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Recovery of molybdenum from spent acid of lamp making industries
Hydrometallurgy, 20 (1988) 147-154
147
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Recovery of Molybdenum from Spent Acid of Lamp Making Industries T.K: MUKHERJEE, A.C. BIDAYE and C.K. GUPTA Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085 (India)
ABSTRACT Mukherjee, T.K., Bidaye, A.C. and Gupta, C.K., 1988. Recovery of molybdenum from spent acid of lamp making industries. HydrometaUurgy, 20: 147-154. The paper reports studies on recovery of molybdic oxide from the molybdenum containing spent acid generated in lamp manufacturing industries. It first gives an account of the efforts made to modify the direct ammonia neutralisation process. Next a new process is described involving adsorption of molybdenum from spent acid on activated carbon and its subsequent desorption with ammonia. Ammonium molybdate crystals are recovered from the eluted liquor by a crystallization/acidificationtechnique. The influence of acidity, molybdenum concentration, amount of carbon, contact time and amount of ammonia as eluant on the process of molybdenum recovery has been studied. A process flowsheet characterised by a large saving in ammonia consumption is presented.
INTRODUCTION
In the present day context of rapidly depleting primary metal resources and ever increasing demand for energy, recovery of metal values from secondary resources has assumed great importance and the metal molybdenum is no exception. In earlier publications [ 1,2 ] from this laboratory, processes were described for recovery of molybdenum metal powder from secondary resources such as low grade molybdenite concentrate which is generated as a byproduct of uranium mining and milling operations, and molybdenum metal scrap. The present paper deals with the extraction of the same metal from yet another secondary resource, namely molybdenum bearing spent acid generated by lamp making industries. During the manufacture of tungsten filaments for electric lamps, tungsten coils are double wound around molybdenum mandrel wire/ rods and the latter is separated from the coil by dissolution of molybdenum in a reagent composed of nitric and sulphuric acids. Such operation results in the generation of large volumes of spent acid containing H2SO4, HNO3 and dissolved molybdenum. 0304-386X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
148
So far, only two papers have been reported on the recovery of molybdenum from the spent acid. Kulkarni [ 3 ] has evaluated a number of techniques such as distillation, solvent extraction with a binary or tertiary amine, ion exchange with anion exxchange resins and direct ammonium molybdate precipitation by neutralisation of the spent acid with ammonia. He concluded that the last was the simplest and the best technique. However, the process is handicapped by its large requirement of costly ammonia. The second reported process came from McCarty [ 4 ], which consisted of boiling the spent acid to crystallize out about 70% of the molybdenum as molybdic acid. Such a process would require reactors and a refluxing system made of special material that could withstand this corrosive spent acid at high temperatures. Moreover, it was recommended that the process would be economic only if the acid residue depleted with respect to molybdenum and nitric acid was reconstituted with fuming nitric acid and small amount of sulfuric acid for reuse as a mandrel dissolving acid. In the present investigation an effort was made first to modify the ammonia-neutralisation technique and then to standardise a carbon adsorption process for the recovery of molybdenum from the spent acid. EXPERIMENTAL
Materials and equipment While molybdenum metal scrap was used for making the synthetic solution, a local lamp industry supplied the spent acid analysing 5 M HNO3, 5.5 M H e S Q and 125 g/1 molybdenum. Reagents of AR grade such as H2SO4, HNO3, NaOH, NH4OH (25%), NaeCO3, CaO and activated carbon ( <47/~m) were used in the experiments. The neutralisation, adsorption, desorption and precipitation experiments on a small scale were conducted in glassware using a magnetic stirrer for agitation. The larger scale experiments for the adsorption-desorption study were conducted in a pair of reactor systems as shown in Fig. 1. Each reactor system consisted of an upper and lower chamber made of stainless steel connected through a ball valve V1/V9 placed in between. The upper chamber had a nylon filter cloth covered perforated plate. The bottom chamber was sealed from the top by means of a gasketted flange which had provision for introducing compressed air or applying vacuum through valve Vz/Vlo. The upper chamber was meant for carrying out the adsorption-desorption cycle by mildly agitating the liquid-carbon slurry with a mechanical stirrer or compressed air blown from the bottom through the perforated plate. The lower chamber was used for collecting the filtered liquid which could be either pressure transferred to the top chamber of the adjacent reactor system, or drawn out as and when required.
149
~SPENT
ACID
H20
AMMONIA~ vI3
BI
V2 AIR/
A21
I
v4
//
\\
I
I e2
v8 _
C AMMONIUMMOLYBDATE SOLUTION
Fig. 1. Experimental set up used for large scale processing of spent acid.
Procedure
Synthetic molybdenum bearing acid was prepared by dissolving 125 g molybdenum scrap in 1 1 of an acid mixture composed of 310 ml of concentrated H2SO4 (98%), 515 ml of concentrated HNO:~ (70%) and balance of water.
150 The acidity of the liquor corresponded to 5 M HNO3 and 5.5 M H2SO 4 with 125 g/1 of molybdenum in solution. In the acid neutralisation experiments, the amounts of ammonia, lime, sodium hydroxide and sodium carbonate required to neutralise 50 ml samples of acid to pH 2 were determined. Next, known quantities of CaO, Na2C03 and NaOH ranging from 25 to 85% of the amounts required for neutralisation to pH 2 were added to 50 ml samples of the acid. Molybdenum from the partially neutralised acid samples was precipitated as molybdic acid by boiling, or as ammonium molybdate by adding ammonia and adjusting the pH to about 2. Both types of salts were calcined at 550 ° C and chemically analysed. In the case of partial neutralisation with lime, CaSO4 precipitate was filtered off prior to separation of molybdenum salts. In the studies on adsorption of molybdenum on carbon from the acidic solution, 50 ml of the molybdenum bearing acid was taken and diluted to known acidity and molybdenum strength before contacting with carbon for a constant duration of 6 h. At the end of this period, the carbon was allowed to settle and the supernatant liquid was analysed by the oxine method to determine the amount of molybdenum adsorbed. In the study of determination of optimum contact time, samples were taken out of the slurry at regular intervals after brief settling periods. In the desorption study, carbon was first loaded with a known amount of molybdenum and then eluted with both sodium hydroxide and ammonical solution at different pH conditions for a constant duration of 3 h. The supernatant liquid was analysed for molybdenum to estimate the extent of desorption. For larger scale tests, 0.5 1 of spent acid was diluted to 5 1 and subjected to a two stage contact with 1 kg of carbon distributed equally in A1 and B1 (Fig. 1 ). The carbon loaded with molybdenum was subsequently treated with aqueous ammonia to bring molybdenum back into solution. This solution was acidified to pH between 1.1 and 0.8 and heated at 85°C to precipitate ammonium molybdate salt. The salt was calcined at 550 ° C to yield pure M o Q . RESULTS AND DISCUSSION Table 1 presents the results of partial neutralisation of the acid with lime, sodium hydroxide and sodium carbonate followed by precipitation of molybdenum salt either by boiling or further neutralisation with ammonia. It can be seen that such a partial neutralisation technique resulted in 90 to 100% molybdenum recovery with considerable savings in ammonia consumption. But in all the variations, the resulting molybdic oxide samples were found to contain rather high amounts of calcium or sodium. Such oxide can in no case be used for the production of high purity molybdenum metal powder. It was next decided to selectively adsorb molybdenum from the spent acid on activated carbon. Sigworth [ 5 ] has already established that molybdenum
151 TABLE1
Results of partial neutralisation of the acid by CaO/NaOH/Na2C03 followed by molybdenum recovery as molybdic acid/ammonium molybdate; vol. of acid=50 ml; vol. of ammonia solution (25%) required for direct neutralisation of the acid to pH 2 = 55 ml Neutralising agent
Amount of neutralising agent added, % of that required for neutralisation to pH
Method of Mo vol. ofNH3 pH of Molybdenum Analyses of precipitation after solution precipitation recovery, % the calcined partial (25%), ml molydic oxide neutralisation
2 CaO
65
Boiling for 2 h
CaO
65
Ammonia addition 2 and heating at
Na2CO:~
70
Boiling for 2 h
Na._,CO:~
70
Ammonia addition 2.1 and heating at
NaOH
60
Ammonia addition 3.5 and heating at
-
-
90
0.9
95
-
90
1.7
99
1.28
100
75°C for2 h -
75°C for 2 h
75°C for 2 h
Ca-2.5% Fe-150 mg/kg A1 < 200 mg/kg Ca-3.2% Fe-155 mg/kg A1< 200 mgkg Na-2.5% Fe-100 mg/kg Al < 200 mg/kg Na-3.65% Fe-100 mg/kg Al < 200 mg/kg Na-3.5% Fe-100 mg/kg Al < 200 mg/kg
can be best adsorbed on carbon from mildly acidic (pH 2) solutions. No information however, is available on molybdenum recovery aspects from highly acidic solutions. The influence of acidity, contact time, molybdenum concentration, and amount of carbon on the percentage of molybdenum adsorbed and the loading factor (wt. of Mo adsorbed/wt, of carbon × 100) are presented in Figs. 2-5. It can be seen from Fig. 2 that a contact time of 6 h is adequate for an equilibrium to be established between molybdenum in solution and molybdenum adsorbed on carbon. Hence, all further small scale experiments on adsorption were conducted for a constant duration of 6 h. Figure 3 shows that carbon adsorption was more suited to molybdenum laden solution of lower acidity. Figure 4 shows that the loading factor rises linearly with the concentration of molybdenum in solution, but the percentage of molybdenum adsorbed increases only marginally from 75% to about 80%. Figure 5 shows that as the amount of carbon was increased, the amount of molybdenum adsorbed improved from 70% to 93% but the loading factor dropped sharply from 13.8% to 5.8%. The spent acid has an acidity of 10.5 M (5 M HNO3+5.5 M H2SO4) and obviously needs reduction in acidity to give adequate adsorption of molybdenum on activated carbon. Diluting the spent acid lowers the acidity but it lowers the molybdenum strength as well. The overall effect of dilution of the acid
152 IOO
I00
80
80
Acid 60 c o
500
mt
Mo content
Carbon
--
o~
12 l ~ g / ~
~
8
6
e
o -o ~f
o
4o
j o ~a o
= o
-- 50 g
4o
g
o -o
60
dsorption
actor
4
o 2O
2O
I
I
=
I
2
4
Contact
time
,
=40 g ~It~.~ Mo content=625g/l
Carbon
I
6
[
I
I
I
2
3
Acidity
hours
2
, M
Fig. 2. Influence of contact time on molybdenumadsorption. Fig. 3. Influence of acidity on molybdenumadsorption and loading factor. I00 Adsorption
4 = o
12
80
60
80
12
60
g
~Loading_~ ~
o o 6
40
Q.
o
40
;
'o
6
o
3
o
factor
3~
20
o J
I
I
I
3.125
6.25
9.375
Mo, Concentration
g/l
12.5
20
Acid - . 5 0 0 Mo
-
12.5
mt
I
I
I
25
50
75
Carbon
o
g/t
j g
Fig. 4. Influence of molybdenumconcentration on its adsorption and loading factor. Fig. 5. Influence of amount of carbon on molybdenumadsorption and loading factor. with water on molybdenum adsorption is plotted in Fig. 6. It can be seen that in spite of decreasing molybdenum strength, the adsorption continuously increased with further dilution. Table 2 presents the results of desorption of molybdenum from activated carbon by using two different eluants, sodium hydroxide and dilute ammonia solutions. W h e n sodium hydroxide was used as eluant, maximum desorption of 90% was achieved at a pH between 11 and 11.5. In comparison with sodium hydroxide, ammoniacal solution was found to be the better eluant as it resulted in almost 97% desorption at a pH close to 9.
153 ~00
60 c
T~ o
Spent 40--
tiquor - 50rnt
Carbon -
40 g
I
I
I
I0
20
30
Dilution
40
factor
Fig. 6. Influenceof dilution factor on molybdenumadsorption. After desorption with ammonia, the carbon could be reused directly without any other treatment. The adsorption and desorption efficiencies of the carbon remained almost unchanged during its use in five consecutive cycles. Based on the above studies, a dilution factor of 10, contact time of 2 h and 100 g of carbon for every liter of the diluted acid were considered optimum for adsorption of molybdenum. Desorption was best carried out with ammonia solution (10%) for a period of 2 h. These parameters were used for the larger scale processing of the spent acid. With a double stage contact, it was found possible to achieve 96 and 99% adsorption and desorption respectively. During the processing of every 5 1 of diluted acid, 9 1 of waste solution analysing 0.3 g/1 Mo and 4 1 of a m m o n i u m molybdate solution analysing 15 g/l Mo were generated. The total ammonia (25%) consumption was 165 ml in comparison to TABLE2 Results of desorptionstudieson activated carbon loadedwith molybdenum;amount of carbon = 20 g containing 2 g molybdenum,volumeof the eluant = 200 ml, contact time---3 h Eluant
pH range
Molybdenumdesorbed, %
NaOH
6.40- 7.00 8.00- 8.50 9.00- 9.50 10.00-10.50 11.05-11.55 12.70-12.8
76.2 84.7 85.2 88.6 90.0 86.4
8.70- 8.93 9.75- 9.37
96.7 97.3
NH~ solution
154 550 ml required for direct neutralisation of the acid. About 85 g of pure MoO3 was recovered from the 4 1 Mo laden solution by precipitation of a m m o n i u m molybdate salt and its calcination. A material balance study indicated an overall molybdenum recovery of 91%. The MoO3 was spectrographically analysed and found to contain < 27 Fe, < 20 Ni, < 100 Cr, < 20 Ca, < 10 Cu, < 100 A1, < 10 Mn, < 20 Mg, < 20 Pb and < 200 Sn in mg/kg. Molybdic oxide of such purity is considered suitable for its hydrogen reduction to powder metallurgical grade molybdenum metal powder. Thus, it can be said t h a t in comparison to the direct neutralistion process with ammonia, the carbon adsorption-desorption technique yielded slightly less molybdenum recovery but about 70% savings in ammonia consumption and excellent product quality could be achieved. CONCLUSIONS An effort was made to modify the existing process of recovery of molybdenum salt from the spent acid by its direct neutralisation with ammonia. Partial substitution of ammonia with cheaper reagents such as CaO, N a O H or Na2CO3 could result in considerable cost saving but the molybdenum salt was found to be highly contaminated with calcium and sodium. An alternative process involving adsorption of molybdenum on activated carbon was found to be suitable. The process is successful if the spent acid is diluted with water before contacting with carbon. The attractive features of the process are a high quality of the end product, excellent molybdenum recovery and over 70% savings in ammonia consumption.
REFERENCES 1 Nair,K.U., Bose, D.K. and Gupta, C.K., 1978. Studies on the processingof molysulphideconcentrate by chlorination. Mining Eng., 30: 291. 2 Bidaye, A.C., Mukherjee, T.K. and Gupta, C.K., 1985. Reclamationof molybdenumpowder from its scrap. In: Proc. 1lth Plansee Seminar on New Applications,Recyclingand Technology of Refractory Metals and Hard Materials held at Reutte, Tirol, Austria during 20-24th May, p. 325. 3 Kulkarni, A.D., 1976. Recoveryof molybdenumfrom spent acid. Met. Trans, 7B: 115. 4 McCarty,L.V., 1985. The recoveryof molybdicoxideand recyclingof acids from molybdenum mandrel dissolving.In: Proc. 1lth Plansee Seminar on New Applications,Recyclingand Technologyof RefractoryMetalsand Hard Materials,held at Reutte,Tirol, Austraia,during20-24th May, p. 45. 5 Sigworth, E.A., 1962. Potentialities of activated carbon in the metallurgicalfield. Am. Inst. Min. Eng. Metal. Soc., Reprint No. 62 B 81.
147
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Recovery of Molybdenum from Spent Acid of Lamp Making Industries T.K: MUKHERJEE, A.C. BIDAYE and C.K. GUPTA Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085 (India)
ABSTRACT Mukherjee, T.K., Bidaye, A.C. and Gupta, C.K., 1988. Recovery of molybdenum from spent acid of lamp making industries. HydrometaUurgy, 20: 147-154. The paper reports studies on recovery of molybdic oxide from the molybdenum containing spent acid generated in lamp manufacturing industries. It first gives an account of the efforts made to modify the direct ammonia neutralisation process. Next a new process is described involving adsorption of molybdenum from spent acid on activated carbon and its subsequent desorption with ammonia. Ammonium molybdate crystals are recovered from the eluted liquor by a crystallization/acidificationtechnique. The influence of acidity, molybdenum concentration, amount of carbon, contact time and amount of ammonia as eluant on the process of molybdenum recovery has been studied. A process flowsheet characterised by a large saving in ammonia consumption is presented.
INTRODUCTION
In the present day context of rapidly depleting primary metal resources and ever increasing demand for energy, recovery of metal values from secondary resources has assumed great importance and the metal molybdenum is no exception. In earlier publications [ 1,2 ] from this laboratory, processes were described for recovery of molybdenum metal powder from secondary resources such as low grade molybdenite concentrate which is generated as a byproduct of uranium mining and milling operations, and molybdenum metal scrap. The present paper deals with the extraction of the same metal from yet another secondary resource, namely molybdenum bearing spent acid generated by lamp making industries. During the manufacture of tungsten filaments for electric lamps, tungsten coils are double wound around molybdenum mandrel wire/ rods and the latter is separated from the coil by dissolution of molybdenum in a reagent composed of nitric and sulphuric acids. Such operation results in the generation of large volumes of spent acid containing H2SO4, HNO3 and dissolved molybdenum. 0304-386X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
148
So far, only two papers have been reported on the recovery of molybdenum from the spent acid. Kulkarni [ 3 ] has evaluated a number of techniques such as distillation, solvent extraction with a binary or tertiary amine, ion exchange with anion exxchange resins and direct ammonium molybdate precipitation by neutralisation of the spent acid with ammonia. He concluded that the last was the simplest and the best technique. However, the process is handicapped by its large requirement of costly ammonia. The second reported process came from McCarty [ 4 ], which consisted of boiling the spent acid to crystallize out about 70% of the molybdenum as molybdic acid. Such a process would require reactors and a refluxing system made of special material that could withstand this corrosive spent acid at high temperatures. Moreover, it was recommended that the process would be economic only if the acid residue depleted with respect to molybdenum and nitric acid was reconstituted with fuming nitric acid and small amount of sulfuric acid for reuse as a mandrel dissolving acid. In the present investigation an effort was made first to modify the ammonia-neutralisation technique and then to standardise a carbon adsorption process for the recovery of molybdenum from the spent acid. EXPERIMENTAL
Materials and equipment While molybdenum metal scrap was used for making the synthetic solution, a local lamp industry supplied the spent acid analysing 5 M HNO3, 5.5 M H e S Q and 125 g/1 molybdenum. Reagents of AR grade such as H2SO4, HNO3, NaOH, NH4OH (25%), NaeCO3, CaO and activated carbon ( <47/~m) were used in the experiments. The neutralisation, adsorption, desorption and precipitation experiments on a small scale were conducted in glassware using a magnetic stirrer for agitation. The larger scale experiments for the adsorption-desorption study were conducted in a pair of reactor systems as shown in Fig. 1. Each reactor system consisted of an upper and lower chamber made of stainless steel connected through a ball valve V1/V9 placed in between. The upper chamber had a nylon filter cloth covered perforated plate. The bottom chamber was sealed from the top by means of a gasketted flange which had provision for introducing compressed air or applying vacuum through valve Vz/Vlo. The upper chamber was meant for carrying out the adsorption-desorption cycle by mildly agitating the liquid-carbon slurry with a mechanical stirrer or compressed air blown from the bottom through the perforated plate. The lower chamber was used for collecting the filtered liquid which could be either pressure transferred to the top chamber of the adjacent reactor system, or drawn out as and when required.
149
~SPENT
ACID
H20
AMMONIA~ vI3
BI
V2 AIR/
A21
I
v4
//
\\
I
I e2
v8 _
C AMMONIUMMOLYBDATE SOLUTION
Fig. 1. Experimental set up used for large scale processing of spent acid.
Procedure
Synthetic molybdenum bearing acid was prepared by dissolving 125 g molybdenum scrap in 1 1 of an acid mixture composed of 310 ml of concentrated H2SO4 (98%), 515 ml of concentrated HNO:~ (70%) and balance of water.
150 The acidity of the liquor corresponded to 5 M HNO3 and 5.5 M H2SO 4 with 125 g/1 of molybdenum in solution. In the acid neutralisation experiments, the amounts of ammonia, lime, sodium hydroxide and sodium carbonate required to neutralise 50 ml samples of acid to pH 2 were determined. Next, known quantities of CaO, Na2C03 and NaOH ranging from 25 to 85% of the amounts required for neutralisation to pH 2 were added to 50 ml samples of the acid. Molybdenum from the partially neutralised acid samples was precipitated as molybdic acid by boiling, or as ammonium molybdate by adding ammonia and adjusting the pH to about 2. Both types of salts were calcined at 550 ° C and chemically analysed. In the case of partial neutralisation with lime, CaSO4 precipitate was filtered off prior to separation of molybdenum salts. In the studies on adsorption of molybdenum on carbon from the acidic solution, 50 ml of the molybdenum bearing acid was taken and diluted to known acidity and molybdenum strength before contacting with carbon for a constant duration of 6 h. At the end of this period, the carbon was allowed to settle and the supernatant liquid was analysed by the oxine method to determine the amount of molybdenum adsorbed. In the study of determination of optimum contact time, samples were taken out of the slurry at regular intervals after brief settling periods. In the desorption study, carbon was first loaded with a known amount of molybdenum and then eluted with both sodium hydroxide and ammonical solution at different pH conditions for a constant duration of 3 h. The supernatant liquid was analysed for molybdenum to estimate the extent of desorption. For larger scale tests, 0.5 1 of spent acid was diluted to 5 1 and subjected to a two stage contact with 1 kg of carbon distributed equally in A1 and B1 (Fig. 1 ). The carbon loaded with molybdenum was subsequently treated with aqueous ammonia to bring molybdenum back into solution. This solution was acidified to pH between 1.1 and 0.8 and heated at 85°C to precipitate ammonium molybdate salt. The salt was calcined at 550 ° C to yield pure M o Q . RESULTS AND DISCUSSION Table 1 presents the results of partial neutralisation of the acid with lime, sodium hydroxide and sodium carbonate followed by precipitation of molybdenum salt either by boiling or further neutralisation with ammonia. It can be seen that such a partial neutralisation technique resulted in 90 to 100% molybdenum recovery with considerable savings in ammonia consumption. But in all the variations, the resulting molybdic oxide samples were found to contain rather high amounts of calcium or sodium. Such oxide can in no case be used for the production of high purity molybdenum metal powder. It was next decided to selectively adsorb molybdenum from the spent acid on activated carbon. Sigworth [ 5 ] has already established that molybdenum
151 TABLE1
Results of partial neutralisation of the acid by CaO/NaOH/Na2C03 followed by molybdenum recovery as molybdic acid/ammonium molybdate; vol. of acid=50 ml; vol. of ammonia solution (25%) required for direct neutralisation of the acid to pH 2 = 55 ml Neutralising agent
Amount of neutralising agent added, % of that required for neutralisation to pH
Method of Mo vol. ofNH3 pH of Molybdenum Analyses of precipitation after solution precipitation recovery, % the calcined partial (25%), ml molydic oxide neutralisation
2 CaO
65
Boiling for 2 h
CaO
65
Ammonia addition 2 and heating at
Na2CO:~
70
Boiling for 2 h
Na._,CO:~
70
Ammonia addition 2.1 and heating at
NaOH
60
Ammonia addition 3.5 and heating at
-
-
90
0.9
95
-
90
1.7
99
1.28
100
75°C for2 h -
75°C for 2 h
75°C for 2 h
Ca-2.5% Fe-150 mg/kg A1 < 200 mg/kg Ca-3.2% Fe-155 mg/kg A1< 200 mgkg Na-2.5% Fe-100 mg/kg Al < 200 mg/kg Na-3.65% Fe-100 mg/kg Al < 200 mg/kg Na-3.5% Fe-100 mg/kg Al < 200 mg/kg
can be best adsorbed on carbon from mildly acidic (pH 2) solutions. No information however, is available on molybdenum recovery aspects from highly acidic solutions. The influence of acidity, contact time, molybdenum concentration, and amount of carbon on the percentage of molybdenum adsorbed and the loading factor (wt. of Mo adsorbed/wt, of carbon × 100) are presented in Figs. 2-5. It can be seen from Fig. 2 that a contact time of 6 h is adequate for an equilibrium to be established between molybdenum in solution and molybdenum adsorbed on carbon. Hence, all further small scale experiments on adsorption were conducted for a constant duration of 6 h. Figure 3 shows that carbon adsorption was more suited to molybdenum laden solution of lower acidity. Figure 4 shows that the loading factor rises linearly with the concentration of molybdenum in solution, but the percentage of molybdenum adsorbed increases only marginally from 75% to about 80%. Figure 5 shows that as the amount of carbon was increased, the amount of molybdenum adsorbed improved from 70% to 93% but the loading factor dropped sharply from 13.8% to 5.8%. The spent acid has an acidity of 10.5 M (5 M HNO3+5.5 M H2SO4) and obviously needs reduction in acidity to give adequate adsorption of molybdenum on activated carbon. Diluting the spent acid lowers the acidity but it lowers the molybdenum strength as well. The overall effect of dilution of the acid
152 IOO
I00
80
80
Acid 60 c o
500
mt
Mo content
Carbon
--
o~
12 l ~ g / ~
~
8
6
e
o -o ~f
o
4o
j o ~a o
= o
-- 50 g
4o
g
o -o
60
dsorption
actor
4
o 2O
2O
I
I
=
I
2
4
Contact
time
,
=40 g ~It~.~ Mo content=625g/l
Carbon
I
6
[
I
I
I
2
3
Acidity
hours
2
, M
Fig. 2. Influence of contact time on molybdenumadsorption. Fig. 3. Influence of acidity on molybdenumadsorption and loading factor. I00 Adsorption
4 = o
12
80
60
80
12
60
g
~Loading_~ ~
o o 6
40
Q.
o
40
;
'o
6
o
3
o
factor
3~
20
o J
I
I
I
3.125
6.25
9.375
Mo, Concentration
g/l
12.5
20
Acid - . 5 0 0 Mo
-
12.5
mt
I
I
I
25
50
75
Carbon
o
g/t
j g
Fig. 4. Influence of molybdenumconcentration on its adsorption and loading factor. Fig. 5. Influence of amount of carbon on molybdenumadsorption and loading factor. with water on molybdenum adsorption is plotted in Fig. 6. It can be seen that in spite of decreasing molybdenum strength, the adsorption continuously increased with further dilution. Table 2 presents the results of desorption of molybdenum from activated carbon by using two different eluants, sodium hydroxide and dilute ammonia solutions. W h e n sodium hydroxide was used as eluant, maximum desorption of 90% was achieved at a pH between 11 and 11.5. In comparison with sodium hydroxide, ammoniacal solution was found to be the better eluant as it resulted in almost 97% desorption at a pH close to 9.
153 ~00
60 c
T~ o
Spent 40--
tiquor - 50rnt
Carbon -
40 g
I
I
I
I0
20
30
Dilution
40
factor
Fig. 6. Influenceof dilution factor on molybdenumadsorption. After desorption with ammonia, the carbon could be reused directly without any other treatment. The adsorption and desorption efficiencies of the carbon remained almost unchanged during its use in five consecutive cycles. Based on the above studies, a dilution factor of 10, contact time of 2 h and 100 g of carbon for every liter of the diluted acid were considered optimum for adsorption of molybdenum. Desorption was best carried out with ammonia solution (10%) for a period of 2 h. These parameters were used for the larger scale processing of the spent acid. With a double stage contact, it was found possible to achieve 96 and 99% adsorption and desorption respectively. During the processing of every 5 1 of diluted acid, 9 1 of waste solution analysing 0.3 g/1 Mo and 4 1 of a m m o n i u m molybdate solution analysing 15 g/l Mo were generated. The total ammonia (25%) consumption was 165 ml in comparison to TABLE2 Results of desorptionstudieson activated carbon loadedwith molybdenum;amount of carbon = 20 g containing 2 g molybdenum,volumeof the eluant = 200 ml, contact time---3 h Eluant
pH range
Molybdenumdesorbed, %
NaOH
6.40- 7.00 8.00- 8.50 9.00- 9.50 10.00-10.50 11.05-11.55 12.70-12.8
76.2 84.7 85.2 88.6 90.0 86.4
8.70- 8.93 9.75- 9.37
96.7 97.3
NH~ solution
154 550 ml required for direct neutralisation of the acid. About 85 g of pure MoO3 was recovered from the 4 1 Mo laden solution by precipitation of a m m o n i u m molybdate salt and its calcination. A material balance study indicated an overall molybdenum recovery of 91%. The MoO3 was spectrographically analysed and found to contain < 27 Fe, < 20 Ni, < 100 Cr, < 20 Ca, < 10 Cu, < 100 A1, < 10 Mn, < 20 Mg, < 20 Pb and < 200 Sn in mg/kg. Molybdic oxide of such purity is considered suitable for its hydrogen reduction to powder metallurgical grade molybdenum metal powder. Thus, it can be said t h a t in comparison to the direct neutralistion process with ammonia, the carbon adsorption-desorption technique yielded slightly less molybdenum recovery but about 70% savings in ammonia consumption and excellent product quality could be achieved. CONCLUSIONS An effort was made to modify the existing process of recovery of molybdenum salt from the spent acid by its direct neutralisation with ammonia. Partial substitution of ammonia with cheaper reagents such as CaO, N a O H or Na2CO3 could result in considerable cost saving but the molybdenum salt was found to be highly contaminated with calcium and sodium. An alternative process involving adsorption of molybdenum on activated carbon was found to be suitable. The process is successful if the spent acid is diluted with water before contacting with carbon. The attractive features of the process are a high quality of the end product, excellent molybdenum recovery and over 70% savings in ammonia consumption.
REFERENCES 1 Nair,K.U., Bose, D.K. and Gupta, C.K., 1978. Studies on the processingof molysulphideconcentrate by chlorination. Mining Eng., 30: 291. 2 Bidaye, A.C., Mukherjee, T.K. and Gupta, C.K., 1985. Reclamationof molybdenumpowder from its scrap. In: Proc. 1lth Plansee Seminar on New Applications,Recyclingand Technology of Refractory Metals and Hard Materials held at Reutte, Tirol, Austria during 20-24th May, p. 325. 3 Kulkarni, A.D., 1976. Recoveryof molybdenumfrom spent acid. Met. Trans, 7B: 115. 4 McCarty,L.V., 1985. The recoveryof molybdicoxideand recyclingof acids from molybdenum mandrel dissolving.In: Proc. 1lth Plansee Seminar on New Applications,Recyclingand Technologyof RefractoryMetalsand Hard Materials,held at Reutte,Tirol, Austraia,during20-24th May, p. 45. 5 Sigworth, E.A., 1962. Potentialities of activated carbon in the metallurgicalfield. Am. Inst. Min. Eng. Metal. Soc., Reprint No. 62 B 81.