REDUCTION OF THE CADMIUM CONTENT OF
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REDUCTION OF THE CADMIUM CONTENT OF
PHOSPHORIC ACID AND CALCIUM SULFATE
REDUCTION OF THE CADMIUM CONTENT OF
PHOSPHORIC ACID AND CALCIUM SULFATE
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 dinsdag 8 september 1987 te 14.00 uur door
Tjay Tjien Tjioe
scheikundig ingenieur,
geboren te Pekalongan
Druk: 1987 Sneldruk EnschedeTR diss
1557
Prof. dr. ir. G.M. van Rosmalen
Prof. ir. J.A. Wesselingh
STELLINGEN
1. Het bestaan van v e r s c h i l l e n d e m o d i f i c a t i e s van calclumsulfaat i s e e r d e r een zegen dan een l a s t b i j de p r o d u c t i e van schoon c a l c l umsulfaat en schoon fosfor zuur.
2. Gezien het wijd v e r b r e i d e optreden van (ongewenste) a a n k o r s t i n g en d e p o s i t i e d i e n t meer onderzoek v e r r i c h t t e worden aan h e t e r o gene kiemvorming en hechting van aankorstingvormende, k r i s t a l l e n of verbindingen.
3. Bij de e l e c t r o l y t i s c h e verwijdering van cadmium u i t "zwart" fosforzuur spelen naast de overspanning van andere veront r e i n i g i n g e n en w a t e r s t o f , de d e p o s i t i e en de hechting van v e r o n t r e i n i g i n g e n aan de e l e c t r o d e een b e l a n g r i j k e r o l . Een u i t s p r a a k over de toepasbaarheid van een e l e c t r o d e m a t e r i a a l zonder rekening t e houden met het bovenstaande heeft weinig waarde, aangezien de oppervlakte-eigenschappen van de e l e c t r o d e s t e r k beïnvloed kunnen worden door de d e p o s i t i e van veront r e i n i g i n g e n .
1. Het gebruik van een a d s o r p t i e m a t e r i a a l met een h y d r o f i e l e bui-t e n l a a g en een hydrofobe kern voor de i m m o b l l i s a bui-t i e van de organische fase b i j L/L e x t r a c t i e van cadmium u i t fosforzuur b i e d t meer p e r s p e c t i e f dan een homogeen m a t e r i a a l .
5. De bewering van Baechle en Wolsteln, dat l o n e n w l s s e l l n g n i e t g e s c h i k t i s voor de v e r w i j d e r i n g van cadmium u i t fosforzuur wegens de aanwezigheid van een groot a a n t a l andere v e r o n t r e i n i g
ingen, i s o n j u i s t .
(H.-T. Baechle and F. Wolsteln i n : Cadmium compounds in mineral f e r t i l i s e r s , The F e r t i l i s e r Society of London ( 4 t h Oct 1984)).
6. Bij het ontwikkelen van regeneratlemethodes van i o n e n w i s s e l a a r s beladen met s t e r k adsorberende ionen d i e n t aan andere methodes dan ionenwisseling gedacht t e worden.
en r e d u c t i e van de cadmiuminbouw i n c a l c i u m s u l f a a t ( d i r e c t of door o m k r i s t a l l l s a t l e ) i s a a n t r e k k e l i j k e r dan de v e r w i j d e r i n g van cadmium vóór de k r i a t a l l l s a t i e van c a l c i u m s u l f a a t , teneinde de k r i s t a l l i s a t i e in een cadmiumTarm m i l i e u u i t t e voeren.
8. Het groeiende gebruik van complexe mathematische modellen en k r a c h t i g e computers gaat n i e t gepaard met een evenredige groei van f y s i s c h e kennis en b e g r i p van de beschreven processen.
9. Onderzoekers dreigen i n t i j d e n van bezuinigingen en inkrimpingen t e v e r v a l l e n i n k o r t e t e r m i j n denken zoals de meeste p o l i t i c i in v e r k i e z i n g s t i j d e n .
10. Mensen zijn geen l o g i s c h handelende en denkende wezens; ze proberen s l e c h t s z i c h z e l f en de wereld l o g i s c h t e doen l i j k e n .
p r o j e c t van het M i n i s t e r i e van V o l k s h u i s v e s t i n g , R u i m t e l i j k e O r d e n i n g en Milieubeheer, DSM Research BV en de Technische U n i v e r s i t e i t D e l f t , met a l s doel de o n t w i k k e l i n g van t e c h n o l o g i e voor een schoon f o s f o r zuur p r o c e s . Dit p r o j e c t wordt g e f i n a n c i e r d door de M i n i s t e r i e s van V o l k s h u i s v e s t i n g , Ruimtelijke Ordening en M i l i e u b e h e e r , Verkeer en W a t e r s t a a t , Economische Zaken en Landbouw en V i s s e r i j en door DSM.
Contents
Chapter 1 General introduction
Cadmium in the environment Cadmium in phosphates Phosphoric acid processes
Existing cadmium removal techniques Fields of research 1 1 2 H 9 13
Chapter 2 Analytical techniques 19
Chapter 3 Crystallization of calcium sulfate 26 3.1 Cadmium incorporation in the crystallization of
calcium sulfate hemihydrate from phosphoric acid 26 3.2 Cadmium incorporation in the recrystallization of
calcium sulfate dihydrate from phosphoric acid 45 3-3 Impurities in calcium sulfate hemihydrate
determined by instrumental neutron activation
analysis 51
Chapter H Removal of cadmium by electrodeposition,
precipitation and liquid-liquid extraction 51
Chapter 5 Extraction of cadmium from phosphoric acid with
diorganyldithiophosphinic acids 63
Chapter 6 Removal of cadmium by anion exchange in a wet
phosphoric acid process 112 6.1 Part 1: Chemistry 112 6.2 Part 2: Equilibria, kinetics and regeneration 136
Chapter 7 Final Remarks 189
Summary Samenvatting Nawoord 193 196 199
CHAPTER 1
GENERAL INTRODUCTION
CADMIUM IN THE ENVIRONMENT
The element cadmium i s p r e s e n t i n very small c o n c e n t r a t i o n s i n 2+
t h e environment as t h e Cd i o n . Cadmium c o n c e n t r a t i o n s varying from 0.05 ppb t o 0.20 ppb ( 1 , 2) has been r e p o r t e d f o r non-contaminated f r e s h w a t e r s i t u a t e d far from ore l a y e r s or m i n e r a l s o u r c e s . The cadmium c o n c e n t r a t i o n s in ground water a r e i n t h e ppb range ( 3 ) . Cadmium c o n c e n t r a t i o n s in t h e a i r of urban a r e a s a r e about 0.005 ug/m3 ( 1 ) .
Although cadmium has many t o x i c e f f e c t s , i t i s an e s s e n t i a l element for many h i g h e r organisms. Cadmium d e f i c i e n c y , however, w i l l not occur under normal l i f e c i r c u m s t a n c e s , s i n c e food and d r i n k i n g w a t e r always c o n t a i n cadmium. E s t i m a t e s of t h e d a i l y i n t a k e of cadmium by man v i a food and w a t e r vary from 40 yg t o 70 yg ( t ) . About 5-10$ of t h e i n t a k e i s absorbed by t h e body r e s u l t i n g in amounts of 2-7 ug Cd/day which e n t e r t h e l i v i n g c e l l s i n the o r g a n i s m . The d a i l y i n t a k e via a i r i n non-contaminated a r e a s i s e s t i m a t e d t o be 0 . 0 2 - 0 . 2 yg/day, from which 50? i s absorbed by the body. The World Health O r g a n i z a t i o n has recommended a maximum t o t a l i n t a k e of 70 yg per day corresponding with a maximum a b s o r p t i o n of 7 yg Cd per day.
At e l e v a t e d uptake of cadmium t h i s element accumulates i n the human body, e s p e c i a l l y i n the l i v e r and k i d n e y s , and has a t o x i c c h a r a c t e r . The t o x i c i t y i s caused by t h e a f f i n i t y of cadmium for n i t r o g e n and sulphur c o n t a i n i n g groups i n t h e a c t i v e c e n t e r s of enzymes. Some e f f e c t s and symptoms of cadmium t o x i f i c a t i o n a r e : - Replacement of Zn by Cd in enzymes r e s u l t i n g i n Zn d e f i c i e n c y symptoms, a l t h o u g h Zn i s p r e s e n t i n a s u f f i c i e n t amount in t h e body. - A d i s t u r b e d calcium r e g u l a t i o n r e s u l t i n g in a decay of the bones
2
- Disturbance of the copper r e g u l a t i o n r e s u l t i n g i n d i s e a s e s of t h e blood v e s s e l s .
Contamination of the environment with cadmium occurs i n many ways. Some examples a r e harbour s l u d g e , urban w a s t e , s c r a p i r o n , zink s o u r c e s , f o s s i l e f u e l s and phosphate c o n t a i n i n g f e r t i l i z e r s . In 197-4 the y e a r l y i n c r e a s e of t h e cadmium content of The N e t h e r l a n d s was estimated t o be 140-200 t o n ( 1 ) .
CADMIUM IN PHOSPHATES
As mentioned above one of the cadmium sources i s phosphate c o n t a i n i n g f e r t i l i z e r . I n t e n s i v e use of t h e s e f e r t i l i z e r s may i n c r e a s e the cadmium c o n t e n t of a g r i c u l t u r a l land s i g n i f i c a n t l y . Williams (5) r e p o r t e d an i n c r e a s e of t h e cadmium c o n t e n t i n a g r i c u l t u r a l s o i l from 9 ppb t o 162 ppb a f t e r a d d i t i o n of s u p e r phosphates during 40 y e a r s . Estimates of the annual cadmium i n p u t t o a r a b l e land i n some European c o u n t r i e s a r e given i n t a b l e 1 ( 6 ) . The q u a n t i t y of Cd e n t e r i n g a r a b l e l a n d from f e r t i l i z e r s i s of t h e same order of magnitude as t h e atmospheric d e p o s i t i o n o r i g i n a t i n g from metal p r o d u c t i o n and r e f u s e i n c i n e r a t i o n .
TABLE 1 .
Estimated annual cadmium input to arable land in 1980.
Country g/ha Country g/ha The Netherlands 4.6
Belgium 9.4 Denmark 2.4 West Germany 4.6 Aerial sources in uncontaminated areas of Europe: 3 g/ha
The cadmium content of phosphate fertilizers depends on that of the phosphate ore and on the production process. The cadmium content of some phosphate rocks are given in table 2 (7).
Italy France united Kingdom Irish Republic 1.6 5.4 6.5 6.6
TABLE 2 . Cadmium c o n t e n t of phosphate r o c k s . Name, c o u n t r y Kola, USSR Oron, I s r a e l Z i n , I s r a e l Khouribga, Marocco J o u s s o u f f i a , Marocco A l g i e r , A l g e r i a ppm 1 3 20 1-17 4 23 Name, c o u n t r y F l o r i d a , USA N. C a r o l i n a , USA Taiba, Senegal Togo, Togo Gafsa, T u n i s i a BuCraa, Span. Sahara
ppm 3-12 36 68-110 38-60 56 13
Phosphate o r e i s a calcium c o n t a i n i n g rock and t h e main component i s u s u a l l y f l u o r o a p a t i t e , 3Ca (PCO-.CaF . Most of the phosphate rock i s converted i n t o phosphoric a c i d by a s o - c a l l e d wet p r o c e s s , which i s an i n t e r m e d i a t e s t e p i n the production of phosphate f e r t i l i z e r s . In most wet phosphoric a c i d p r o c e s s e s t h e ore i s t r e a t e d w i t h s u l f u r i c a c i d g i v i n g the s o l i d byproduct CaSOü.nH?0
and an aqueous s o l u t i o n of the product H-PCL. The i m p u r i t i e s in the phosphate o r e end up i n both t h e byproduct and i n t h e phosphoric a c i d s o l u t i o n . The c o n c e n t r a t i o n s of some i m p u r i t i e s in t h e o r e , t h e phosphoric a c i d s o l u t i o n and t h e byproduct CaS0^.2K 0 (gypsum or d i h y d r a t e ) i n a s o - c a l l e d h e m i h y d r a t e - d i h y d r a t e process a r e shown in t a b l e 3 ( 8 ) .
Both t h e phosphoric a c i d s o l u t i o n and the d i h y d r a t e are contaminated with t h e i m p u r i t i e s o r i g i n a t i n g from the phosphate o r e . In t h e h e m i h y d r a t e - d i h y d r a t e process 20% of the cadmium ends up in t h e byproduct gypsum. This contamination may hamper t h e a p p l i c a t i o n of phosphogypsum and may i n c r e a s e t h e cadmium burden of the environment i f the gypsum i s disposed of. These problems i n i t i a t e d t h e r e s e a r c h on t h e development of a new phosphoric a c i d p r o c e s s . The aim of t h i s study i s t h e development of methods for the p r o d u c t i o n of phosphoric a c i d and calcium s u l f a t e both with a low c o n t e n t of cadmium.
4
TABLE 3 .
Composition of t h e phosphate o r e (Khouribga/Zin 3 0 / 7 0 ) , the phosphoric a c i d s o l u t i o n (40 w$ H PO J and t h e byproduct gypsum. The major components a r e expressed as t h e i r o x i d e s .
component ■
(?)
P2 ° 5 CaOso
3 F S i 02 A 12 ° 3 F e2 ° 3 Na20 ore 31 52 2 4.2 2.2 0.54 0.19 0.73 a c i d 29 0.5 2.0 1.7 0.33 0.18 0.09 gypsum 0.40 32.5 45.3 1.1 0.6 0.14 0.005 0.47 element (ppm) Cr Mn Ni Cu Zn As Cd Pb V U Ra ore 170 14 34 32 310 10 23 4 240 118 40ppb acid 160 10 30 25 300 10 19 <1 15 100 gyp 5 3 2 4 <10 1 2-3 2 -<15PHOSPHORIC ACID PROCESSES
Several designs of s o - c a l l e d wet p r o c e s s e s e x i s t for t h e production of phosphoric a c i d . Figures 1a-1c show t h e b a s i c elements of t h r e e process d e s i g n s . These p r o c e s s e s can be d e s c r i b e d with t h e following s t e p s , which may occur s i m u l t a n e o u s l y .
1. Digestion ( d i s s o l u t i o n ) of t h e phosphate o r e i n phosphoric a c i d .
C ai 0( P 04)6F2 + U H3P 04 = = > 1 0 C a ( H2P 04)2 + 2 H F
2 . C r y s t a l l i z a t i o n of calcium s u l f a t e hemihydrate (HH). Ca(H2P01J)2 + H2S0^ + 1/2 H20 ==> CaSOJ4.1/2H 0 + 2 H PO^
3 . R e c r y s t a l l i z a t i o n of HH i n t o calcium s u l f a t e d i h y d r a t e (DH). CaSO^.1/2H20 + 1.5 H20 ==> CaS04.2H 0
The r e c r y s t a l l i z a t i o n of HH i s performed t o r e d u c e t h e amount of i n c o r p o r a t e d phosphate ions and t o improve t h e f i l t e r a b i l i t y and w a s h a b i l i t y of the c r y s t a l s .
S S " * " " H
2S0
42
E
®
DH digestion • cryst. HH recryst. H H - » D H 90°C 70-50°C"®
recycle acid©
product acid M W % H3P 04 FIGURE 1a. S i n g l e f i l t e r h e m i h y d r a t e - d i h y d r a t e p r o c e s s phosphate ore H2S04 digestion cryst.HH 90°C recycle acid HH H2S04i®_r
recryst. H H - * D H®
.©
70-50°C product acid 40-60W%H3P04 OH FIGURE 1b. T w o - f i l t e r h e m i h y d r a t e - d i h y d r a t e p r o c e s s6 phosphate ore
®
H2S04 digestion ♦ cryst. of HH MCP - (D CSL 90°C H2S04 c r y s h HH HH 90°C recryst. HH-»DH®
70-50°C OH recycle acid HH©
product acid 55W%H3POi(FIGURE 1c. Clean technology phosphoric acid (CTPA) process
In figure 1a a single f i l t e r hemihydrate-dihydrate (HD) process
i s shown (9). The digestion of the ore and the c r y s t a l l i z a t i o n of HH
occur more or l e s s simultaneously at about 90°C. Then the slurry i s
cooled and r e c r y s t a l l i z a t i o n of HH into DH occurs. The dihydrate i s
f i l t e r e d off and part of the acid i s recycled t o the digestion
section. The phosphoric acid concentration in the solution is about
40 w? H.PCv. Higher phosphoric acid concentrations cannot be
obtained with t h i s process, because the r e c r y s t a l l i z a t i o n of HH into
DH by j u s t lowering the temperature of the slurry proceeds very
slowly in more concentrated acid. Since a phosphoric acid
concentration of 55%
H?
POii
i srequired for the ' production of
f e r t i l i z e r s , concentration of the produced acid by evaporation i s
needed.
Concentrated phosphoric acid can d i r e c t l y be obtained with a
two-filter hemihydrate-dihydrate process ( f i g . 1b) (9). Two
phosphoric acid loops e x i s t : a loop of concentrated phosphoric acid
in which HH is crystallized and a second loop of d i l u t e phosphoric
acid in which HH is recrystallized into DH. A disadvantage of t h i s
process scheme i s the necessity of two f i l t e r s resulting in an
enhancement of the investment and of operational c o s t s . These costs
can be reduced by deleting the r e c r y s t a l l i z a t i o n . This so-called
hemihydrate process has a lower phosphate efficiency caused by the
high phosphate content of the byproduct. The lower phosphate
efficiency i s compensated more or less by the direct production of
concentrated phosphoric acid.
In figure 1c a process design which contains elements of both a
hemihydrate and a hemihydrate-dihydrate process i s shown. This
so-called clean technology phosphoric acid (CTPA) process has been
developed and investigated on a laboratory scale by Van der Sluis
(10). In t h i s process the digestion of the phosphate ore and the
c r y s t a l l i z a t i o n of the* major part of the HH are separated and
performed in different reactors. In t h i s way both processes can be
optimized separately. An advantage of this procedure i s , that
hemihydrate with a low phosphate content and a good f i l t e r a b i l i t y
and washability can be obtained resulting in a high phosphate
efficiency without the need to r e c r y s t a l l i z e a l l hemihydrate.
Concentrated phosophoric acid i s directly produced.
Because of the presence of excess s u l f u r i c acid in the recycle
stream some hemihydrate i s also formed in the digestion section
(about 30% of the t o t a l amount). This HH has a high phosphate
content and must be r e c r y s t a l l i z e d into DH. A b o t t l e neck of t h i s
process i s the s o l i d / l i q u i d separation after the digestion section,
since the solid material has a poor f i l t e r a b i l i t y . Another
disadvantage i s the large recycle stream, which i s about eight times
the product acid stream (w/w). This large stream i s necessary to
dissolve the soluble part of the phosphate ore completely giving a
monocalcium phosphate (MCP) solution. The main goal of t h i s process
design i s , however, the production of phosphoric acid and calcium
s u l f a t e both with a low cadmium content. This feature will be
discussed below.
Reduction of the cadmium content of either the phosphoric acid,
or the byproduct calcium s u l f a t e , or both can be obtained at
different locations in the process. These locations are indicated
with the l e t t e r s A to D in figures 1a-1c. To obtain calcium sulfate
8
with a r e l a t i v e l y low cadmium content the process parameters in the
r e c r y s t a l l i z a t i o n section (location A) as well as in the
c r y s t a l l i z a t i o n section (location D) can be adapted. Subsequently
the product acid can be purified in location C. The f l e x i b i l i t y of
the ( r e ) c r y s t a l l i z a t i o n process of the single f i l t e r HD process
( f i g . 1a) and the CTPA process ( f i g . 1c) are more limited compared
with the two-filter process ( f i g . 1b), where the r e c r y s t a l l i z a t i o n
i s performed in a separate loop.
Another strategy i s the purification of the MCP solution or the
acid in the loop (location B). If the cadmium content of t h i s
solution can be reduced far enough, the ( r e ) c r y s t a l l i z a t i o n i s
performed in a clean solution giving clean calcium s u l f a t e . Removal
of cadmium at location B in the s i n g l e - f i l t e r HD process ( f i g . 1a)
i s inefficient, since the phosphate ore i s introduced after the
cadmium removal section giving a high cadmium concentration after
the digestion s t e p . Location of the cadmium removal section between
the digestion section and the r e c r y s t a l l i z a t i o n section is not
feasible, because of the high solid content (30 w?) of the s l u r r y .
Removal of cadmium in location B in the two-filter HD process ( f i g .
1b) is more e f f i c i e n t . However, an additional cadmium removal
section C is then necessary to obtain clean product acid. In the
CTPA process the cadmium removal section can be located between the
digestion section and the HH c r y s t a l l i z a t i o n section, since the MCP
solution has a low solid content. Since the r a t i o (recycle a c i d ) /
(phosphate ore) i s large, the cadmium concentration in the solution
in the digestion section i s s t i l l low and l i t t l e cadmium i s
transported to the r e c r y s t a l l i z a t i o n section. Besides the cadmium
uptake by the HH crystals formed in the digestion section i s
particularly low due to the prevailing process conditions (high
calcium concentration). So one cadmium removal section i s sufficient
to obtain phosphoric acid and calcium sulfate both with a low
cadmium content. A disadvantage of t h i s location i s the large stream
to be treated and the high temperature (90°C). Figure 2 shows some
mass flows and cadmium concentrations in a CTPA process with a
cadmium removal section located between the digestion and the
c r y s t a l l i z a t i o n section.
Cd 32ppm I ■- digestion MCP 7ppm H2S04 Cd removal 4ppm H20 . recrystallisation of HH to DH
3 L
H20 crystallisation of HH H2S04 4ppm ore MCP Cd removal 120ton/h 720m3/h 3.5kg/h —"-0H <1ppm -■►HH <1ppm fluoride removal product acid 40w%P2OsFIGURE 2 . CTPA process with a cadmium removal s e c t i o n l o c a t e d between t h e d i g e s t i o n and t h e c r y s t a l l i z a t i o n s e c t i o n . The v a l u e s i n ppm a r e cadmium c o n c e n t r a t i o n s .
EXISTING CADMIUM REMOVAL TECHNIQUES
In t h e l a s t decade s e v e r a l methods f o r t h e removal of cadmium from phosphoric a c i d and phosphate o r e have been developed. These t e c h n i q u e s w i l l be d i s c u s s e d b r i e f l y and some r e p r e s e n t a t i v e examples w i l l be g i v e n . The methods w i l l be d i v i d e d i n t h e following g r o u p s .
1. P r e - t r e a t m e n t of t h e phosphate r o c k . 2 . Cadmium s u l p h i d e p r e c i p i t a t i o n .
3 . E i t h e r L/L e x t r a c t i o n or p r e c i p i t a t i o n of cadmium with o r g a n i c s u l p h u r - c o n t a i n i n g compounds.
1). L/L e x t r a c t i o n of cadmium with o r g a n i c amines + h a l i d e s .
10
?C§~£r.맣2e.!2£_2£_.£Öë_E!225Et!2ië_r.2c.!$i
The cadmium content of some phosphate rocks can be reduced by c a l c i n a t i o n of the rock in an oxygen c o n t a i n i n g atmosphere a t 900-1000°C, u s u a l l y i n a f l u i d i z e d bed ( 6 , 7, 1 1 ) . Cadmium i s removed as the v o l a t i l e CdO compound. A r e d u c t i o n of the cadmium content from 80 ppm to 20 ppm has been r e p o r t e d f o r Nauru phosphate rock ( 6 ) . However, t h i s t e c h n i q u e i s only s u i t a b l e for phosphate rocks w i t h e x c e p t i o n a l l y low alumina, s i l i c a and i r o n oxide c o n t e n t s . C a l c i n a t i o n of other rocks gave an agglomeration of fused r o c k s r e s u l t i n g in a poor phosphate r e c o v e r y i n t h e d i g e s t i o n .
Another t e c h n i q u e i s s e l e c t i v e l e a c h i n g of f i n e l y ground o r e (< 100 ym) with a c h l o r i d e c o n t a i n i n g d i l u t e s u l f u r i c a c i d s o l u t i o n ( 1 2 ) . This t e c h n i q u e i s based on t h e s e l e c t i v e d i s s o l u t i o n of CaCO , which o f t e n c o n t a i n s a s u b s t a n t i a l amount of t h e cadmium i n t h e o r e . The presence of c h l o r i d e i o n s p r e v e n t s t h e a d s o r p t i o n or 2-p r e c i 2-p i t a t i o n of cadmium by t h e formation of the s o l u b l e CdCl. complex. The presence of s u l f a t e p r e v e n t s t h e decomposition of t h e calcium phosphate p a r t of the o r e by keeping t h e e q u i l i b r i u m calcium c o n c e n t r a t i o n i n t h e s o l u t i o n low. Chemische Fabrik Budenheim (12) has r e p o r t e d a r e d u c t i o n of the cadmium c o n t e n t from 36 ppm t o 8 ppm using a s o l u t i o n c o n t a i n i n g 11.3 w% c h l o r i d e and 3.9 w% s u l p h a t e (ore : leach s o l u t i o n = 1:2 w/w, 56°C). The phosphate l o s s was 0 . 2 kg P.0,_/ton o r e .
C a o ^ i u m _ s u l p h i d e _ g r e c i p i t a t i o n / f i l t r a t i o ni
The e a r l i e s t methods for t h e removal of cadmium from wet-process phsophoric a c i d were based on t h e p r e c i p i t a t i o n of CdS with H S or a s o l u b l e s u l p h i d e s a l t ( 1 3 - 1 7 ) . In some of t h e p a t e n t a p p l i c a t i o n s an o v e r p r e s s u r e was used t o o b t a i n low r e s i d u a l cadmium c o n c e n t r a t i o n s . Chemische Fabrik Kalk GmbH (14) r e p o r t e d a r e d u c t i o n of t h e cadmium content of 75 w? H P 0 . from 65 ppm t o 13 ppm u s i n g an o v e r p r e s s u r e of 5 bar at 20°C. The p r e c i p i t a t e was s e p a r a t e d by f i l t r a t i o n .
More r e c e n t p a t e n t a p p l i c a t i o n s r e p o r t a p a r t i a l n e u t r a l i z a t i o n of t h e acid before t h e a d d i t i o n of t h e s u l p h i d e s o u r c e ( 1 8 - 2 1 ) . Boliden AB (19) r e p o r t e d a r e d u c t i o n of t h e cadmium c o n t e n t of 62 w?
H PO. from 20 mg/l t o 2 . 5 mg/l a f t e r n e u t r a l i z a t i o n with NaOH t o Na:P = 1:10 mol/mol (60°C). Hoechst AG (20) r e p o r t e d t h e removal of cadmium from Odda a c i d (30-35? HNO , 21-28% H.PO^, 7-10? CaO) a f t e r n e u t r a l i z a t i o n with ammonia u n t i l pH = 1 . The cadmium content was reduced from 3-6 t o 0 . 2 ppm.
Either_L/L e x t r a c t i o n _ o r _ g r e c i g i t a t i o n _ o f cadmium_with o r g a n i c §yiEbyC;£9Qtaining_cornpoundsi
A) P r e c i p i t a t i o n / F i l t r a t i o n ( 2 2 ) .
S o l i d d i p h e n y l d i t h i o c a r b a m a t e s a l t s , Ph?NCSSM, or t h e i r aqueous
s o l u t i o n s were used t o remove cadmium ions from 65? H-POj.. The Cd-c o n t a i n i n g p r e Cd-c i p i t a t e was s e p a r a t e d by f i l t r a t i o n .
B) P r e c i p i t a t i o n / A d s o r p t i o n ( 2 3 - 2 7 ) .
A d i o r g a n y l - 0 , 0 - e s t e r of d i t h i o p h o s p h o r i c a c i d adsorbed on a c t i v e carbon or s i l i c a (20-60? w/w (RO) PSSH on a d s o r b e n t ) was used t o remove cadmium i o n s from a 40? H-P0Ü s o l u t i o n . The Cd complexing
l i g a n d and t h e adsorbent can a l s o be added s e p a r a t e l y t o t h e FUPOj. s o l u t i o n . Apart from d i o r g a n y l - 0 , 0 - e s t e r s of d i t h i o p h o s p h o r i c a c i d , 0 - e s t e r s of d i t h i o p h o s p h o n i c a c i d ((RO)RPSSH) and d i o r g a n y l d l t h i o p h o s p h i n i c a c i d s (R PSSH) were used f o r the removal of cadmium from Odda-acid. For a p p l i c a t i o n at high t e m p e r a t u r e s (50-80.°C) a d d i t i o n of a l a r g e amount of a r e d u c t i v e compound i s n e c e s s a r y ( 2 6 ) . 1 w? Fe powder was used t o remove Cd from 72? H PO. at 70°C.
C) P r e c i p i t a t i o n / F l o t a t i o n ( 2 8 - 3 0 ) .
2+ 3 +
After r e d u c t i o n of the phosphoric a c i d ([Fe ] / [ F e ] > 6) a d i o r g a n y l - 0 , 0 - e s t e r of d i t h i o p h o s p h o r i c a c i d was added t o t h e phophoric a c i d . The Cd-containing p r e c i p i t a t e was removed by
f l o t a t i o n .
D) L i q u i d - l i q u i d e x t r a c t i o n ( 3 1 - 3 5 ) .
A 10? w/w s o l u t i o n of a d i a l k y l - 0 , 0 - e s t e r of d i t h i o p h o s p h o r i c a c i d , (RO)-PSSH, i n kerosene was used t o remove cadmium i o n s from 40-70? w/w H,P0^. R e - e x t r a c t i o n of the loaded o r g a n i c phase was performed w i t h 30? w/w HC1 or 48? w/w HBr s o l u t i o n s . Apart from d i o r g a n y l - 0 , 0 - e s t e r s of d i t h i o p h o s p h o r i c a c i d , (RO) PSSH, 0 - e s t e r s of d i t h i o p h o s p h o n i c a c i d , R(R0)PSSH, and d i o r g a n y l d l t h i o p h o s p h i n i c a c i d s , R PSSH, were used for t h e e x t r a c t i o n of cadmium i o n s from
12
Odda a c i d (30-35? HNO_, 7-10? CaO; 20-28? H-PO^). D i - 2 - e t h y l h e x y l d i t h i o - O . O - p h o s p h o r i c a c i d was a l s o used for t h e e x t r a c t i o n of Ni ( 3 5 ) .
A l l compounds mentioned above a r e not s t a b l e i n t h e phosphoric a c i d s o l u t i o n a t higher t e m p e r a t u r e s (> 80°C) and decompose more or l e s s .
L/L e x t r a c t i o n _ o f cadmium with_organic_ammonium_halides
Cadmium forms n e g a t i v e l y charged CdL complexes ( i > 2) i n the p r e s e n c e of h a l i d e s . These complexes have a high a f f i n i t y for p o s i t i v e l y charged o r g a n i c ammonium ions and t h u s cadmium can be e x t r a c t e d with an o r g a n i c amine i n a c i d i c media (36-42) . The e f f e c t i v e n e s s of the amine i n c r e a s e s i n t h e s e r i e s primary < secondary < t e r t i a r y . An a l c o h o l with more than 10 carbon atoms can be used t o improve t h e s o l u b i l i t y of the amine s a l t i n t h e o r g a n i c p h a s e . Usually a c o n c e n t r a t i o n of 3 w? amine s a l t i n t h e o r g a n i c phase i s used. The r e - e x t r a c t i o n of cadmium i s performed with the ammonium or a l k a l i s a l t s of H SO., HNO or H PO.. The cadmium c o n c e n t r a t i o n i n 72? H.PO^ could be reduced from 60 ppm t o 1 ppm in a four stage c o u n t e r - c u r r e n t o p e r a t i o n at a mass flow r a t i o of o r g a n i c : a c i d = 1 : 5 . The r e g e n e r a t i o n was performed with a 5? NaSO. s o l u t i o n ( o r g a n i c : 5? NaSO. = 1 : 10, w/w).
§°iYËDt_extraction_of_ghgsghoric_acid_followed_b^_an
In the p a t e n t a p p l i c a t i o n s of Toyo Soda (43-16) h i g h - p u r i t y H_P0^ was obtained w i t h two p u r i f i c a t i o n s t e p s : s o l v e n t e x t r a c t i o n of wet-process phosphoric a c i d followed by anion exchange t r e a t m e n t of the a c i d . After s o l v e n t e x t r a c t i o n of t h e w e t - p r o c e s s a c i d (45 w? H P0 , 8 ppm Cd, 200 ppm Zn) with BuOH/HCl and r e - e x t r a c t i o n of the p h o s p h o r i c a c i d with H O / H P Cv/HCl t h e remaining BuOH i n t h e H P 0 . s o l u t i o n was removed by d i s t i l l a t i o n . Then t h e a c i d c o n t a i n i n g 29 g HCl/1 was passed through a f i x e d - b e d of a s t r o n g l y b a s i c a n i o n exchanger t o remove Zn ( 4 3 - 4 6 ) . After c o n c e n t r a t i o n of t h e acid and s i m u l t a n e o u s e v a p o r a t i o n of t h e remaining HCl an 85? H PCv s o l u t i o n c o n t a i n i n g l e s s than 0.01 ppm Cd and l e s s than 0 . 2 ppm Zn was
o b t a i n e d . The c a p a c i t y of the r e s i n for the removal of Zn was 35 t 3
P„0,-/m wet a n i o n exchange r e s i n .
FIELDS OF RESEARCH
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The c r y s t a l l i z a t i o n and f i l t r a t i o n of calcium s u l f a t e are
important unit operations in the phosphoric acid process. Three tons
of CaSO^ are produced per ton of H PCv and the following aspects are
of importance:
- good f i l t e r a b i l i t y
- good washability
- l i t t l e incorporation of phosphate ions
- l i t t l e incorporation of cadmium ions
The f i r s t three points are conditions which are required to obtain a
commercially feasible phosphoric acid process. The l a s t point i s an
environmental condition, which i s the main objective of the CTPA
process.
The aspects which are of importance for the incorporation of
cadmium in calcium s u l f a t e can be divided into two groups:
- aspects concerning the crystal growth (nucleation, growth r a t e ,
supersaturation, s o l u b i l i t y )
- aspects concerning cadmium (concentration, complexation,
adsorption)
Understanding of these physical aspects may give an indication of
the influences of process parameters such as H-POj. concentration,
HpSOj. concentration, temperature, residence time, s o l i d / l i q u i d r a t i o
and additives upon the uptake. In t h i s research the incorporation of
cadmium ions and phosphate ions during the c r y s t a l l i z a t i o n of
calcium sulfate hydrates has been investigated. The r e s u l t s are
presented in chapter 3. A study on the f i l t e r a b i l i t y and washability
of the c r y s t a l s has been performed by Van der Sluis (10).
14
Removal_of_cag^ium_from_phosphoric_acid
The cadmium removal techniques which are applicable in the CTPA
process must operate in concentrated phosphoric acid (55-65?
HPO^)
at high temperatures (85
_95°C). The conditions in the other H,POj.
processses are l e s s severe. Since the phosphoric acid contains
several metal ions with much higher concentrations than cadmium, the
cadmium removal technique must have a high s e l e c t i v i t y . In addition,
i f an auxiliary phase i s used, the Cd d i s t r i b u t i o n coefficient must
also be high, because of the large phosphoric acid stream to be
t r e a t e d . The Cd d i s t r i b u t i o n coefficient i s defined as the r a t i o
[Cd] t . / [Cd] „ .
concentrate phosphoric acid
The following cadmium removal techniques will be discussed in this thesis: - Electrodeposition - Precipitation - L/L extraction - Ion exchange A) Electrodeposition
Electrodeposition is an elegant method for the removal of Cd, since the element is obtained in a concentrated form without the use of additional chemicals. At the start of this study no patents concerning the electrodeposition of Cd from concentrated phosphoric acid was found. In this thesis the results of a preliminary study (47) performed at the Netherlands Organization for Applied Scientific Research (TNO) in cooperation with our laboratory is presented and discussed (chapter 4 ) .
B) Precipitation
Selective precipitation of cadmium from concentrated phosphoric
acid can be performed with sulphide or organic sulphui—containing
compounds. Sulphide i s a cheap ligand, but i t i s d i f f i c u l t to handle
because of the formation of US in acidic media. The s o l u b i l i t y of
CdS also increases with increasing acidity of the solution. The
residual cadmium concentration can be reduced by using an organic
ligand. In t h i s way the formation of H.S i s also eliminated.
However, organic ligands are more expensive than sulphide.
A disadvantage of the formation of s l i g h t l y soluble compounds
i s the poor f i l t e r a b i l i t y of the precipitate because of the small
s i z e of the precipitate p a r t i c l e s . The size of the crystals can be
influenced by controlling the precipitation conditions (seed
c r y s t a l s , slow release of ligand). Other ways to obtain larger
p a r t i c l e s are coprecipitation and the use of adsorbents.
Instead of f i l t r a t i o n or adsorption the p r e c i p i t a t e can also be
concentrated by f l o t a t i o n . Then a much smaller volume remains to be
f i l t e r e d (separation of precipitate from the foam).
In t h i s thesis only the precipitation of cadmium as CdS will be
discussed (chapter *0 .
C) L/L extraction
The organic phase of a liquid-liquid extraction system for the
removal of metal ions usually consists of two components: the
e x t r a c t a n t , which binds to the metal ion, and the diluent which
influences the hydrodynamic behaviour of the organic phase. The
diluent f a c i l i t a t e s the contacting between the organic phase and the
aqueous phase by reducing the viscosity or increasing the potential
i n t e r f a c i a l area. In some cases a third component (a modifier) may
be added to the organic phase to prevent formation of a third-phase.
However, third-phase formation may also be used as an advantageous
phenomenon, since in t h i s third phase the extracted element is
present in a highly concentrated form.
The economical and technical f e a s i b i l i t y of a liquid-liquid
extraction system i s strongly determined by the losses of the above
mentioned components by entrainment, chemical breakdown, solubility
and evaporation. Emulsification i s a potential problem of
liquid-liquid extraction and the occurrence of emulsions may already be
caused by tiny amounts of impurities or residual fine p a r t i c l e s .
This problem can sometimes be prevented by adsorption of the organic
l i q u i d phase upon a porous solid material.
In t h i s research two types of extractants were investigated:
sulphur-containing acidic extractants (dithiophosphinic acids) and
16
i o n - p a i r forming e x t r a c t a n t s ( o r g a n i c a m i n e s ) . The r e s u l t s o a r e p r e s e n t e d in chapter i| and 5. The influence of d i f f e r e n t types of d i l u e n t s and t h e i n f l u e n c e of modifiers were not i n v e s t i g a t e d .
D) Ion exchange
An ion-exchange r e s i n c o n s i s t s of a s o l i d porous m a t r i x and a c t i v e groups a t t a c h e d t o t h e m a t r i x . Usually p o l y s t y r e n e c r o s s -l i n k e d with d i v i n y -l b e n z e n e i s used as a m a t r i x . The same types of compounds as in l i q u i d - l i q u i d e x t r a c t i o n can be used as a c t i v e g r o u p s . Apart from i t s dependence on t h e chemical s t a b i l i t y of t h e a c t i v e groups, the l i f e time of t h e r e s i n i s a l s o determined by i t s p h y s i c a l s t a b i l i t y for p r e s s u r e , a t t r i t i o n and osmotic shock.
Most commercially a v a i l a b l e c a t i o n exchangers have s u l p h o n i c a c i d or c a r b o x y l i c a c i d groups as t h e i r a c t i v e component. These r e s i n s have a low a f f i n i t y for Cd in c o n c e n t r a t e d phosphoric a c i d . Resins with a n e g a t i v e l y charged sulphur atom ( s o f t Lewis base) have a high a f f i n i t y for cadmium. However, t h e s e compounds a r e u s u a l l y not s t a b l e in a c i d i c and o x i d a t i v e media at high t e m p e r a t u r e s .
Another way t o remove cadmium i s t h e use of anion e x c h a n g e r s . These r e s i n s u s u a l l y have an amine as t h e i r a c t i v e component and a r e s t a b l e in a c i d i c media. Cadmium can be removed as an anion by u s i n g a h a l i d e as an a d d i t i v e g i v i n g n e g a t i v e l y charged cadmium complexes. In t h i s t h e s i s the removal of cadmium by anion exchange from c o n c e n t r a t e d phosphoric a c i d i s coverd in chapter 6.
Some a n a l y t i c a l t e c h n i q u e s has been developed or adapted f o r a p p l i c a t i o n in t h i s r e s e a r c h . These t e c h n i q u e s a r e d e s c r i b e d i n chapter 2 .
REFERENCES
1. W.J. Chardon, Cadmium ( I n t e r n a l Report of the Wageningen U n i v e r s i t y of A g r i c u l t u r a l S c i e n c e s ) , Wageningen, 1980 2. M.I. Abdullah, L.G. Royle, Communications of Eur. Comm., Eur.
3. H.W. Fassbender, G. Seekamp, Geoderma J_6 (1976) 55
4. L. Friberg et. al., Cadmium in the Environment, CRC Press, 1974 5. C.H. Williams, D.J. David, Austr. J. of Soil Research _H_ (1973)
13
6. M. Hutton, Phosphorus & Potassium No. 123, Jan/Feb 1983, p. 33 7. H.-T. Baechle, F. Wolstein, Cadmium Compounds in Mineral
Fertilizers, The Fertiliser Society of London, London (4 October 1984)
8. UKF IJmuiden, personal communication
9. P. Becker, Phosphate and Phosphoric Acid, Marcel Dekker Inc., New York, 1983
10. S. van der Sluis, Y. Meszaros, J.A. Wesselingh and G.M. van Rosmalen, A Clean Technology Phosphoric Acid Process, The Fertiliser Society of London, London (19 October 1986) 11. F.L. Smidth & Co., EP 151 551 (7 March 1985)
12. Chemische Fabrik Budenheim, DE 3332 698 (21 March 1985) 13. Mitsui Toatsu Chem. Inc., J5 0075 115 (20 June 1975) 14. Chemische Fabrik Kalk GmbH, DE 2422 902 (20 Nov 1975) 15. Hoechst AG, DT 2447 390 (8 April 1976)
16. Central Glass KK, J5 3075 196 (4 Aug 1978)
17. Mitsui Toatsu Chem. Inc., J5 3110 997 (28 Sept 1978) 18. Chisso Corp., J5 4067 597 (31 May 1979)
19. Boliden AB, WP 80/02418 (13 Nov 1980) 20. Hoechst AG, DE 3134 847 (17 March 83) 21. Hoechst AG, EP 0087 065 (31 Aug 1983)
22. Unie van Kunstmestfabrieken BV, EP 116 989 (29 Aug 1984) 23. Hoechst AG, DE 3 202 658 (4 Aug 1984) and EP 85344 (5 March
1986)
24. Hoechst AG, DE 3 227 202 (26 Jan 1984) 25. Hoechst AG, DE 3 212 675 (6 Oct 1983) 26. Hoechst AG, DE 3434 611 (3 April 1986) 27. Hoechst AG, DE 3442 142 (22 May 1986)
28. Societe Uranium Pechiney, NL 8 403 788 (16 July 1985) 29. E. Jdid, J. Bessiere and P. Blazy, Industrie Minerale - Les
Techniques, May 1984, p.389
18
Process., J_6 (1986) 63
31. Hoechst AG, DE 3 127 900 (3 Feb 1983) 32. Hoechst AG, DE 3 142 666 (5 May 1983) 33. Hoechst AG, DE 3 209 183 (15 Sep 1983) 34. Hoechst AG, EP 78 051 (4 May 1983)
35. Kaza Phosphor Ind., SU 912 637 (15 March 1982) 36. Chem. Fab. Budenheim, EP 94 630 (23.11.83) 37. Chem. Fab. Budenheim, DE 3218 599 (1 Dec 83) 38. Chem. Fab. Budenheim, DE 3327 394 (14 Feb 85) 39. Chem. Fab. Budenheim, DE 3330 224 (14 March 85) 40. Chem. Fab. Budenheim, DE 3341 073 (23 May 85) 41. Chem. Fab. Budenheim, DE 3342 211 (30 May 85)
42. S. Stenstrom and G. Aly, Hydrometallurgy jj4 (1985) 257 43. Toyo Soda KK, BE 860 176 (27 April 78)
44. Toyo Soda KK, DE 2748 279 (20 Dec 79) 45. Toyo Soda KK, J5 3056 189 (22 May 78) 46. Toyo Soda KK, J5 3056 190 (22 May 78)
47. A. de Jong and D. Schmal, Electrolytical removal of cadmium ions from phosphoric acid (Internal Report of TNO), Delft (1985)
CHAPTER 2
ANALYTICAL TECHNIQUES
In t h i s r e s e a r c h s e v e r a l a n a l y t i c a l t e c h n i q u e s were used t o determine t h e c o n c e n t r a t i o n s of i m p u r i t i e s and a d d i t i v e s i n H PCv s o l u t i o n s (30 - 6 5 w$ H P O J and in calcium s u l f a t e h y d r a t e s . Most a t t e n t i o n was focussed on Cd and Cu ( 0 . 5 - 100 ppm). The a d d i t i v e s i o d i d e , bromide and c h l o r i d e were p r e s e n t i n t h e c o n c e n t r a t i o n range from 50 t o 5000 ppm.
INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)
INAA ( 1 , 2) i s a q u a n t i t a t i v e elemental a n a l y s i s based on the measurement of c h a r a c t e r i s t i c r a d i a t i o n from n u c l i d e s formed
d i r e c t l y or indirectly by radiation of the sample with neutrons.
Procedure: The samples and Zn standards were weighed in poly
ethylene capsules and the capsules were put in a polyethylene
r a b b i t . 100-200 mg sample was used for the analysis of the elements
present in CaS0
u.nH
?0, phosphoric acid solutions and phosphate ore.
A 10-20 mg sample was used for the analysis of the elements present
in ion-exchange r e s i n s (air-dry weight), solvents from liquid-liquid
extractions and p r e c i p i t a t e s . Ion-exchange resins and precipitates
were spread out on a tape before insertion in the capsule. Solvents
from liquid-liquid extractions were absorbed on f i l t r a t i o n paper in
the capsules.
The rabbits containing the samples and Zn standards was
irradiated at a position 8 cm from the nuclear reactor core. The
i r r a d i a t i o n s were carried out in the swimming pool 2 MW nuclear
reactor of the Interuniversity Reactor I n s t i t u t e (IRI) at Delft.
After the i r r a d i a t i o n and the appropriate waiting time the Y-speetra
of the samples and Zn standards were measured with a Ge(Li)-detector
and the spectra were analyzed with a computer. A scheme of the
i r r a d i a t i o n , waiting and measuring time i s given in table 1. The
samples were irradiated twice with different exposure times and
20
measured three times. The results were combined and analyzed with a computer.
Most elements with an atomic number higher than the element Ne can be analyzed with INAA. The detection limits of many elements in samples with only a few impurities and little background radiation vary between 1 to 10 ppm. The detection limits of many elements in samples with a large number of impurities or high background radiation vary between 10 to 100 ppm. This technique is powerful for the multi-elemental analysis of solid and liquid samples without pretreatment of the samples. The disadvantages of this technique are the long waiting time and the high costs.
TABLE 1.
Irradiation, waiting and measuring times used in INAA. Irradiation Waiting Measuring 1. 8-20 s 2-15 min 3-10 min
2. 30-120 min 1-6 days 30-60 min 3. 20-25 days 30-60 min
INDUCTIVELY COUPLED ARGON PLASMA SPECTROSCOPY (ICP-AES)
ICP-AES i s a q u a n t i t a t i v e elemental a n a l y s i s based on t h e measurement of c h a r a c t e r i s t i c emission l i n e s from elements i n an argon plasma.
The following a p p a r a t u s were used :
Nebulizer : Home-made Babington-type ( P . v . d . P l a s / L . de Galan) P e r i s t a l t i c pump : Pharmacia P-1
Monochromator : J o b i n Yvon JY 3 8 , type HR 1000
Generator: Kontron, type LINN FS H with home-made power s t a b i l i z e r . After t a k i n g i n t o account s p e c t r a l i n t e r f e r e n c e s ( e s p e c i a l l y caused by Fe) and enhanced background s i g n a l ( e s p e c i a l l y caused by by Al and Mg) the c o n d i t i o n s shown i n t a b l e 2 were chosen. The l i n e a r dynamical range of the c a l i b r a t i o n l i n e i s four decades
starting from the detection limit (x + 3o of noise). The obtained detection limits are given in table 3 (3).
Procedure : 1 g of CaSO^.nH.0 is dissolved in a HCl solution and diluted to 100 ml (1.5 M HCl) before measurement. Sometimes clogging of the nebulizer occurred. This could be prevented by regularly passing a diluted HCl solution between the measurements. The standard solutions for the calibration line contained only 1.5 M HCl as the matrix and no CaSCv.nH 0. Recalibration of the line was performed each hour.
Phosphoric acid solutions were diluted until they contain about
7% H,P0Ü before the measurement. The standard solutions for the
calibration line contained 1% H P0. as the matrix (matrix matching).
Recalibration of the line was performed each hour.
TABLE 2.
Conditions for the measurement of Cu, Cd and Zn (ICP-AES).
wave l e n g t h (nm) background l e f t (nm) background r i g h t (nm) Cu 327.396 -0.042 +0.042 Cd 228.802 -0.042 +0.042 Zn 206.200 -0.070 +0.034 TABLE 3.
Detection limits of Cu, Cd and Zn in different solutions (ICP-AES). All values are in vig/1.
Cu Cd** Zn 1 wï CaSCy 12 10 10 .nH20* 7 w? ^ P O ^ 13 24 7 H20 6 5 6 * In 1.5 M HCl
22
P a r t of the samples were analyzed with a Perkin Elmer Plasma I I s p e c t r o m e t e r . The measuring procedure and t h e o b t a i n e d r e s u l t s a r e about t h e same as mentioned above for the Jobin Yvon. The e l e m e n t s Ca ( i n MCP), P ( i n H PO s o l u t i o n s ) and S ( i n H SO c o n t a i n i n g product a c i d ) were a l s o analyzed with a Perkin Elmer Plasma I I . Usually 5 g of sample was d i l u t e d with water u n t i l 500 g b e f o r e a n a l y s i s .
ICP-AES i s a powerful technique for a f a s t m u l t i - e l e m e n t a l a n a l y s i s of d i l u t e d aqueous s o l u t i o n s . A d i s a d v a n t a g e of t h i s t e c h n i q u e are the high investment c o s t s and t h e l a r g e consumption of argon.
TABLE 4.
Detection limits (x + 3o) and maximum concentrations used, c , of Cd, Cu and Zn in different solutions
max
using flame atomisation AAS.
The concentrations are given in ug/1. 13 w? H PO^ 0.25$ CaS04.2H20 in 1 M HC1 c max Cd 8 5 1300 Cu 13 31 5000 Zn 33 10
ATOMIC ABSORPTION SPECTROMETRY (AAS)
AAS measurements were performed with a Perkin Elmer 460 AAS (flame atomisation) and a Perkin Elmer 2280 AAS (electrothermal atomisation = ETA). A deuterium background correction was applied. Detection limits (x + 3o) and the maximum concentration used for determinations (c ) of Cd, Cu and Zn in phosphoric acid and in max calcium sulfate solutions analyzed with flame atomisation are given in table 4 (4). For the determinations in phosphoric acid solutions
standard addition was applied. A calibration line was used for the analysis of Cd, Cu and Zn in CaSO..
Only very diluted calcium sulfate solutions (< 0.3 w$ CaSO.) gave a stable flame. Higher calcium sulfate concentrations probably caused scaling in the burner resulting in a decreasing signal with time.
Cd and Cu in a 1 w% CaS0ü.2H 0 solution in 1 M HC1 were also
determined with ETA-AAS. The temperature program, the detection
TABLE 5.
ETA-AAS t e m p e r a t u r e program, d e t e c t i o n l i m i t s (x + 3o) and c of Cd and Cu i n a 1 w? CaS0„.2H„0 s o l u t i o n in 1 M HC1. max 4 2 The c o n c e n t r a t i o n s a r e given in u g / 1 . Cd Cu s t e p drying ashing a t o m i s a t i o n drying ashing a t o m i s a t i o n T (°C) 120-130 300 1300 120-130 1100 2300 t ( s ) 55 40 4 55 35 5 d e t . l i m . 0.14 1.6 c max 6 50 TABLE 6.
Detection limits (x + 3a) of Cd, Cu and Pb in different solutions using DPP or ASV. The concentrations are given in pg/1.
Cd Cu Pb 6.5 w? DPP 10 200 20 H3P 04 1 w? CaSCv. DPP 30 100 30 ,2H 20 i n 1 M HC1 ASV 0 . 1 2 1.5 1.4
24
limits and the maximum concentration used in the determinations are given in table 5 ( 1 ) . Standard addition was applied.
DIFFERENTIAL PULSE POLAROGRAPHY AND ANODIC STRIPPING VOLTAMMETRY
Differential Pulse Polarography (DPP) and Anodic Stripping Voltammetry (ASV) were performed with a Princeton Applied Research (PAR) Model 171 Polarographic Analyser and a PAR Model 303 Smooth Mercury Dropping Electrode. A PAR Model 315 Automated Electro-analysis Controller was also used for ASV. The detection limits (x + 3o) of Cd, Cu and Pb in diluted phosphoric acid and in a 1 w? CaSO^.nH 0 in 1 M HC1 are given in table 6 (4). In the ASV measurement the deposition was performed at - 800 mV SCE during 120 seconds.
CATHODIC STRIPPING VOLTAMMETRY OF HALIDES
Principle: Anodic electrodeposition of Hg X (X = Cl—, Br—,I—) on a hanging mercury drop electrode is followed by a cathodic dissolution (stripping) of the mercury halide. The current voltage curve will show a peak-current, which might be proportional to the halide concentration in the solution (5).
Apparatus and procedure : The measurements were performed using a Metrohm 646 VA processor with a 647 electrode standard (Metrohm). A 6% w/w H-POj. solution was used as a liquid junction for the
calomel reference electrode. The halide containing phosphoric acid solution was diluted to about 6% H PCv before analysis. The
electrodeposition of iodide was performed at + 220 mV SCE during 120 seconds. The stripping process was performed at a scan rate of 2 mV/s. A calibration line up to 10 mg/1 was used. The electro deposition of bromide was performed at + 250 mV SCE during 120 seconds. The stripping process was performed at a scan rate of 4 mV/s. A calibration line up to 10 mg/1 was used.
REFERENCES
M. de Bruin, P.J.M. Korthoven and P. Bode, J. Radioanal. Chem., 70 (1982) 197
M. de Bruin, Instrumental Neutron Activation Analysis - A Routine Method (thesis), Delft University of Technology, Delft
(1983)
A. Klok, J.J. Tiggelman, P. Weij, J.P.J. van Dalen and L. de Galan, Proc XXIV Colloq. Spectrosc. Intern. 1985, p. 98
P. Weij, Internal report, Delft University of Technology, Delft (1981))
G. Colovos, G.S. Wilson, and J.L. Moyers, Anal. Chem., H6_
26
CHAPTER 3 3.1
CADMIUM INCORPORATION IN THE CRYSTALLIZATION OF CALCIUM SULFATE HEMIHYDRATE FROM PHOSPHORIC ACID
T.T. Tjioe, H. van der Woude, J. Verbiest, P.F.M. Durville and G.M. van Rosmalen
Department of Chemical Engineering Delft University of Technology
De Vries van Heystplantsoen 2, 2628 RZ Delft, The Netherlands
ABSTRACT
Semi-batch c r y s t a l l i z a t i o n experiments of CaS0..1/2H 0 (HH) were performed in c o n c e n t r a t e d phosphoric a c i d a t v a r i o u s H-SOn c o n t e n t s in t h e absence and p r e s e n c e of a d d i t i v e s . The i n f l u e n c e of the o p e r a t i n g c o n d i t i o n s and a d d i t i v e s on t h e cadmium and phosphate i n c o r p o r a t i o n in t h e HH l a t t i c e were d e t e r m i n e d . The Cd uptake i n c r e a s e d with r a i s i n g H SOj. c o n t e n t . Addition of HNO caused a reduced cadmium and phosphate i n c o r p o r a t i o n . Addition of i o d i d e a l s o lowered t h e cadmium i n c o r p o r a t i o n .
The obtained r e s u l t s f i t s i n t o a model for t h e i n c o r p o r a t i o n of f o r e i g n ions during c r y s t a l growth. According t o t h i s model t h e i n c o r p o r a t i o n i s p r i m a r i l y a k i n e t i c a l l y c o n t r o l l e d p r o c e s s .
INTRODUCTION
The production of phosphoric a c i d by wet p r o c e s s i n g i s d i r e c t l y r e l a t e d t o t h e need for f e r t i l i z e r s . In t h e s e processes huge amounts of e i t h e r calcium s u l f a t e d i h y d r a t e , a l s o c a l l e d gypsum, or hemi-h y d r a t e a r e p r e c i p i t a t e d as b y p r o d u c t . Bothemi-h calcium s u l f a t e hemi-h y d r a t e s
metal ions l i k e cadmium ions o r i g i n a t i n g from the phosphate o r e . This not only r e s t r i c t s i t s d i s p o s a l as waste p r o d u c t , but a l s o l i m i t s t h e development of new a p p l i c a t i o n s .
At the moment a new hemihydrate process i s being developed at our l a b o r a t o r y aiming a t the production of p u r i f i e d a c i d and hemi h y d r a t e . A s i m p l i f i e d scheme of t h e process i s given in F i g . 1. The phosphate o r e i s converted i n t o a monocalcium phosphate s o l u t i o n (MCP) , w h e r e a f t e r hemihydrate i s p r e c i p i t a t e d , according t o :
C a1 0( P 04)6F2. x C a C 03 + H PO^ ==> C a C H ^ O ^ + CC>2 + H20 + HF
Ca(H2P01 ))2 + H2S0M + H20 ==> CaSCy 1 /2H20 + H PO^
The r a t i o r e c y c l e s t r e a m / p h o s p h a t e o r e stream i s about 12 w/w. The s o l u t i o n s c o n t a i n 55-65? w/w H PO.. Heavy m e t a l s o r i g i n a t i n g from t h e phosphate o r e e n t e r t h e process i n t h e p r e d i g e s t i o n s t e p . The p u r i f i c a t i o n s t e p can be l o c a t e d e i t h e r i n t h e MCP stream or i n t h e r e c y c l e s t r e a m . In e i t h e r case i n c o r p o r a t i o n of phosphate ions and r e s i d u a l heavy metal ions i n t h e hemihydrate l a t t i c e should be minimized by an a p p r o p r i a t e choice of o p e r a t i n g c o n d i t i o n s for t h e c r y s t a l l i z a t i o n . If the r e d u c t i o n of the i n c o r p o r a t i o n i s s t i l l i n s u f f i c i e n t , a d d i t i v e s should be a p p l i e d . phosphate , ore H2SOi/H3P04 HH — 1 _ I
T
predigestion —•— crystallizationX
recycle stream H3PO4 FIGURE 1 . Process scheme
In t h i s study t h e i n c o r p o r a t i o n of both cadmium and phosphate ions a t d i f f e r e n t c r y s t a l l i z a t i o n c o n d i t i o n s were determined. Cadmium was s e l e c t e d for t h i s study because of i t s t o x i c c h a r a c t e r and phosphate ions because of t h e economics of the p r o c e s s . The degree of i n c o r p o r a t i o n in dependence on t h e s u p e r s a t u r a t i o n , t h e
28
free sulfuric acid content and the phosphoric acid concentration were determined. The free sulfuric acid content is defined as the amount of sulfate ions in excess of the calcium ions, given as weight percentage of free H_SO^ in solution. This percentage can thus be either positive for excess sulfate ions or negative for excess calcium ions. Additionally the influence of additives like nitric acid, a sulphonic acid and iodide on the incorporation were also studied. The approach of the problem is based on simplified models for the incorporation of foreign ions during crystal growth. The selection of the additives will also be elucidated from this model.
MODEL DESCRIPTION
A simple way of incorporation occurs if a lattice ion is directly replaced by a single foreign ion during the crystal growth process. This is most likely the case for cadmium ions. The CdSO. hydrates are far more water soluble than the CaSC,. hydrates and their incorporation is not predictable on this ground. The radii of
2+ 2+
Cd and Ca , however, are about equal (table 1; Huheey,1975). Little lattice strain will therefore be introduced into the
p-hemihydrate lattice during cadmium uptake. The radii of SO. and
2-HPO. are also about equal and little disturbance of the lattice is caused by the replacement of sulfate ions by phosphate ions.
Another way of incorporation can occur if a lattice ion is replaced by a complex of foreign ions. It is not excluded that Al
— ( n — 1^ —
and F a r e i n c o r p o r a t e d as A1F i> complexes. I n c o r p o r a t i o n of
^+ n
s e p a r a t e Al ions and F ions i s very u n l i k e l y , because of the i n t r o d u c t i o n of l a r g e l a t t i c e s t r a i n s .
A t h i r d way of i n c o r p o r a t i o n can be achieved i f t h e f o r e i g n ion can form a l e s s s o l u b l e s a l t than CaS0ü.1/2H 0. The f o r e i g n ion w i l l
probably be i n c o r p o r a t e d as ion p a i r s or as c l u s t e r s , s i n c e the bonding between t h e c a t i o n and anion i s very s t r o n g . This p o s s i b l y
2+ 2+
occurs for Sr and Ba ( s e e t a b l e 1 ) . L a t t i c e s t r a i n w i l l be i n t r o d u c e d , but t h i s s t r a i n i s compensated by t h e f a v o u r a b l e l a t t i c e e n t h a l p y .
In attempting to reduce the incorporation of a foreign ion, the process of incorporation and the parameters controlling this process should be known. For this reason a simplified model for the incorporation of cadmium ions will be presented as a starting point. This model is based on commonly used models for describing crystal growth processes (Bennema, 1973 and 198-M). The incorporation of foreign ions during crystal growth can be described with the following simplified model (see Fig. 2 ) .
TABLE 1 . Water s o l u b i l i t i e s of MSCL and i o n i c r a d i i of M M C d2 + C a2 + S r2 + 2 + Ba SO 2 _ water solubili of ty MS04 (mole/1) >0.4 4.7 * 10- 3 8.7 * M3~M 1.0 * 10- 5 ionic radius (pm) 95 • 00 116 136 2 3 0 2 3 8 1 ^ AGldiff " 'r bulk solution M ( L )L^ M ( L )a d S ) S t e p— M ( L )i n c- Mi n c
FIGURE 2 . I n c o r p o r a t i o n of a f o r e i g n ion during c r y s t a l growth.
Step 1. Adsorption of the ion from the l i q u i d phase at the c r y s t a l s u r f a c e with l o s s of the second c o o r d i n a t i o n s p h e r e of the i o n .
30
M ( L ) i <===> M ( L )a d s
M = c e n t r a l ion
(L) = c o o r d i n a t i n g ions and/or molecules
S t e p 2 . Surface d i f f u s i o n of t h e adsorbed ion t o an a c t i v e growth s i t e ( s t e p , kink) and a d s o r p t i o n on t h i s s i t e .
M ( L )a d s < =" > M ( L )s t e p
Step 3. Partial incorporation of the ion caused by the crystal growth with partial desolvation of the first coordination sphere of the ion.
a
M(L) . <===> ML. step . inc
The d r i v i n g f o r c e f o r c r y s t a l growth i s the r e l a t i v e s u p e r s a t u r a t i o n a, which i s defined as
i1/2 o = [Ca][S04]
[ca]
rso.]
\ ~
1 eq 4 eq J[Ca] and [SCL] a r e t h e t o t a l calcium and s u l f a t e c o n c e n t r a t i o n s in t h e mother l i q u i d . S u b s c r i p t eq means e q u i l i b r i u m v a l u e .
Step *J. Total i n c o r p o r a t i o n of the ion in t h e growing c r y s t a l with t o t a l d e s o l v a t i o n of t h e i o n .
o
ML. ===> M, i n c . i n c
The f i r s t t h r e e s t e p s i n t h i s model a r e r e v e r s i b l e p r o c e s s e s , whereas the t o t a l i n c o r p o r a t i o n of t h e f o r e i g n ion i s almost i r r e v e r s i b l e . The i n c o r p o r a t e d ion w i l l u s u a l l y not be a b l e t o l e a v e t h e l a t t i c e i f the s u p e r s a t u r a t i o n i s m a i n t a i n e d . The r e a c t i o n r a t e s of t h e d i f f e r e n t s t e p s a r e determined by t h e f r e e e n t h a l p i e s of a c t i v a t i o n , AG . The a d s o r p t i o n and s u r f a c e d i f f u s i o n a r e f a s t p r o c e s s e s compared with t h e p a r t i a l d e s o l v a t i o n and i n c o r p o r a t i o n . Thus t h e f i r s t two s t e p s can be t r e a t e d as quasi e q u i l i b r i u m s t a t e s . The i n c o r p o r a t i o n i s assumed t o be p r i m a r i l y a k i n e t i c a l l y c o n t r o l l e d process which i s mainly determined by t h e f o l l o w i n g p a r a m e t e r s :
1) The c o n c e n t r a t i o n of t h e f o r e i g n ion a t t h e a c t i v e growth s i t e (ML ,_ ) , which i s determined by t h e c o n c e n t r a t i o n of M(L),, t h e
s t e p 1 s o l u b i l i t y of CaSO..1/2H20, t h e Ca/SO. r a t i o i n t h e s o l u t i o n as well
a s t h e a d s o r p t i o n e q u i l i b r i a . The s o l u b i l i t y and t h e Ca/SO^ r a t i o d e t e r m i n e t h e impingement r a t e of t h e calcium and s u l f a t e ions on t h e c r y s t a l s u r f a c e . Thus t h e r a t i o Cd(ads)/Ca(ads) w i l l change i f t h e s o l u b i l i t y or t h e Ca/SCL r a t i o c h a n g e s , r e s u l t i n g i n a d i f f e r e n t c o n c e n t r a t i o n of ML . . The s o l u b i l i t y of hemihydrate can e . g . be i n c r e a s e d by adding a s t r o n g a c i d t o t h e mother l i q u o r .
2) The r a t e s of d e s o l v a t i o n ( s t e p 3 and 4 ) , which determine t h e amount of p a r t i a l l y i n c o r p o r a t e d i o n s . If t h e r a t e of p a r t i a l d e s o l v a t i o n of t h e f o r e i g n ion i s low, l i t t l e p a r t i a l i n c o r p o r a t i o n ( s t e p 3) w i l l occur and t h e amount of t o t a l l y i n c o r p o r a t e d ions ( s t e p I) w i l l be r e d u c e d . If t h e t o t a l d e s o l v a t i o n of ML, proceeds
F i n c
s l o w l y , c r y s t a l growth w i l l be r e t a r d e d . The r a t e s of d e s o l v a t i o n can be reduced by u s i n g a complexing a g e n t .
3) The growth r a t e of t h e c r y s t a l , which i s governed by t h e r e l a t i v e s u p e r s a t u r a t i o n ( s t e p 3 and 4 ) . The growth r a t e determines t h e time a v a i l a b l e t o pursue t h e thermodynamic e q u i l i b r i u m value of t h e i n c o r p o r a t e d f o r e i g n i o n s . This e q u i l i b r i u m c o n c e n t r a t i o n w i l l be r e a c h e d i f the c r y s t a l growth r a t e i s very low. The i n c o r p o r a t i o n w i l l be s t r o n g l y dependent on t h e c r y s t a l growth r a t e , i f the e q u i l i b r i u m c o n c e n t r a t i o n i n t h e s o l i d phase d i f f e r s s t r o n g l y from t h e c o n c e n t r a t i o n of the p a r t i a l l y i n c o r p o r a t e d ions ML.
i n c EXPERIMENTAL
APPARATUS: Semi-batch experiments were performed i n a t h e r m o s t a t e d r e a c t i o n v e s s e l of 3 l i t e r c l o s e d w i t h a polypropylene l i d . The v e s s e l was provided with b a f f l e s and a t u r b i n e s t i r r e r t o achieve good mixing of the s u s p e n s i o n . Both t h e H SO/H PO. s o l u t i o n and t h e MCP s o l u t i o n were pumped a t f i x e d r a t e s i n t o t h e r e a c t i o n v e s s e l t h r o u g h h e a t e d supply l i n e s . During r e a c t i o n t h e s o l i d / l i q u i d r a t i o of t h e suspension was kept n e a r l y c o n s t a n t a t a l e v e l of about 0.1 by withdrawing c o n t i n u o u s l y s o l u t i o n through a g l a s s f i l t e r . The
32
i n c r e a s e in c r y s t a l mass with time was taken i n t o a c c o u n t . Hardly any l o s s of c r y s t a l mass through t h i s f i l t e r was n o t i c e d .
CHEMICALS: All chemicals were chemically pure or r e a g e n t grade u n l e s s o t h e r w i s e s t a t e d .
SEED CRYSTALS: Pure hemihydrate o b t a i n e d from Ca(NO,) and H SO^ i n a HNO, s o l u t i o n at 50°C was used for t h e p r e p a r a t i o n of seed c r y s t a l s (Weiser, 1937). From t h e s e c r y s t a l s 20 g were suspended in 200 g of 64? w/w H,P(K with 2? f r e e H SO^ in t h e r e a c t i o n v e s s e l at 90°C. After suspending t h e c r y s t a l mass was i n c r e a s e d up t o 180 g by feeding s i m u l t a n e o u s l y i n t o the v e s s e l a MCP s o l u t i o n prepared from CaCO and 65? Hop oii ( g i v i n g 3.2? CaO) and a H SO^/H PO^ s o l u t i o n
c o n t a i n i n g 17? H 2 S 0i i prepared from 96? HpSOj, and 65? H P 0 . . The
r a t i o of the feed r a t e s was chosen t o give a s o l u t i o n of 2? f r e e H SO., while t h e H P 0 , content i n s o l u t i o n a f t e r mixing was about 6 4 ? .
CRYSTALLIZATION: The experiments w i l l be d i v i d e d i n 4 types ( t a b l e 2 and 3 ) :
Type A : For each c r y s t a l l i z a t i o n experiment 20 g of the above d e s c r i b e d seed c r y s t a l s were suspended in 200 g of 61? H,P0. c o n t a i n i n g t h e s e l e c t e d f r e e H-S0. p e r c e n t a g e . Immediately h e r e a f t e r t h e c r y s t a l l i z a t i o n process was s t a r t e d by t h e simultaneous dosage of t h e MCP and H S O / H PO. s o l u t i o n s at c o n s t a n t r a t e s . The MCP s o l u t i o n was prepared by mixing ground Kouribgha phosphate o r e (51? CaO, 32? P„0r and 23 ppm Cd) with 65? phosphoric a c i d a t 90°C. For
t h e f i r s t two s e r i e s of experiments (A.I and A . I I ) MCP s o l u t i o n s were used c o n t a i n i n g about 3.3? CaO and t h u s about 1.4 ppm Cd. For t h e t h i r d s e r i e s (A. I l l ) MCP s o l u t i o n s were a p p l i e d with h a l f the CaO and Cd c o n t e n t . The H SO./H P0,. s o l u t i o n s used i n a l l t h r e e s e r i e s contained i n v a r i a b l y 17? H.SO.. The r a t i o of t h e MCP and FLSO./H-PO. feed r a t e s was adapted t o g i v e t h e amount of f r e e HpS0j.
s e l e c t e d for the experiment. The c r y s t a l l i z a t i o n experiments were performed a t 90 C under continuous s t i r r i n g a t a f i x e d s t i r r e r s p e e d . The d e n s i t y of the s o l u t i o n s was about 1450 kg/rrr a t 90°C. To v e r i f y t h e percentage of f r e e H SO. d u r i n g t h e experiment a l i q u i d sample was drawn each 15 minutes and a t t h e end of the e x p e r i m e n t . After each experiment, which l a s t e d about 60 m i n u t e s , t h e c r y s t a l s
