Mercury extraction from cinnabar ores using hydrobromic acid

HydrometaUurgy, 21 (1988) 127-143

127

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Mercury Extraction from Cinnabar Ores Using Hydrobromic Acid A. B A L L E S T E R , E. O T E R O and F. GONZ.A,LEZ

Department of Materials Science and Metallurgical Engineering, Faculty of Chemical Sciences, Complutense Universi(v, 28040 Madrid (Spain) (Received June 3, 1987; accepted April 18, 1988)

ABSTRACT Ballester, A., Otero, E. and Gonzdlez, F., 1988. Mercury extraction from cinnabar ores using hydrobromic acid. HydrometaUurgy, 21: 127-143. Studies to develop a new hydrometallurgical process for cinnabar treatment have been carried out. This process deals with an ore attack using hydrobromic acid-like reagent. Mercury was dissolved in the form of tetrabromomercuriate ion (HgBr~ ~- ). The best conditions to leach cinnabar were 303 or 313K and 7 k m o l ' m :~HBr. After leaching, solution was treated in order to recover mercury and to regenerate hydrobromic acid. Mercury recovery was carried out by electrolysis and cementation (mercury was obtained in elemental form) and by precipitation with sodium hydroxide (mercury was obtained as red mercury oxide). Electrolysis was carried out at 1.2 V and 303K using concentrated solutions (175 mol.m :~ Hg~+ ). Cementation was performed from diluted solutions, at different temperatures between 293 and 333K, with iron powder. Precipitation from concentrated solutions (up to 190 mol.m :~ Hg ~+ ) was performed by pH change at 283K. Efficiencies were: 90% for electrolysis, 100% for cementation, and 99.9% for basic precipitation. Hydrobromic acid was regenerated after electrolysis through the following reaction: B r , + H , S ~ H B r + S ~. Acid and sulphur were separated by using organic solvents.

INTRODUCTION

Pyrometallurgical processes to obtain metals from sulphide raw materials have very important problems: the control of the release of gases such as sulphur dioxide to the atmosphere and the energy requirements of this treatment. These are important in mercury recovery because its ores are very lean. The most important ore, cinnabar (red mercury sulphide, HgS), is mined from deposits with low (0.8-1.0%) or very low grade (0.1-0.2%). In exceptional deposits like Almaden (Spain), medium grades can amount to several percent, but even in this mine it is usual to find grades below 1%.

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© 1988 Elsevier Science Publishers B.V.

128

Thus, due to these pollution and energy problems, several researchers have developed hydrometallurgical alternatives to obtain mercury. There are three possible hydrometallurgical routes to dissolve cinnabar ores: (a) displacement of sulphur using a strong non-oxidizing acid, producing hydrogen sulphide (H2S); (b) oxidizing attack to obtain elemental sulphur or sulphate; and (c) complexing attack to form a very stable mercury complex. These methods have been applied both singly and in a combined manner. The use of oxidizing agents has been described in several works [ 1 ], in particular the action of ferric ion [2]. The combined action of an oxidant with a strong acid has been proved through use of hot, concentrated nitric acid [3]. The use of complex agents, mainly sodium sulphide in a basic medium, has been commonly used. Various investigations have been carried out by the Bureau of Mines (U.S.A.). Town et ah have studied both cinnabar leaching [4] and the electrolytic precipitation of mercury from solution [5]. Town and Stickney [6 ] have carried out an economic study both for mercury ore leaching and mercury cementation with aluminium. Obeso and Pdrez [7] have studied the leaching of mercury ores mined in Caunedo (Spain) using the same reagent. The combination of oxidizing and complexing methods has been also extensively studied. In this case, a reagent used very frequently is hypochlorite lye. Parks [8] proposed a process where, after attack, metal is recovered by cementation or adsorption on active carbon. In other experiments [9], this author regenerated hypochlorite solutions by electrolysis of brines containing sodium chloride. Other studies with this reagent have been carried out by the Bureau of Mines (U.S.A.). Scheiner et al. [10] have studied the ore attack in an electrolytic cell with a sodium chloride electrolyte and with the ore in suspension. Sodium hypochlorite is produced at the anode which, at the same time, forms oxygen which oxidizes the sulphur in the ore to form mercury sulphate. In the presence of chloride ions, this compound forms a very stable and soluble mercury complex. Metallic mercury is precipitated from the leach solution by cementation with zinc. The same authors have applied the method to sixty-two different ores at pilot scale [11]. Calvo and Forn [12,13] have worked on a process where ore suspensions in water were attacked with gaseous chlorine in the presence of calcium carbonate. Mercury was then precipitated as an oxide with sodium hydroxide, or as elemental metal by cementation. In recent years, our group has developed a process based on the use of an acid and complexing action to attack cinnabar. The reagent used is hydrobromic acid through the following reaction: HgS~s,,,)+4Br~aq)+2 H(aq) + --~ HgBr4 ~,S~)• (aq) + H ;~,

(1)

which transforms sulphide sulphur to H~S gas (thus the reaction kinetics are not blocked by elemental sulphur), and mercury passes into solution. Cinnabar insolubility can be overcome by taking advantage of the high activity coeffi-

129 cients that concentrated solutions of halogen, in general [14], and of bromides in particular, show. Under these conditions, activities are very high. Mercury appears in the resultant solutions as a very stable complex and this also iraproves the attack on the mineral. The solution is treated to obtain metal either in the elemental state (by electrolysis or cementation) or in the tbrm of an oxidized compound, red mercury oxide (by pH change and precipitation). In addition and as a consequence of anodic bromine evolution during electrolysis, it is possible to recover the leaching reagent by the following reaction: H.2S+Br~2 ~ H B r + S ° Nuflez et al. [15-17] have studied a process using acid solutions of potassium iodide. Metal was obtained in elemental form by electrolysis or in oxidized form, as mercury oxide, by an indirect mechanism with a final step which involved basic precipitation. In this paper, we show the fundamental conditions defining a hydrometallurgical process which uses hydrobromic acid as the leaching reagent. The metal was precipitated in elemental form and also as red mercury oxide (HgO) because this compound is in strong demand among mercury derivatives: in 1982, 51!i~ of mercury production was used in the form of' red mercury oxide for electrical purposes [18]. The precipitation in elemental form was carried out by electrolysis of concentrated solutions and by cementation for dilute solut ions. Oxide was precipitated with sodium hydroxide. Recovery of the leaching agent through reaction (2) using hydrogen sulphide produced during leaching and hromine produced at the electrolytic cell anode, was also studied. CINNABAR LEACHING The study of ore leaching at a laboratory scale was first carried out. Then, a pilot reactor was designed the results of which have been described elsewhere [ 19, 20 ]. Working conditions for the reactor were determined from laboratory scale experiments.

Experimental The experimental apparatus used at laboratory scale consisted of a continuous stirred spherical glass reactor (700 mL capacity) submerged in a thermostatic bath with continuous temperature control. A low grade ore and a flotation concentrate were leached. The first contained 1.9% mercury and its particle size was 46 wt% less than 63/lm. The second had 31% mercury and its particle size was 80 wt% less than 63 /Ira. Leaching solutions were made from 48~i (1840 kmol.m -:~) hydrobromic acid (analytic grade) and distilled water. According to the laboratory results, reaction (1) takes place in two steps,

130

the first being very fast and the second slower. Thus the pilot reactor (Fig. 1 ) has three parts: A, B and C. Ore and acid reagent are fed into A. During the fast period of reaction, cinnabar and solution react in part B. Finally, during the slow period, reaction takes place in part C where there is turbulence produced by agitation. After attack, pulp is collected in two tanks where it separates by sedimentation. Clarified solution is recirculated to part A and heated. Exhausted ore is collected in both tanks. The hydrogen sulphide produced in the reaction is fixed in sodium hydroxide using an air flow. A low grade cinnabar ore with 1.98% mercury, 2.00% iron and 0.002% copper {particle size was 36 wt% less than 6 3 / l m ) and a flotation concentrate with 34.7% mercury, 9.3% iron and 0.05% copper (particle size was 93 wt% less than 63 pm ) were leached in the pilot reactor. The leaching solutions had small quantities of mercury, iron and copper, because they came from laboratory experiments on cinnabar ore. The concentration of hydrogen and bromide ions was adjusted with hydrobromic acid and sodium bromide. Metal analysis in solution was carried out by atomic absorption and hydrogen and bromide analysis by volumetric analysis. Results and discussion The influence of concentration and temperature, as classical kinetic variables, of pulp density and mercury concentration in solution and the possible dissolution of metals other than mercury (copper and iron) on leaching efficiency were studied at laboratory scale. The stirring speed was fixed at 3.33 s 1. Variation limits for the other variables were fixed by carrying out previous experiments with different concentrations and temperatures. Such limits were: (a) concentrate: temperature H S

--I Solid feed

F m

t ~0,5m

i '

Tank collecting putp after attack

f ;,rt:c2:_ Fig. 1. Experimental pilot scale apparatus fi)r leaching cinnabar.

131

between 293 and 323K and hydrobromic acid concentration between 6 and 7 kmol. m-:~; (b) low grade ore: temperature between 303 and 323K and concentration between 5 and 8 kmol-m -:~. As stated above, the reaction took place in two steps, which was due to ore particle size. Mercury sulphide liberated from silica gangue (particle size < 63 /ira ) was attacked easily and quickly (first period), whereas occluded mercury sulphide (particle size > 63 ,um) was either dissolved with difficulty or not dissolved (second period). The following results were obtained with the concentrate: when the acid concentration was low (6 kmol-m-:~), there was a large influence of temperature; when the acid concentration was 7 k m o l ' m -:~, the temperature hardly influenced the leaching rate provided that the temperature was higher than 293K. These results suggest a possible change in the leaching mechanism, because the process may be controlled by a chemical reaction step at low temperatures and by reagent boundary layer diffusion at high temperatures. Thus as temperature had no effect over 303K, using 7 kmol-m -:~ hydrobromic acid (Fig. 2), the conditions selected in order to leach ore in the pilot reactor were these. By using these conditions, the fraction reacted was above 90% after 600 s of reaction. For low grade ore, when the acid concentration was 5 or 6 kmol-m -:~, the reaction did not go to completion. With 7 k m o l ' m :~ hydrobromic acid, 95% reaction both at 313 and 323K was achieved (Fig. 3). No more mercury was recovered because milling did not allow mercury sulphide release from the gangue. Acid concentration was the most important variable on the leaching rate, suggesting that cinnabar dissolution is controlled by reagent diffusion and not by chemical reaction on the solid surface. In any case, the optimum conditions tbr low grade ore attack were 7 kmol-m -:~ hydrobromic acid and 313K. The influence of other factors such as mercury in solution or solid concentration in the pulp was very small or non-existent. In addition, iron and copper dissolution was more important from the low grade ore than from the concentrate. This is explained by the different leaching times.

-~ 0.8 ¢J d

co

293 K

o

3 0 3 ', 3 1 3 ,,

a

323 "

0.5

g *~0.2 kL

1 0.96

Time.10-3(s _.J

1.68

I

2 .z,O

t

L

3.12

3.84

) _

Fig. 2. P l o t of t h e f r a c t i o n r e a c t e d of c i n n a b a r c o n c e n t r a t e as a f u n c t i o n of t i m e for d i f f e r e n t t e m p e r a t u r e s (3.33 s ~ a n d 7 k m o l . m :~ H B r ) .

132

0.9 0.6

o 303 K A 313 ,, o 323 "

~o.3 LL

Time.10-3(s ) 0/48 ~

- -

0.96 ~

1.44 ~

--

Fig. 3. Plot of the fraction reacted of cinnabar ore as a function of time for different temperatures (3.33 s J and 7 k m o l . m -:~HBr).

of Br- and H30÷.10-3 [mot. m-3 )

Concentration of mercury~iron and copper (mo[.m "3)

Concentration

71 -

Bromide ions

5 - ions ~ons M

e

r

H9

,Cu

99.7

.1 !0.31

~ 89.7- 32Z.3 ~0.28

4

S

Copper

59.8 i214.9 10.'g 0~

2.4

4.8

0.09

7.2 .I0 -3 (s)

Time

Pig. 4. Pilot scale attack of a cinnabar ore. Plot of concentration of bromide and hydrogen ions and mercury, iron and copper in solution as a function of time (313K and 7 kmol-m -:~ HBr).

Leaching conditions determined from laboratory experiments were applied to the pilot reactor. When low grade ore was leached, the solids flow in the reactor was 3.67 g-s-1. The solution contained 27.42 mol'm-:~ mercury, 17.92 mol.m -:~ iron and 0.11 mol.m :~copper and its flow was 33.3 mL-s -1. Results are plotted in Fig. 4. Both hydrogen and bromide ion concentrations decreased much more than expected from the stoichiometry of reaction (2); thus, the leaching solution loses its dissolution capacity too quickly, probably due to reaction between the reagent and gangue or pyrite and chalcopyrite contained in the ore. 90% of the mercury was dissolved and a solid residue containing 0.27% mercury was obtained. The remaining 10%, which was not dissolved, was occluded inside gangue ore particles (silica) where hydrobromic acid was not able to

133

react. In order to dissolve this cinnabar it was necessary to mill the ore to a particle size less than 60 pm. The cinnabar flotation concentrate (34.7% mercury) was attacked using a smaller solid flow (1.58 g ' s - 1 ) . Liquid flow was 33.3 m L ' s -1. In this case, bromide and hydrogen ion concentrations evolved according to the stoichiometry of reaction ( 1 ). The mercury content in the solid residue was 4.9%, thus, the efficiency of the process was 86%. Microscopic studies of' the solid residue have shown that the undissolved cinnabar was liberated from the silica matrix. Therefore, the leaching efficiency could be increased by lengthening the reaction time. MERCURY PRECIPITATION

Mercury enriched electrolyte obtained in the leaching step must be suitably treated in order to obtain metal and to recover leaching reagent. From an economic and industrial point of view, the metal can be obtained either in its elemental state or as a compound, because 80% of mercury is consumed as metal or oxide [18]. Thus, the main objective of this work was to precipitate mercury from leaching solutions in elemental and oxidized forms. In order to obtain metallic mercury two methods were tested: electrolysis of concentrated solutions and cementation of dilute solutions.

Precipitation by electrolysis During electrolysis, the following chemical and electrochemical reactions take place: In solution:

HgBr4 2- -~ Hg2+ + 4 B r -

(3)

Cathode:

Hg 2+ + 2 e - ~ Hg °

(4)

Anode:

Br2+2 e

(5)

-~ 2 B r -

The selected electrodes were liquid mercury cathode and an anode made of titanium coated with titanium and ruthenium oxides. This cathode was chosen fbr its high overpotential for hydrogen evolution. Since electrolysis takes place in a very acid solution it was necessary to use a cathode not causing hydrogen evolution. A titanium anode was chosen because this type of electrode is widely used in brine electrolysis with very good results. The study of the precipitation step began with potentiostatic drawing of anodic and cathodic potential-current curves. The decomposition potential for the experimental system was obtained from these curves, and its value was about 1 V. It was possible to precipitate mercury from leaching solutions without hydrogen evolution, however bromine was always reduced at the cathode. Therefore, it was necessary to use an electrolytic cell with the anode and the

134

cathode separated by a polypropylene membrane in order to obtain high anodic and cathodic current efficiencies. With this device, anodic bromine produced through reaction (5) was not reduced at the cathode. In order to find the best conditions for electrolysis, initial experiments where temperature was varied between 298 and 323K and the applied potential between 1.2 and 1.5 V were performed. When the recirculation speed of the electrolyte was 20 mL. s- 1, the best conditions were 1.2 V and 303K. These potential and temperature values were tested in a continuous process with the following steps: ( 1 ) Electrolysis. The products, mercury and bromine, were not in contact (reactions (4) and (5). (2) Transformation of bromine contained in the solution by reaction with hydrogen sulphide from the leaching reaction (reactions (1) and (2)). (3) Recovery of elemental sulphur produced during the second step with organic solvents. Before designing an experimental device where three steps occur, it was necessary to study both the kinetics and the best conditions for reaction (2) and the sulphur recovery step. Transformation of bromine by reduction with hydrogen sulphide is a very fast reaction with efficiencies close to 100%. Working conditions were 333K and 0.66 mL's-1 hydrogen sulphide. After 600 s, all bromine saturated acid solution was transformed to hydrobromic acid. The temperature was the optimum in order to extract sulphur with organic solvents (third step). This extraction was carried out using commercial organic solvents made from alkylbenzenes: "Shell-Soil AB" (from Shell) and "Solvesso 150" (from Esso). Efficiencies were close to 100%. A flow sheet for the electrolytic precipitation of mercury is shown in Fig. 5. Regulator tank 1 for [ Electrolysis cell Reactor F - ~ - l S e d i m e n t a t

P~Tj~Feed (aqueous phase)

tank t

Fig. 5. Electrolysis circuit schematic drawing.

or I

135

:-L I

• Hg 2+ o

~

F . . . . C F - t ~

~ Br-

~

J°' 7,0

+

.

,,,

~

%.8,~

-

oi?

3 9

s

>, :2

6.0 [

,

T i m e " I o - L Is) 10,8

L

T o --

d~

Fig. 6. Electrolysis of leaching solutions. Plot of bromide and hydrogen ions and mercury concentrations as a function of time (1.2 V and 303K }.

A closed cell was used. The electrodes had a surface area of 45 cm 2 each and they were separated by a membrane. The electrolyte leaving the cell was enriched in bromine and impoverished in mercury, and it was fed to a reactor where the bromine was reacted with hydrogen sulphide to yield sulphur and hydrobromic acid. Reaction between sulphur and a suitable organic solvent took place in the same reactor. Later, both phases, aqueous and organic, were separated. The organic phase was recirculated to the reactor and the aqueous phase to the electrolytic cell. The circuit was fed with 6.75 L electrolyte coming from leaching experiments on Almaddn cinnabar. Electrolysis was carried out at 1.2 V and 303K. The time for each experiment was 80 h. In order to know the anodic and cathodic efficiencies, solutions were analysed for mercury (by atomic absorption) and for hydrogen and bromide ions (by volumetry). The results in Fig. 6 show that the mercury recovery efficiency and the hydrobromic acid regeneration efficiency were very high, 90.0% and 96.6%, respectively. These good results could be ascribed to the fact that bromine did not reduce at the cathode together with mercury ions. Nevertheless, the cathodic efficiency was not as high, because there were other ions in solution, such as ferric and cupric, which could be reduced at the cathode simultaneously with mercury. Ferrous and cuprous ions will be reoxidized to ferric and cupric ions by reaction with bromine in solution; thus, the iron and copper concentrations remained constant during electrolysis.

Precipitation by cementation The main objective was to find the best conditions for cementation of mercury from dilute solutions. Concentrated solutions were precipitated by elec-

t36

trolysis (Hg) or pH change (red HgO) because cementation does not allow hydrobromic acid regeneration. Cementation can only be used to recover the mercury contained in secondary and marginal solutions. The best cementing agents for mercury are iron, aluminium and zinc. Previous experiments had shown that aluminium and zinc react violently with hydrogen ions in solution, with an increase of temperature. However, by using iron powder reaction was not violent and produced easily controllable temperature changes. Mercury was quantitatively precipitated only when iron was the reducing agent (Fig. 7). Iron is cheap and does not tbrm amalgams with mercury. All these reasons justified the use of iron as cementing metal. The acid concentration of the cementation solution was 6 kmol-m :~, containing small quantities of mercury (between 2.5 and 15.0 mol-m-:~). Sufficient iron powder was used so that its weight reduction during the reaction time was smaller than 5 %. The variables tested were: temperature (293-333K) (Fig. 8), stirring speed (4.17-16.7 s -t ) (Fig. 9) and initial concentrations of mercury (2.5-15.0 mol-m -:~) (Fig. 10). Generally, the cementation reactions fit a first order model:

10 "~0.8~

~0.6

• s -1 O C e m e n . w i t h Fe (T=303 K) a .... AI(T=303*333 K}

/~ -~0.4

/

I,p

~

z~(r:3o3-3~aK

. . . .

0.2 Time.lO_2( s ) f

1.2

i

L

3.6

6.0

l__

8.4

i

10.8

Fig. 7. Cementation experiments with aluminium, zinc and iron.

1.0

osis Z ooij/ o~/ff

I ~I /

0 2 H/

0.8 293K

0.6

g

o313 ,.

~

323

I I

"

[] 3 3 3 . ,

;z 0A

r LL

z~ 12.50 [] 16.68

/

0.2 [

"

Time .lO-2(s ) 1.2

3.6

5.0

8.4

10.8

1.2

3.5

6.7

8.4

10,8

Fig. 8. Plot of the fraction reacted as a function of time for different temperatures at 8.34 s stirring speed. Fig. 9. Plot of the fraction reacted as a function of time for different stirring speeds at 313K.

137

1ol ~0.8 O 15 m o l m -3 Hg 2÷ o10 D 5 a, 2 5

0.5 o

o OA

U

U-

02 Time-t0 -2 (s) 1'.2

3.6

6.0

8.4

10.8

Fig. 10. P l o t of t h e f r a c t i o n r e a c t e d as a f u n c t i o n of t i m e for d i f f e r e n t initial c o n c e n t r a t i o n s of m e r c u r y (12.5 s ' s t i r r i n g s p e e d ) .

1.00 ~ _

o 4 1 7 s -I

%."4,'%,

o

2.s0 0.10

.. ..

[] 16.6B ,,

oo,

Time.lO -2 (s) I

I

I

I

I

1.2 3,6 50 8.4 10.8 Fig. 11. P l o t of log ( C / C . ) as a f u n c t i o n of t i m e for d i f f e r e n t s t i r r i n g s p e e d s at 293K.

in ( ( H g ~ + ) / ( H g 2+)o) = - K ( A / V ) t

(6)

where (Hg 2+ ) is the mercury concentration at time t, (Hg 2+ )0 is the mercury concentration at zero time, A is the iron surface area, V is the solution volume and K is the kinetic constant. The experimental data fit well to the proposed model (Fig. 11 ). The kinetic constants obtained from the slopes of the lines varied from 1.44×10 -7 to 3.73× 10 -7 m . s -1. The influence of stirring speed and temperature on cementation kinetics can be quantified from the kinetic constant values. Figure 12 shows these values versus stirring speed to the power of 1/2 at different temperatures. The straight lines suggest that both variables are proportional. Therefore, process control is diffusional [23 ]. With respect to the influence of temperature on the kinetic constant, Fig. 13 shows an Arrhenius dependence. Since the slope of the lines is proportional to process activation energy it can be observed that activation energy decreases when the stirring speed increases. This apparently contradictory behaviour may be explained by the following considerations:

138

K 107(m.s-1)

4

~

• 293 K

o 303 "

.

4313 3

,,

[3 323 ,, o 333 "

2 1

i 1

speed) 1/2 (s)-1/2 I i

{Stirring

I

~

2

3

4

5

Fig. 12. Plot of kinetic c o n s t a n t as a function of stirring speed for different temperatures.

In K

o 417 s -I

-15,5 -15.0[

~

t,,,_t ~ 30

I

i( ]/T)'103(K, 3.2

3.4

-I} 3.5

Fig. 13. Arrhenius plot for mercury cementation for different stirring speeds. (1) Mercury is liquid at the cementation temperature. The metal does not wet iron because it has a very high surface tension and therefore it forms many small droplets on the iron surface. (2) The reaction occurs in acid medium, thus small hydrogen bubbles are formed on the iron surface when there is a low stirring speed. When agitation increases, hydrogen bubbles are eliminated more easily and the effectiveness of the reaction at the surface increases. Under these conditions, the proportion of nucleation sites for mercury droplets also increases. These small mercury droplets obstruct contact between mercury ions and solid iron, decreasing the reaction rate due to a diffusion step through liquid droplets on the solid surface.

Precipitation by pH change The last alternative for mercury precipitation is to obtain it in an oxidized state, as red mercury oxide. There are two reasons to obtain this product: ( 1 ) Red mercury oxide has a very important market. (2) The oxide price is 30-40% higher than the price of elemental mercury. The hydrometallurgical method allows the direct precipitation of the oxide from the leaching solutions. Experimentation was carried out from solutions with mercury contents between 25 and 150 mol.m -:~, in 6 kmol.m -:~ hydrobromic acid. Three precipi-

139

rating agents were investigated: sodium hydroxide, sodium carbonate and calcium hydroxide. Sodium hydroxide was chosen because of the basicity required to precipitate mercury oxide. Temperatures were varied between 283 and 323K. Experiments were carried out with continuous control of pH. A yellow substance precipitated during the process and later became red. The yellow compound was a non-crystalline mercury oxybromide and the red compound was identified as red mercury oxide by X-ray diffraction. Precipitation pH and crystalline structures are shown in Table 1. It can be seen that the precipitation pH decreases when the temperature increases. This decrease was expected because decomposition of the tetrabromo-mercuriate complex is favoured when the temperature rises. Different structures for mercur>, oxide were obtained: ortho-rhombic, hexagonal or both. The hexagonal structure was obtained at higher temperatures (323K) and the ortho-rhombic structure at lower temperatures (283K). The influence of the temperature, solution seeding with mercury oxide and excess of precipitating agent on precipitation kinetic were also studied. Figure 14 shows that a temperature decrease improved the precipitation efficiencies. Efficiencies were 77.1% at 323K and 89.9% at 283K. When the temperature was decreased, the oxide solubility also decreased, increasing the oversaturation (driving force for precipitation). On the other hand, with low temperatures, the nucleation process is favoured because the critical nucleus size decreases and therefore energy requirements of the process also decrease. The effect of solution seeding on precipitation efficiency and kinetics, at different temperatures (283 and 323K) was studied. The results at 323K show greater exhaustion of the solution when it was seeded (82.7% efficiency) than TABLE 1 E x p e r i m e n t a l p r e c i p i t a t i o n p H a n d c r y s t a l l i n e s t r u c t u r e of m e r c u r y oxide at d i f f e r e n t t e m p e r a tures and concentrations Temperature

H g ~+ C o n c e n t r a t i o n (tool. m - :~)

(K) 283 303 313 323

25

50

100

150

--H,O 14.00 H 13.05 H,O 12.75

--H,O 13.60 H 13.10 H 12.65

--H,O 14.00 H 13.00 H 12.65

O 14.00 H,O 13.90 H,O 13.10 H 12.70

N()te: O = o r t h o - r h o m b i c ; H = h e x a g o n a l .

140

1.5

Weight of Hg2+ .103 (kg}

1.2

" T=283 K o T=323 K

0.9 0.6 0.3

-

Time.10-3(s)

36 7.2 10.8 Fig. 14. Influence of temperatures on the precipitation kinetic of red mercury oxide with sodium hydroxide (initial solution: 150 m o l ' m -:~ Hg '-'+ and 6 kmol-m :~HBr).

1.51Weight of Hg2*.103(kg) 1.2 i

l 1

0.g0.6i

~

0.3

owith seeding t

seeding

'~-~-- Ti me.lO_3(s)

0

72

Fig. 15. Influence of' seeding on the precipitation kinetic of red mercury oxide with sodimn hydroxide at 323K (initial solution: 150 mol-m :{Hg ~+ and 6 kmol-m :{HBr).

when it was not ( 77.1% efficiency) (Fig. 15 ). Nevertheless, the level of seeding had no influence. Seeding influenced precipitation mechanism steps are nucleation and growth. In the absence of seeding, precipitation involved both steps. When there was seeding the first step was not necessary and the process was favoured. However, seeding did not improve the results when the temperature was 283K due to increasing oversaturation which enhances nucleation of primary mercury oxide particles. On the other hand, in order to obtain total exhaustion of solutions it was necessary to add an excess of precipitating reagent. Figure 16 shows that the amount of mercury in solution was similar when the excess of basic reagent was 25 or 40 mL fbr each 50 mL of initial volume. When the excess was lower than 25 mL, mercury levels in solution remained appreciably greater. This experimentation allowed the establishment of the best conditions to precipitate mercury oxide with maximum efficiencies: 283K, without seeding, and by adding an excess of basic reagent (25 mL of 12 k m o l . m -:~ NaOH). These conditions were used in a solution obtained from a cinnabar ore leach which, in addition to important quantities of mercury (189.43 mol-m -:~), had 34.01 mol-m :~iron. These metals were separated by fractional precipitation, because their precipitation pHs are different: iron precipitates at pH 4 and

141

1.5 W ~ g h t

of Hg2*.103 ( k g )

k mot. m -3) = NaOH excess (12 ° 10 crn 3 • 20 ,, ~ 2 5 ,,

1.2 0.9 0.6 0.3

o

r.

"-

A

0

12

i

•

2.&

3,6

a

t_

- - L ~

4.8 6.0 7.2 Tirne .10-3 (s)

Fig. 16. I n t l u e n c e of excess s o d i u m h y d r o x i d e on p r e c i p i t a t i o n k i n e t i c s of red m e r c u r y oxide at 2 8 3 K (initial solution: 150 m o l ' m :~H g e+ a n d 6 k m o l ' m :~ H B r ) .

mercury at basic pH. Firstly, all the iron was separated and then the mercury, although 2% of the latter metal was retained in the iron hydroxide. CONCLUSIONS

Cinnabar leaching: (1). Cinnabar attack is clearly favoured by temperature and hydrobromic acid concentration and the best conditions in order to leach cinnabar are: (a) Low grade ore: 313K and 7 k m o l . m -:~ HBr; (b) Concentrate: 303K and 7 kmol. m :~. (2). With respect to leaching in a pilot reactor, the leaching efficiency for a low grade ore was 90%, with the remaining 10% mercury occluded in the gangue. This result could be improved by milling ore to 60 llm. When a concentrate was leached, 86% mercury was dissolved and the efficiency could be improved by increasing the leaching time, because cinnabar was not occluded in the gangue.

l'recipitation t)3' electrolysis: {3). Mercury precipitation from tetrabromomercuriate solutions by electrolysis must be carried out in a cell with the anode and cathode separated by a membrane, in order to avoid bromine reduction at the cathode together with mercury ions. (4). Working at 303K and 1.2 V, efficiencies were 90% at the cathode and 96.6% at the anode.

Precipitation by cementation: ( 5 ). Iron was the most suitable metal to cement mercury from dilute solutions. (6). Mercury cementation with excess iron followed first order kinetics. Values of' the kinetic constant vary between 1.44 × 10 v and 3.73 × 10 -7 m - s - ' . Reaction is controlled by a diffusion step.

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Precipitation by pH change: (7). Red mercury oxide was precipitated by adding sodium hydroxide to the leaching solution. The precipitation pH varied between 12.75 and 14.00 and precipitates were obtained with either ortho-rhombic or hexagonal structures. (8). The precipitation efficiency increased when the temperature was decreased: 77.1% at 323K and 89.9% at 283K. (9). Solution seeding improved efficiency when the temperature was higher (323K). (10). Fractional precipitation of metals (iron and mercury) from cinnabar leaching solutions was possible, because iron precipitated at pH about 4 and mercury at basic pHs. ACKNOWLEDGEMENTS

Many thanks are given to Comisidn Asesora de Investigacidn Cientffica y T6cnica (Spain) for their financial help given to carry out this work.

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