], who came to the same conclusion from a thermodynamic approach.
i The crystallographic structure of fluoroapatlte has been described in detail by Beevers and Mclntyre [2]. Montel e.a. [2*4] report the fluoroapatlte crystal lattice to have a hexagonal structure with two series of parallel channels. One series of narrow channels has a diameter of about 2 A, while the other series consists of wide channels with a diameter of about 3.5 A, each channel containing the fluoride ions centered on its axis. The fluoroapatite lattice therefore has a very open structure. According to Kreidler [20] almost all ions seem to fit to some extent in the apatite lattice, due to its open structure.
Not only the fluoroapatite lattice itself has an open structure on an atomic scale, but also the fluoroapatite as a mineral has an open structure on a microscopic scale. Caro e.a. [6] investigated the pore structure of phosphate ore with mercury porosimetry and nitrogen desorption. By comparing the pore size distributions of large particle size fractions with small particle size fractions, they found the fine pore structure to be uniformly distributed throughout the material, irrespective of particle size. Hill e.a. [15] measured specific surface areas and found, for particle size fractions smaller than 150
2
um, these areas to be varying between about 1 m /g for Virginia phosphate ore 2
and 37 m /g for Tunisian phosphate ore. This relatively large surface areas are attributed to pore space within the particles. These particles are often viewed as aggregates of fine crystals. The average dimension of these elementary grains is deduced from the broadening of X-ray diffraction bands and compared with a figure calculated from surface area measurements of several suitably fine samples (10 y m ) . The results obtained by both methods are within one order of magnitude. The elementary grain size for Moroccan and Tunesian phosphate ore was found to be about 100 - 500 A, while for Virginian phosphate ore, it was approximately 1000 - 3000 A. The average dimension of the elementary grains is one of the most important factors determining the reactivity of the ore [15].
S I C » • COM • H U T * PHOSPHORUS ncm SUM ACE •vntnm
Q
VBCOSITT COITML ABUTSFigure 1: Uses of phosphoric a c i d produced by dry and wet p r o c e s s e s [ 3 2 ] .
1.3 Phosphoric acid production processes
1.3.1. Introduction of process routes
Phosphoric acid can be produced from phosphate ore via two major process
routes: the so-called wet processes, using strong mineral acids for digestion of
the ore and the dry processes, producing elemental phosphorus as an intermediate
by burning of the ore in an electric furnace or in a rotary kiln [3]
An outline of the field of applications of phosphoric acid produced by dry
processes as well as by wet processes is given in figure 1 [32]. Since
phosphoric acid produced through dry routes contains less impurities its
application lies mainly in the "high added value" areas such as detergents and
food additives, while phosphoric acid from wet processes is mainly further
processed into fertilisers.
1.3.2. The dry processes
In the dry processes, the phosphate ore is reduced, by addition of silica
and carbon, to phosphorus and slag at about 1500 °C in a furnace:
Ca
1Q(PO
l))
6F
2+ 20 Si0
2+ 28 C — > 20 CaSiO + 28 CO + 3 Pj,
+2
?
2AH = 27.9 MJ/kg of
?
kproduced [22]
The heat, AH needed for this reaction is substantial and could until 1985 only
be achieved by internal resistence heating of the molten charge in an electric
furnace. In 1985, however, a direct fired rotary kiln was found to be able to
supply the heat [22]. A flowsheet of the KPA process, based on this way of
supplying the necessary heat is shown in figure 2.
After the combustion of the evaporating phosphorus, the obtained phosphorus
pentoxide is absorbed in water or in moderately concentrated phosphoric acid, to
obtain concentrated phosphoric acid (> 60 w$ P-0,-).
The direct production of clean and concentrated phosphoric acid is possible
with the dry processes, because most impurities from the ore, remain in the slag
[3].
Although the heat of combustion of the phosphorus and the carbon monoxide
is recovered in the KPA process, these processes are still too expensive for use
in the fertiliser industry, because of their high energy consumption. Moreover,
fine ore particles cannot be used directly in these processes, since they hamper the interstitial gas flow through the ore bed.
The product of the dry processes can, however, be used in applications, where the product from the wet processes is not sufficiently pure. This does, sometimes, require additional removal of arsenic and fluoride.
vent
phosphate ore coke ^ silicaf
raw feed preparation1
i i wafert
balling preheatingA
i i fuel ---1 — — I air — 1 rotary kiln reactor ' heat recovery * j spent gas scrubbing i , acid absorption acid cleaning solids » -70 w % P205Figure 2: Flowsheet of the KPA process [22].
1.3.3. The wet processes
1.3.3.1. General procedure
Phosphoric acid can be released from phosphate ore by the action of strong mineral acids, such as nitric acid, hydrochloric acid and sulphuric acid. Sulphuric acid is the only acid, which forms an insoluble precipitate with the calcium from the phosphate ore, thus allowing the phosphoric acid to be separated directly by filtration. The chlorides and nitrates of calcium are both soluble, so special techniques, like solvent extraction [28], ion exchange [19] or cooling crystallisation [1,36] are required to recover the phosphoric acid. In spite of this phosphoric acid is being commercially produced using nitric acid as well as hydrochloric acid. The economical feasibility of these processes is mainly determined by the availability and price of the mineral acid and by local production facilities and by the desired products.
1.3.3.2. Acidulation with hydrochloric acid
The acidulation of phosphate ore in hydrochloric acid can be represented by the following reaction [28]:
Caln(P0„),Fo + 20 HC1 — > 6 H_PO.. + 10 CaCl. + 2 HF 1 0 4 O £ j 1 2 phosphate ore m gas
i r
dissolution recyc le wash solids separation to s extr solids washing dissolution liquor olvent action to » aste dissolution liquor recycle stripped liquor aqueous acids t o " l » e n t to distillation recovery water ' reflux i JJ1"""J-L
•^extraction)—■»] pufification] —) washing] —|
stripping}-extracted aqueous liquid circulating solvent
- solvent streams - aqueous streams
Figure 3= Flowsheet of the ore digestion section [28].
Figure 4: Flowsheet of the solvent extraction section [28].
The hydrogen chloride can be supplied, either as a gas entering an aqueous suspension of phosphate ore or as hydrochloric acid. The phosphoric acid can be separated from the calcium chloride solution by solvent extraction with C,. and C_ alcohols or with a mixture of these solvents.
In figure 3 a flowsheet of the ore digestion section of the IMI process is shown and in figure 4 the corresponding flowsheet of the solvent extraction section. The phosphoric acid and the quantity of hydrochloric acid dissolved in the solvent are recovered from the solvent by washing with water. Thereafter the solvent is distilled off and subsequently the phosphoric acid and hydrochloric acid are separated by evaporation.
An additional advantage of this method is the reduced impurity level of the obtained concentrated phosphoric acid (> 50 w$ P _ Oc) , because the impurities are
less easily soluble in the solvent than in phosphoric acid. A disadvantage, however, is the severe corrosion.
1.3.3.3. Acidulation with nitric acid
There are three main processes in which nitric acid is used for acidulation of phosphate ore: the Odda process, the DSM sulphate-recycle process [5,36] and the Superfos process [19].
In the Odda process, the phosphate ore is digested in 60 w% HNO. at 50-70 °C. Thereafter, a large part of the calcium nitrate is removed by cooling crystallisation and separation of the crystals. The calcium nitrate crystal slurry is further processed to obtain a fertiliser, while the filtrate is partly recycled. This is shown in figure 5.
phosphate HNO, dissolution
-L^-N°x.F
- ■ - i n e r t s I crystallisation | filter . . . — HN03 1 Ca(N03)2AH20 ' Ito conversion) neutralisation — NH3 r » H20 ■L—NH2NO3 (from conversion) evaporation | granulationFigure 5: Flowsheet of the Odda process [5].
In the DSM process, the phosphate ore is digested with nitric acid at 65 °C, whereafter the solution, containing insoluble phosphate ore particles, reacts with ammonium sulphate at 55 °C. The calcium ions are precipitated as gypsum crystals and filtered off. Subsequently the moisture content of the filtrate is reduced after neutralisation by steam evaporation to obtain an ammonium phosphate fertiliser. The gypsum cake is fed, together with a 52-53 w$ ammonium carbonate solution into a crystalliser, in which calcium carbonate is precipitated and ammonium sulphate is recovered for recycling. A flowsheet of this process is shown in figure 6.
The third process is the so-called Superfos process, in which the phosphate ore is digested with nitric acid. The so obtained calcium nitrate and phosphoric acid containing solution is fed into an ion exchange section, where the calcium
ions are exchanged against potassium ions. The ion exchange r e s i n , loaded with
calcium, is regenerated with a potassium chloride solution, giving an CaCl
effluent stream. The product, consisting of a KNO and H PO^ solution i s used as
an intermediate in the manufacturing of f e r t i l i s e r s . In a process modification,
where not a l l the calcium ions are removed, these residual calcium ions are
precipitated with the major part of the phosphate as CaHPO.. A feed grade
quality i s claimed, which i s only possible if the impurities were removed before
or in the ion exchange s e c t i o n . This process is i l l u s t r a t e d in figure 7.
Phosphate
OPEJ I " ™ *
I V. \ ,. h * N0X.F | dissolution [_~ i n e r t s
JGypsum precipitation}—— INHJ3SQ1, (From conversion)
granulation
Figure 6: Flowsheet of the DSM process [5].
potassium chloride water phosphate ore nitric acid phosphoric acid ammonia I 1 l_.
♦
'
L_
1
__K
1 ion exchanqe ] L CaCl2 | concentration! "I * ^neutralisation! | granulation ] chlorid1
>-freefi PK pre-neutrs separation |r
| crystallisation f - « — | separation | 1 ■ | drying | dicalcium ammonia to NPK plant phosphateFigure 7: Flowsheet of the Superfos ion exchange NPK process with CaHPO. coproduction [19].
1.3-3.1). Acidulation with sulphuric acid
Although all acidulation processes are refered to as wet processes, the term wet process is mainly reserved for processes in which sulphuric acid is applied.
During the production of phosphoric acid from phosphate ore by acidulation with sulphuric acid, huge amounts of calcium sulphate are precipitated as a byproduct. Depending on the temperature as well as on the phosphate and sulphate content of the solution, either calcium sulphate dihydrate (DH), hemihydrate (HH) or anhydrite (AH) is formed, as shown in figure 8 [1,35].
The solid lines in figure 8 represent quasi equilibrium curves, indicating which phase will initially precipitate under the given conditions. The broken line in figure 8, is the border of the regions, where either AH or DH is stable. The HH phase only exists as a metastable phase. The influence of sulphuric acid can be taken into account by assuming one mole of sulphuric acid to be equivalent with about 1.5 mole of phosphoric acid [32].
100
temp°[
80
60-40'
20
20 30 40 50w % P2
0
5 ^Figure 8: Phase diagram of calcium sulphate in phosphoric acid.
Commercial processes have been developed, producing either DH, HH or AH as a byproduct. An overview of these processes is given in table 2 [31], where the process is named after its byproduct.
AM precipitation HH precipitation DH precipitation - - C A H stable / D H stable"^N 16
process | temperature | concentration | company/name
|
w*
P2°5
DH HH DH/HH doublef i l t e r
HH/DHsingle
f i l t e r
HH/DH double f i l t e r 70 - 8 5 90 - 100 stage 1 : 65 stage 2 : 95 stage 1 : 90 stage 2 : 55 -9 0 - 1 0 0 70 100 100 65 28 35 35 30 -30 - 32 - 50 - 38 ■32 - 50Prayon, Jacobs Dorco, SIAPE Kellog-Lopker, Norsk Hydro, IITPIC, Rhone Poulenc
Norsk Hydro, Jacobs Dorco, O c c i d e n t a l
Prayon
Nippon Kokan, Nissan M i t s u b i s h i
Singmaster and Breyer, Norsk Hydro, N i s s a n , Nippon Kokan
AH | 100 -240 | 4 0 - 5 0 | (Nordengren)
Table 2: Wet phosphoric a c i d p r o c e s s e s .
The advantages and d i s a d v a n t a g e s of t h e v a r i o u s p r o c e s s e s a r e summarized in d e t a i l by Becker [ 1 ] . The main d i s a d v a n t a g e of t h e DH, t h e s i n g l e f i l t e r HH/DH and the DH/HH p r o c e s s e s , i s t h e production of r e l a t i v e l y d i l u t e (about 30 wj P-0,.) phosphoric a c i d , which has t o be c o n c e n t r a t e d for use i n f e r t i l i s e r a p p l i c a t i o n s . Due t o the e n e r g y consuming c o n c e n t r a t i o n s t e p , t h e HH and t h e two f i l t e r HH/DH p r o c e s s e s , a l l o w i n g d i r e c t production of 40 w$ P 0 or even higher c o n c e n t r a t e d acid from t h e f i l t e r , a r e r a p i d l y g a i n i n g f i e l d .
AH processes are not operational at this moment [1], due to the serious problems observed in small scale commercial batch plants [25], such as enhanced corrosion at the required high temperature as well as a prolonged residence time following from the low growth rate of AH.
Each process has its own requirements regarding raw materials, utilities, product and byproduct quality and last but not least, overall phosphate efficiency. An important disadvantage of the HH processes for instance, is the relatively low (< 95$) phosphate efficiency. An advantage of the HH/DH and DH/HH processes is the production of a relatively clean byproduct especially with regard to P 0 incorporation.
According to Becker [1], the ultimate process route in phosphoric acid production processes is the double filter HH/DH process route, because:
- concentrated phosphoric acid is directly produced from the filter - the highest overall phosphate efficiency is obtained,
- the lowest sulphuric acid consumption is needed,
- relatively clean phosphoric acid is produced, due to the reduced solubility of impurities in the concentrated acid and
- phosphogypsum with a low phosphate content is produced.
ISULPHURIC
ACI0|-WASTE GAS
N°1 H°2 COOLING PUMP DIGESTER TANK TANK
N°1 N"2 N"3 HYDRATION TANKS
Figure 9: The Nissan C p r o c e s s .
In figure 9, one of the commercially available double filter HH/DH processes is shown [1].
Additional aspects, which have to be taken into account before a proper process selection can be made are: capital cost, maintenance cost and on-line time, raw material and utility cost and also product as well as byproduct quality. These aspects depend largely on the local situation.
1.1. Impurities in relation to the applications of the calcium sulphate byproduct
The phosphate ore contains a lot of impurities, as is shown in table 3 [to].
In all wet processes impurities, such as radium and heavy metal ions, like cadmium, originally present in the phosphate ore, are distibuted between the phosphoric acid and the byproduct. Several methods have been developed to remove impurities from the phosphoric acid, such as flotation [33], ion exchange [9,37] and solvent extraction [29]. One of the recently published solvent extraction processes is the BESA-2 process [30], which claims to produce a phosphoric acid quality comparable with the acid produced by the dry process for almost half the price of the dry process acid.
One of the most important environmental problems in phosphoric acid production is the cadmium content of the phosphoric acid and of the calcium sulphate byproduct. Cadmium concentrations as low as 1 mg/1 in water have led to-a pto-ainful bone diseto-ase, known to-as Itto-ai-Itto-ai [ 1 ] .
18 ppm 250 ppm 10 ppm 30 ppm 20 ppm 50 ppm <0.1 ppm
Table 3: Indicative concentrations of heavy metal ions and radio-active elements in phosphate ores from sedimentary deposits.
40 K 238(J 2 3 2T h 2 2 6R a Ra-equi valence 6 pCi/g 40 pCi/g 1.2 pCi/g 38 pCi/g to pCi/g Cd Zn Cu Pb As Ni Hg
In the last years, there is a growing concern, particularly in Europe, that this toxic metal ion could enter the food chain in increasing amounts. Some countries intend to adopt a cadmium limitation of no more than 90 mg cadmium per kg P?0(- in the fertiliser product, as a preventive measure [17,38]. The
Netherlands plans legislation restricting the levels of heavy metal ions in phosphate fertilisers as well as in other products used in agriculture, and also the levels in the byproduct hydrated calcium sulphate. The disposal of this byproduct is limited by its cadmium content, while its application as an indoor buiding material is also hampered by its radium content. In phosphoric acid processes operating nowadays the calcium sulphate byproduct is either disposed in water or stacked on land [3]. In urban countries, however, the land stacking is no good alternative. Another possibility is to use the so-called phosphogypsum [40].
Applications already operational or still under investigation are [8,40]: - in the building industry for the production of gypsum board, blocks etc. - as a settling retarder for cement
- in super sulphated cement
- as a filling material for paper, plastics, paint, etc. - for conversion into sulphuric acid and cement additives - for conversion into sulphur
- for conversion into ammonium sulphate - for agricultural use
- as a substitute for sand in road works - in glass production
- as new building materials
- in the production of gypsum ammonium nitrate as a substite for calcium ammonium nitrate
In most applications, the cadmium content of the phosphogypsum remains a problem, because leaching of heavy metal ions cannot be totally prevented [18]. If the phosphogypsum is used for manufacturing of building materials, the radium content is also a major problem, because the radio-active radon gas, a decay product of radium, can build up in the atmosphere. Methods to reduce the radium content of the phosphogypsum, however, have already been developed [26,34,40].
1.5. Aim of this study
In the foregoing paragraphs the importance of the impurity content of both phosphoric acid and its byproduct, phosphogypsum, has been elucidated. From the viewpoint of environmental pollution action is required.
The aim of this study is the development of a new commercially competitive process for the production of concentrated phosphoric acid, where the following requirements must be fulfilled:
- a low cadmium content and a low fluoride content of the phosphoric acid and - a low cadmium content and a low phosphate content of the calcium sulphate
byproduct as well as a low radium content to make the phosphogypsum suitable for building purposes.
1.6. Literature
1. Becker, P., Phosphates and Phosphoric Acid, Fertiliser Science and
Technology Series, Vol. 3, New York, Marcel Dekker Inc. (1983). 2. Beevers, C.A. and Mclntyre, P.B., Min. Mag. 27 (1947) 254.
3. British Sulphur Corporation, Phosphoric Acid, Outline of the industry, British Sulphur Corporation Limited, London, 4th. Ed. (1984).
4. Burova, M.S. and Kazak, V.G., Khim. Prom., 1 (1985) 29.
5. Calis, G.H.M., "The role of impurities in nitrophosphate fertilizer production," 192th A.C.S. national meeting, september 7-12 (1986) Anaheim, U.S.A.
6. Caro, J.H. and Freeman, H.P., J. Agr. Food Chem., 9, 3 (1961) 182. 7. Caro, J.H. and Hill, W.L., J. Agr. Food Chem., 4 (1956) 684.
8. Chang, W.F., Ed. "Condensed Papers of the Second International Symposium on Phosphogypsum, December 1986, University of Miami, Florida, USA. 9. Chemische Fabrik Budenheim, DE 3218599A1 (1-2-1983),
DE 3327394A1 (14-2-1985).
10. Chien, S.H., Soil Science 123, 2 (1977) 117. 11. Chien, S.H. and Black, C.A.,
Soil Science Soc. Am. J., 40 1976) 234. 12. Deer, W.A., Howie, R.A. and Zussman, J.,
13. Dufour, P., Predali, J.J. and Ranchin, G., US. patent 4.436.616., mar. 13, 1984. 14. Gremillion, L.R. and McClellan, G.H.,
Trans. Soc. Mining Engineers of AIME, 270 (1981) 1975. 15. Hill, W.L., Caro, J.H. and Wieczorek, G.A.,
J. Agr. Food Chem., 2, 25 (1954) 1273.
16. Houot, R., Int. J. of Min. Processing, 9 (1982) 353-17. Hunter, D., "Low-Cadmium H-PO. for fertiliser use,"
Chemical Week, 136, 8 (1985) 25.
18. Jansen, M., Waller, A., Verbiest, J., Van Landschoot, R.C. and Van Rosmalen, G.M., Industrial Crystallisation v8 4 , The Hague, pg 171.
Ed. Jancic and De Jong, Elsevier, Amsterdam (1984).
19. Knudsen, K.C., Proc. of the Fertiliser Society of London* 3 oktober 1985. 20. Kreidler, E.R., "Stoichiometry and Crystal Chemistry of Apatite,"
PhD Thesis, Department of Ceramic Science, Pennsylvania State University, Michigan, USA (1967).
21. Lawver, J.E., McClintock, W.0. and Snow, R.E., Minerals Sci. Engng., 10, 4 (1978) 278.
22. Leder, F., Park, W.C., Chang, P.W., Ellis, J.D., Megy, J.A., Hard, R.A., Kyle, H.E., Mu, J. and Shaw, B.W.,
Ind. Eng. Chem. Process Des. Dev., 24 (1985) 688.
23. McClellan, G.H. and Lehr, J.R., Am. Mineralogist, 54 (1969) 1374. 24. Montel, G., Bonel, G., Trombe, J.C., Heughebaert, J.C. and Rey, C ,
er
Proc. 1 Congr. Int. des Composes Phosphores, Rabat, 17-21 oct., (1977). 25. Nordengren, S., Francia, I. and Nordengren, R.,
Proc. of the Fertiliser Society of London, nr. 33 (1955). 26. Olin Corporation, US patent 4,146,568, Mar. 27, (1979). 27. Philipson, T., Lantbrukshogskolans Annaler, 29 (1963) 267.' 28. Phosphoric Acid Manufacturing using Hydrochloric Acid,
Phosphorus and Potassium, 125 (1983) 29. 29. Purifying wet-process Phosphoric Acid,
Phosphorus and Potassium, 139 (1985) 34. 30. Rubin, A.G., "The BESA-2 Process,"
31. Scott, W.C., Patterson, G.C. and Hodge, C.A., Fert Solutions, 18 (1974) 62.
32. Slack, A.V., "Phosphoric Acid", Fertiliser Science and Technology Series, Vol. 1, Marcel Dekker, New York, (1968).
33. Societe Uranium Pechiney, NL patent 8103788 (26-06-1985). 34. Stamicarbon B.V., NL patent 8006946 (16-06-1982).
35. Taperova, A.A. and Shulgina, M.N., J. Appl. Chem. USSR, 23 (1950) 27. 36. The nitro-phosphates Alternative, Fertiliser International, 209 (1985) 8. 37. Tjioe, T.T., PhD Thesis, To be published,
Technical University of Delft.
38. UKF/DSM, Private Communication with D.C. Oosterwijk. (1986).
39. Vaman RaO, D., Narayanan, M.K., Nayak, U.B-., Anantnapadmanashan, K. and Somasundaraan, P., Int. J. of Mineral Processing, 11 (1985) 57.
40. Weterings, K., "The Utilisation of Phosphogypsum,"
2. SCOPE OF THIS INVESTIGATION
Especially in Europe their is a growing concern about the influence of heavy metals on human health. One of the results of extensive discussions between the members of the European Community is to prohibit the use of cadmium containing materials and to forbid the disposal of cadmium in the environment.
Some companies, however, have a license for disposal of their cadmium containing byproducts which expires in the near future. Due to the new environmental restrictions their licences will not be renewed under the same terms. So they have to look for solutions of their problems. One of these companies is DSM which license for disposal of phosphogypsum into the river ends in 1988.
The use of the phosphogypsum is also limited by its impurity content. Moreover, the total quantity of phosphogypsum produced in the Netherlands by far exceeds its potential use in known applications.
Studies initiated and sponsered by the Government, were therefore started to look for new applications of phosphogypsum and for new processes to produce clean phosphogypsum, suitable for application. Additionally, the cadmium content of fertilisers, made from phosphoric acid should be reduced as well to prevent the build up of cadmium in the soil and thus an increase in cadmium in the food chain.
This study is part of the development of a new process route for the production of clean phosphoric acid and clean gypsum. There are three basically different routes.
First of all the amount of cadmium introduced into the process can be reduced by either using phosphate ore with a low cadmium content, of which the availability is limited, however, or by removal of cadmium from the phosphate ore prior to its use. This last possibility is attainable by either calcination of the ore or by selective digestion of the carbonate fraction of carbonate rich phosphate ores with a mineral acid with a relatively high halogenide content. Although the carbonate fraction of these ores is known to contain an appreciable quantity of the total amount of cadmium present in the ore, a reduction with a factor two of the total cadmium content of the ore is the best result obtained so far. The calcination procedure consumes much energy and moreover, the CdO is released into the atmosphere, because it is difficult to recover the cadmium completely from the off gasses.
The second method is to prevent the incorporation of cadmium in the phosphogypsum by addition of complexing agents for cadmium and to remove it from the phosphoric acid afterwards, by either solvent extraction, flotation, flocculation or ion exchange. If the cadmium content of the ore is high, the free cadmium level in the solution can only be reduced to an acceptable low level with the addition of relatively large amounts of complexing agents. The addition of complexing agents for cadmium is therefore, a good solution for phosphate ores with a relatively low cadmium content.
The third alternative is the integral method by which the cadmium is removed from the process stream and thus simultaneously reducing its
incorporation in the phosphogypsum. This alternative can be obtained in two ways. The first one is the removal of cadmium, ions from the solution before the precipitation of gypsum. Therefore a predigestion step is necessary to obtain a clear calcium-di-hydrogen-phosphate solution, from which the cadmium ions can be removed by either ion exchange, extraction or another technique. The second way is based on the relatively high cadmium uptake of calcium sulphate anhydrite (chapter 5 ) . The cadmium is removed from the solution by its incorporation in a small amount of calcium sulphate anhydrite, precipitated from the recycled phosphoric acid by addition of calcium ions.
To produce concentrated phosphoric acid and phosphogypsum both with a low cadmium content the alternative with the predigestion step has been chosen, because this process route also has the possibility to optimize the digestion step and the crystallisation step independently from each other, and moreover complexing agents can be applied for a further reduction of the cadmium content of the phosphogypsum.
This thesis covers important parts of the development of a "clean technology phosphoric acid" (CTPA) process along a predigestion route. It is composed of a series of self consistent articles, some of which have already been published in the course of the investigation. A small overlap in the contents of the various chapters as well as some inconsistency in the notations were therefore unavoidable. The various articles have been arranged according to the sequence in process steps, when possible.
Chapter 3 gives a brief overview of the Clean Technology Phosphoric Acid (CTPA) process, as developed at the Technical University of Delft. Also the separate process steps are indicated and briefly discussed. This chapter can be regarded as an extended summary of the investigation. It links the separate process steps together.
One of the new and essential steps of the CTPA process is treated in chapter 4. Here the digestion of phosphate ore in phosphoric acid is treated. The digestion conditions were systematically varied by changing the phosphoric acid concentration, temperature and particle size of the Zin phosphate ore.
The subject of chapter 5 concerns the second new and important step of the CTPA process, the crystallisation of calcium sulphate hemihydrate from a clear solution. The crystallisation conditions were varied over a wide range by changing the phosphoric acid concentration, the sulphuric acid concentration, the stirrer speed, the solid over liquid weight ratio and the residence time in the crystalliser. The cadmium as well as the phosphate incorporation in the precipitated calcium sulphate hemihydrate (HH) were measured.
Chapter 6 describes the filtration experiments, performed with the phosphoric acid-HH slurries obtained from the experiments described in chapter 5. The permeability of the calcium sulphate hemihydrate (HH) cake obtained by filtration of the phosphoric acid-HH slurry from the crystalliser is determined as a function of process parameters like the sulphate content and the residence time of the solution in the crystalliser.
Chapter 7 deals with the fluoride distribution between the phosphoric acid and the gas phase. Because of environmental restrictions the fluoride removal must take place in a controlled way. The fluoride distribution coefficient is studied as a function of temperature, phosphoric acid concentration, sulphuric acid concentration and fluoride content of the solution. Moreover, an attempt is made to develop an equation, which can be used to predict the fluoride distribution coefficients as a function of the process parameters.
Most of the results obtained in the foregoing chapters are used in chapter 8 to estimate the mass and heat balances of a 1000 tons P„0C per day producing
plant, operating according to the CTPA process. Most attention a priori was focussed on energy and phosphate efficiencies. The flowsheeting program TISFLO, developed dy DSM is used for the calculation of the composition of all process streams and of the overall mass and heat balances.
To study the influence of impurities, a bench-scale plant of the more relevant parts of the CTPA process was build. The influence of impurities on the phosphate and cadmium incorporation in the HH as well as on the permeability of the HH cake obtained by filtration of the HH slurry was studied.
The results obtained in this study should be combined with the results obtained from a simultaneous study of the cadmium removal techniques [1] and with the results obtained from a reasonably sized pilot plant; operating according to the CTPA process, to give a clear view of the possibility to produce "clean" phosphoric acid and "clean phosphogypsum" with the CTPA process in an economically feasible way.
[1] Tjioe, T.T., PhD Thesis, To be published, Technical University of Delft
3. OVERVIEW OF THE CTPA PROCESS
3.1. Summary
Phosphoric acid for use in fertiliser applications is mainly produced by a 'wet process', i.e. by digestion of phosphate ore with sulphuric acid. In such wet processes, however, impurities like cadmium and radium, originating from the phosphate ore are distributed between the phosphoric acid and the byproduct, a calcium sulphate modification. The use or disposal of the byproduct and probably the use of the phosphoric acid in future as well is limited by its impurity content.
The aim of the currently developed process is the direct production of concentrated phosphoric acid with a low cadmium content as well as the production of a major amount of calcium sulphate as hemihydrate (HH), with a low phosphate, cadmium and radium content, in a commercially feasible way.
For this purpose the phosphate ore is first completely digested in recycled phosphoric acid, containing about 40 w? P?0,- a n d 1«8 w? H?S0j,. After separation of the insoluble ore residue together with a minor amount of calcium sulphate hemihydrate, precipitated in the digester, a clear calcium-di-hydrogen-phosphate
2+
(CDHP) solution is obtained. From this solution the Cd -ions can be removed by e.g. ion exchange. Thereafter the calcium ions are removed by adding concentrated sulphuric acid to the CDHP solution in the crystalliser at 90 °C. In this way a clean calcium sulphate hemihydrate can be obtained. In order to optimize the individual process steps, each step had to be investigated separately.
By performing HH crystallisation experiments, a linear relationship was found between the molar phosphate over sulphate ratio in the crystals and in the solution. The phosphate content of the crystals decreases with increasing sulphate concentration in the crystalliser. It was found that above 3 w$ H S O . the phosphate content of the HH was lower than 0.1 vi% P2°c:- T n e cadmium incorporation was also measured as function of the operating conditions and appears to increase significantly with raising sulphate concentrations above 2 w* H2S Or
Filtration studies showed, that the HH crystals obtained during digestion of the ore, are difficult to filter, while the HH crystals developed in the crystalliser, filter quite well. A maximum filtration rate was reached, if a sulphate content of about 1.8 wï H SO. was maintained in the crystalliser.
A study of the fluoride emissions in the process led to the development of an expression for the fluoride distribution coefficient as a function of the operating conditions.
Preliminary results from a continuously operated bench-scale plant, in which part of the process is studied, were in agreement with the results obtained from the separate studies.
A. preliminary process flowsheet and mass balance, in combination with our experimental results, show that concentrated phosphoric acid (40 wJ'P-Cv) with less than 5 ppm Cd as well as clean calcium sulphate hemihydrate with less than 1 ppm Cd and less than 0.3 w% P?0_ can be produced.
3.2. Introduction
Phosphoric acid is mainly used for the manufacturing of fertilisers. The phosphoric acid production is almost directly linked to the world phosphate fertiliser consumption, which is expected to rise from 30,7 million tons P-CL in 1982 to about 11,7 million tons P„0,. in 1990. This represents an increase of the wet phosphoric acid production from 19,7 million tons P-CL in 1982 to 27,8 million tons P.0,. in 1990 [ 1 9 ] .
Phosphoric acid can be released from phosphate ore by the action of strong mineral acids such as nitric acid, sulphuric acid and hydrochloric acid. Sulphuric acid is the only acid which forms an insoluble precipitate with the calcium of the phosphate ore, thus allowing the phosphoric acid to be separated directly by filtration. The chlorides and nitrates of calcium are both soluble, so special techniques, like solvent extraction [18] ion exchange [14] or cooling crystallisation [ 2 ] , [31] are required to recover the phosphoric acid. In spite of this, phosphoric acid is being commercially produced using nitric acid as well as hydrochloric acid. The economical feasibility of these processes is mainly determined by the availability and price of the mineral acid and the local production facilities.
Although these acidulation methods are all referred to as "wet processes", the term "wet process" is mainly reserved for processes in which sulphuric acid is applied.
During the production of phosphoric acid from phosphate ore by wet processes, e.g. by applying sulphuric acid, huge amounts of calcium sulphate are precipitated as a byproduct. Depending on the temperature as well as on the phosphate and sulphate content of the solution, either calcium sulphate
dihydrate (DH), hemihydrate (HH) or anhydrite (AH) will be formed, as is shown in figure 1 [1, 29].
100-temp°C
80-
60-40'
20
20 30 40 50w % P
20
5 ^Figure 1: Phase diagram of calcium sulphate in phosphoric acid.
The solid lines in figure 1 represent quasi-equilibrium curves, which indicate which phase will initially precipitate under the given conditions. The broken line in figure 1 bounds the regions, where either DH or AH is stable. The HH phase only exists as a metastable phase. The influence of the sulphuric acid content can be taken into account by assuming that 1 mole of sulphuric acid is equivalent to about 1.5 mole of phosphoric acid.
Commercial wet processes are developed producing either DH, HH or AH or even combinations as a byproduct. Becker [1] summarizes the advantages and disadvantages of such processes.
The main disadvantage of the DH, the single filter HH/DH and the DH/HH processes is the production of a relatively diluted (about 30 w$ P„0_) 2 D
phosphoric acid, which has to be concentrated for use in fertiliser applications. Due to the expensive concentration step, the HH and the two filter HH/DH processes, allowing direct production of 40 - 50 w$ P 0 from the filter, are rapidly gaining field. AH processes are not operational at this moment [1], due to serious problems observed in small scale commercial batch plants [17], such as the severe corrosion at the required high temperature and the low growth rate of AH.
AH precipitation
HH precipitation
DH precipitation
- - C
A H stable
, / D H stable""*N
30
In all wet processes, impurities, such as radium ions and heavy metal ions, like cadmium, originally present in the phosphate ore, are distributed between the phosphoric acid (up to 80$ of the Cd) and the byproduct, 'phosphogypsum'.
Cadmium is responsible for a painful bone disease known as Itai-Itai. Concentrations as low as 1 mg/1 in water have led to that illness [1], Cadmium is one of the most worrisome environmental problems, and in the last years there is growing concern, particularly in Europe, that this toxic metal, which is present in e.g. the wet phosphoric acid, could enter into the food chain in increasing amounts. Some countries intend to adopt a cadmium limitation of no more than 90 mg per kg P20_ in the fertiliser product as a preventive measure [12,33]. The Netherlands plans legislation restricting the levels of heavy metals in phosphate fertilisers as well • as in other products, used in agriculture and also the levels in phosphogypsum, which is disposed into the water. These restrictions may well be taken up by the European Economic Community in the near future.
The byproduct, calcium sulphate, also tends to incorporate phosphate ions, which lowers the overall phosphate efficiency of the process. The use of this byproduct as a building material is hindered by its radium and phosphate content, while the development of new applications for phosphogypsum is limited by the uptake of heavy metal ions, since leaching of heavy metal ions can not be totally prevented [13]. The direct production of concentrated and relatively clean phosphoric acid is so far only possible by applying the furnace process, an alternative to acidulation processes.
In the furnace process the phosphate ore is reduced, by addition of silica and carbon, to phosphorus and slag at about 1500 °C:
2 Ca1 Q(P01 ))6F2 + 20 S i 02 + 28 C - — > 20 CaSiO + 28 CO + 3 P^ + 2 F2 (1)
AH = 27.9 MJ/kg of P^ produced [16].
The heat needed for this reaction is substantial, and could, until a few years ago only be achieved by internal resistance heating of the molten charge in an electric furnace. In 1985, however, a direct fired rotary kiln was used to supply the heat [ 1 6 ] . After combustion of the phosphorus, the P 0, is
2 5 absorbed into water or moderately concentrated phosphoric acid, to obtain concentrated phosphoric acid (>60 w$ P„0,-). The direct production of concentrated and clean phosphoric acid is possible with the furnace process, because most impurities from the ore remain in the slag [19]. This process,
however, is too expensive for use in the fertiliser industry, due to the high energy cost. Moreover, there are other possibilities to remove Cd from wet phosphoric acid, like for instance flotation [28], solvent extraction [21] or the Budenheim process [3]. In all these processes, however, the disposal of the byproduct remains a problem, mainly due to its cadmium content, while the use of the byproduct as e.g. building material, is hampered by its radium content.
In order to produce not only relatively clean and concentrated phosphoric acid for fertiliser applications, at a feasible price, but also pure HH, the wet Clean Technology Phosphoric Acid (CTPA) process is currently being developed.
3.3. Objective of the study
The aim of the CTPA process is the production of concentrated phosphoric acid with a low cadmium and fluoride content as well as the production of calcium sulphate hemihydrate with a low cadmium, radium and phosphate content in a commercially feasible way.
3.4. Description of the CTPA process
3.4.1. Introduction to the process
2+
To meet the foregoing requirements, the Cd -ions have to be removed from 2+ a large process stream. An alternative would be to prevent incorporation of Cd
2+ 2+ '
-ions in the HH or to leach the Cd -ions from the HH, in which cases the Cd -• ions only have to be removed from the much smaller product acid stream. Removal of Cd from the phosphate ore by e.g. calcination, is far too expensive to be a reasonable alternative. Selective leaching of cadmium from ground phosphate ore by a diluted mineral acid with a high chloride concentration is another possibility [4]. Due to the high chloride content, however, severe corrosion will probably occur. So far no methods are known to prevent the Cd uptake during crystallisation sufficiently, to produce HH with less than 0.5 ppm Cd, nor methods to leach the Cd from the byproduct until an acceptable low level is reached. Therefore, removal of Cd from the large process stream, before the major amount of the calcium sulphate is crystallised, is a necessity.
For this purpose the digestion of phosphate ore and the crystallisation of HH, which normally occur more or less simultaneously, have to be divided into two separate stages (see figure 2 ) .
This also makes it possible to control each stage apart and to optimize each stage on its own, instead of the combination of the two. The first step of the process, the digestion of phosphate ore, occurs in a recycle stream of phosphoric acid. Since the recycled acid stream, however, unavoidably contains sulphate ions in an amount dictated by the operating conditions in the crystalliser, some of the HH, up to a maximum of 25 w$, will precipitate during digestion of the ore.
After filtration of the insoluble ore residue, together with the'relatively small amount of HH precipitated, a clear calcium-di-hydrogen-phosphate (CDHP)
2+
solution is obtained, from which the Cd -ions can be removed by ion exchange or extraction. Thereafter the HH is crystallised. The HH precipitated during digestion of the phosphate ore has to be recrystallised into DH in order to raise the overall phosphate efficiency of the process to an acceptable level (> 98 w$ P„0C recovery). Although recrystallisation of this part of the HH is
necessary, concentrated phosphoric acid (> 40 w$ P„0_ ) is still produced, c. D
because the recrystallisation step is fully integrated in the process. The scheme of the process is given in figure 2.
H2SOfc
\
digestior Cd removal recrystallisation H H — — OHt
DH recycle acid HH crystallisation 1 HH productFigure 2: Process scheme.
The most important steps of the CTPA process will be discussed in the next sections, except for the Cd removal step, which will be presented elsewhere [32]. The principle of the Cd removal by ion exchange is the addition of a cadmium specific inorganic complexing agent, like bromide or iodide ions, followed by the removal of the Cd-halogenide complex by anion exchange with commercially available basic ion exchange resins.
3.4.2. The digestion stage
The main phosphate ores used today are sedimentary phosphates or franeolltes. These ores mainly consist of fluoroapatite with part of the phosphate ions replaced by fluoride and carbonate ions. By beneficiation of the
ore, large amounts of waste material are removed. The treated ore, however, still contains residual calcite and dolomite, which cause severe foaming during the digestion [1].
Fluoroapatite is dissolved in pure phosphoric acid according to the following reaction:
C a1 Q( P 04)6F2 + 114 HgPO,, — > 10 C a ( H2P 04)2 + 2 HF (2)
fluoroapatite CDHP
after which the calcium-di-hydrogen-phosphate, further referred to as CDHP, dissolves in the phosphoric acid stream.
6 5 4 3 2 1 0 30 35 40 45 ^ w % P205
Figure 3: Solubility of CaO in phosphoric acid.
The amount of CDHP which can be dissolved in phosphoric acid strongly depends on the temperature and the phosphate content of the solution, as shown in figure 3 [7].
Phosphoric acid to be used for the production of mono- and di-ammonium-phosphate must at least contain 40 w? P„0C [11]. The temperature in the
2 D
digestion stage i s r e s t r i c t e d by the HH-AH phase boundary, because the p r e c i p i t a t e d calcium sulphate has to be HH, which can be e a s i l y r e c r y s t a l l i s e d into DH. In order to minimize the recycle acid stream, necessary for complete digestion of the phosphate o r e , conditions with the highest CaO s o l u b i l i t y have
3t w%CaO
100 °C
to be chosen. The optimum conditions for digestion of the phosphate ore are therefore about 40 w? P-O- and approximately 90 °C.
When a mixture of phosphoric acid and sulphuric acid is used for the digestion of the ore, two processes will occur more or less simultaneously: digestion of the phosphate ore and precipitation of calcium sulphate (HH) [1]. At sulphate concentrations in the digestion stage above 2.5wJ H SO., the HH tends to precipitate not only upon the present HH crystals, but also upon the ore particles. Such coating of the ore reduces the digestion rate and is called blinding [ 5 ] .
Since the highest possible digestion rate should be obtained, the sulphate concentration in the digestion stage must be carefully selected in order to avoid blinding. In a continuous process, blinding can only be avoided by keeping a low sulphate concentration in the digester, which implies maintaining a high calcium concentration. Under such conditions, the continuously added sulphate ions precipitate upon the hemihydrate crystals and the digestion can be treated as if proceeding in pure phosphoric acid. The digestion experiments were thus performed in pure phosphoric acid [25]. At 90 °C, however, the digestion proceeds very fast and is accompanied by severe foaming. Most experiments were thus performed at lower temperatures, with pretreated ore.
The phosphate ore used was a mixture of Khouribga/Zin in a 40/60 \>% ratio.
In order to reduce the foaming, the ore was pretreated to remove calcite and dolomite with a 0.5 N tri-ammonium-citrate solution at pH = 8.1 for a few days [22]. The remaining fluoroapatite was washed with water and dried at 60 °C for a few days. Thereafter, the ore was divided in twelve fractions by sieving through "Twente" sieves. The ore particle sizes range from 150 to 2000 ym. The 1.5 liter reaction vessel was thermostated. About 50 grams of ore were suspended in about 1 kg of acid. To monitor the digestion process with time, periodically samples were taken from the reactor by vacuum withdrawal, filtered over a G^ glass
filter, coated with perlite, and analysed for the CaO content.
From the results obtained, it can be deduced that in a first approach the digestion of phosphate ore in phosphoric acid depends on the concentration of
the phosphoric acid as well as on the temperature and the particle size of the phosphate ore. In figures 4 and 5 it is shown that, as expected, the smaller the particle size of the ore and the higher the temperature, the higher the digestion rate of the calcium present in the ore.
If the rate determining step in the digestion process were a chemical reaction, an increase of the digestion rate with increasing acid concentration is expected, due to the increase of the hydrogen ion concentration. If, however,
diffusion is the rate determining step, a decrease of the digestion rate is expected, due to the increase of the viscosity with increasing acid concentration. conversion 0.00129-0,00151 0,000655-0.000780 0.000462 0,000181-0,000256 time (mini
Figure 4: Influence of the particle size on the digestion rate of the phosphate ore at 50 °C in 40 u% Po0r.
2 D
conversion
emp.PC) particle size (ml 50 0,00185 70 0.00185 50 0,00078 60 0,00078 70 0,00078 10 time (min)
-Figure 5: Influence of the temperature at different particle sizes on the digestion rate of the ore in 40 w% P„0,_.
2 5
In figure 6 the influence of the concentration of phosphoric acid on the digestion rate shows that from 35 up to 45 w? P_0 the digestion rate decreases, while for 30 up to 35 w$ PT) the digestion rate seems to increase.
2 O
This can be explained by assuming that at 50 °C the rate determining step changes from chemical reaction at low phosphoric acid concentrations to diffusion for higher phosphoric acid concentrations. At 90 °C diffusion will be the rate determing step also for 30-35 wj P-O... This study has been continued in
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order to determine the rate controlling mechanism at different operating conditions [23, chapter 4 ] . Preliminary results from our bench-scale plant showed that a residence time of about 1 hour is enough, for ore particles up to 2000 vim, to reach more than 99 % conversion.
t
conversion 1 -50°C 0.000655m f Ii
u 5 10 time (min) •-Figure 6: Influence of the phosphoric acid concentration on the digestion rate of phosphate ore at 50 °C.
3.4.3. The crystallisation stages
In the CTPA process several crystallisation stages exist. The first occurs in the digestion stage. The recycled acid, used for digestion of the phosphate ore, contains approximately 1.8 w$ H_SOü, which almost completely precipitates
as HH. From previous batch experiments [13] the phosphate incorporation in this HH is expected to be high (above 2 w$ P„0_). In our continuously operated bench-scale plant also a phosphate incorporation of approximately 3 w? P_0C was found
in this HH. Because about 25 wj of the total calcium present in the phosphate ore is precipitated during digestion, the overall phosphate efficiency of the process would be unacceptably low. Therefore, this HH must be recrystallised to reduce the phosphate incorporated in the calcium sulphate crystal lattice. It is known [10] that due to recrystallisation of HH into DH, a reduction in the phosphate incorporation of about 90 % can be achieved.
Batch recrystallisation experiments of HH into DH showed that a phosphate incorporation of 0.3 w$ P2°5 i n D H '-35^ i 3 a reasonable estimate of the value to
be obtained in a production plant. A residence time of approximately 5 hrs [10], however, is needed for complete conversion of HH into DH. In order to reduce
temperature = particle size = w% P; 30 35 40
