Kolosnitsyn 2005 - Recovery Of Nickel With Sulfuric Acid Solutionsfrom Spent Catalysts For Steam Conversion Of Methane

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 4, pp. 539!543. + Pleiades Publishing, Inc., 2006. Original Russian Text + V. S. Kolosnitsyn, S. P. Kosternova, O. A. Yapryntseva, A. A. Ivashchenko, S. V. Alekseev, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 4, pp. 551!555.

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INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY

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Recovery of Nickel with Sulfuric Acid Solutions from Spent Catalysts for Steam Conversion of Methane V. S. Kolosnitsyn, S. P. Kosternova, O. A. Yapryntseva, A. A. Ivashchenko, and S. V. Alekseev Ishimbai Branch of Ufa State Technical University of Aviation, Ishimbai, Bashkortostan, Russia Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences, Ufa, Bashkortostan, Russia PALLAD NPF, Salavat, Bashkortostan, Russia Received May 6, 2005; in final form, December 2005

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Abstract Certain fundamental aspects of leaching-out of nickel with sulfuric acid solutions from spent catalysts for steam conversion of methane under static and dynamic conditions were studied. DOI: 10.1134/S1070427206040069

At present, a considerable attention is given to the problem of utilization and processing of solid wastes containing nonferrous metals. Wastes of this kind are toxic substances hazardous to the environment, on the one hand, and a valuable chemical raw material, on the other.

of nickel from raw materials with various acids and solutions of ammonia and ammonium salts. The use of ammonia for leaching-out of nickel relies on the fact that nickel hydroxides readily react with ammonia to give stable complexes of various compositions. This circumstance serves as a basis for recovery of nickel from oxidized ores [3], spent catalysts [4], and worked-out electrodes of nickel3iron batteries [5, 6]:

A widely occurring type of solid wastes from petrochemical and oil refining industries are spent catalysts, including nickel3alumina catalysts for steam conversion of methane. In the course of operation, the catalysts lose activity as a result of changes in their composition and structure. At high temperatures (600 3700oC), the catalyst undergoes sintering and its active surface area decreases.1 In the course of time, a considerable fraction of nickel cations diffuses from the catalyst surface deep inside alumina, active nickel oxide is converted on alumina into inactive nickel aluminate, and spinel-like compounds, inactive in catalysis, are formed [1, 2].

Ni(OH)2 + 6NH3

6 [Ni(NH ) ](OH) . 3 6

2

It was found that addition of ammonium salts to aqueous solutions improves the efficiency of dissolution via the buffer action precluding accumulation of OH! ions: Ni(OH)2 + 4NH3 + 2NH4Cl

6 [Ni(NH ) ]Cl 3 6

2

+ 2H2O.

It has been suggested to leach-out nickel from oxide raw materials with solutions of (NH4)2SO4 and (NH4)2CO3 and with mixtures of these at pH 6 3 8 [7]:

In some cases, deactivated catalysts are regenerated, but this cannot be done for a large number of catalysts. They are removed from the process and reprocessed.

NiO + (NH4)2SO4 + 4NH4OH

Commonly, two methods are suggested for recovery of nickel from ores and raw materials of technological origin: pyrometallurgical technique, whose main product is ferronickel obtained at high temperatures, and hydrometallurgical, based on leaching-out

6 [Ni(NH ) ]SO 3 6

4

+ 5H2O.

Leaching-out of nickel from ores and raw materials of technological origin with solutions of mineral acids was suggested in [8312].

ÄÄÄÄÄÄÄÄÄÄ

However, the leaching methods described in the literature have certain disadvantages. For example, the

1 The temperature of steam conversion of methane is 800oC.

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leaching solutions used are not always harmless: ammonia, which is the most frequently used to leach-out nickel, is a toxic gas. The leaching techniques suggested by certain authors [8, 13] meet technological difficulties. Autoclave leaching requires expensive special apparatus. Use of acid solutions with pH < 1 for leaching-out of nickel complicates the process of selective recovery of nickel from a solid matrix. Nevertheless, the hydrometallurgical method for nickel recovery, compared with the pyrometallurgical technique, is the most promising because it conforms to the principle of combination of environmental and economical interests (markedly diminishes the number of technological procedures, does not require fuel expenditure for high-temperature processes, minimizes the amount of solid wastes and toxic discharges into the atmosphere). One more advantage of the given technique is that it can recover from leaching solutions, at an appropriate choice of methods used, both metallic nickel (electrolytically) and its salts. An analysis of published data demonstrated that, despite the wide variety of methods for leaching-out of nickel from raw materials of technological origin, including spent catalysts, the fundamental aspects of nickel recovery by the hydrometallurgical technique are far from being fully understood. The difficulty encountered in leaching-out of nickel consists in that part of nickel is in the form of compounds that are formed in the course of operation of a catalyst and the composition of these compounds is frequently unknown. Also, a certain part of nickel may be situated deep inside (in pores) catalyst particles, at places inaccessible to the leaching solution. One more difficulty is that, to obtain a product solution enriched with the target component, the leaching solution should be selective with respect to the metal being recovered and should not dissolve the catalyst matrix. The aim of this study was to examine the fundamental aspects of the process of nickel leaching-out with sulfuric acid from spent nickel3alumina catalysts for methane conversion. EXPERIMENTAL In the course of exploitation, a deactivated catalyst is removed from reactors as it loses its activity and is delivered to storage in batches. This gives a mixture of spent catalysts that differ in the content of nickel, degree of coking, porosity, particle shape, and other properties.

As objects of study served spent nickel3alumina catalysts of K-905 and K-87 brands for steam conversion of methane. These catalysts are fabricated by joint precipitation of nickel and aluminum hydroxides, with the subsequent forming and calcination. Therefore, nickel oxide is distributed throughout the catalyst volume. Grains of K-87 catalyst are gray cylinders about 15 mm in diameter and a height of 14 mm, and those of K-905 catalyst are rings with outer and inner diameters of 15 and 10 mm, respectively, and a height of 14 mm. An average sample was originally taken from a batch of mixed catalysts. Then the sample was reduced to 100 3200 g by the ring-and-cone method [10]. It was noticed that the catalysts differ in outward appearance. It was assumed that the degree of nickel leaching-out from these catalysts will also vary. To elucidate this question, part of catalyst samples were classified into portions: portion 1, dark gray particles of K-905 catalyst; portion 2, gray particles of K-905 catalyst; portion 3, black particles of K-87 particles; and portion 4, gray particles of K-87 catalyst. The content of nickel in a catalyst was determined by X-ray fluorescence analysis and by complexometric titration. The X-ray fluorescence analysis was made as follows. A 50-g catalyst sample was calcined in a muffle furnace at 600oC for 2 h and then ground in a ball mill to a dustlike state. The resulting powder was compacted into pellets 10 mm in diameter and 5 mm high. The catalyst samples were analyzed on a Spectroscan-4 automated X-ray spectrometer (SpektronOPTEL, St. Petersburg, Russia). The error in nickel determination did not exceed 2.5%. To determine the content of nickel by complexometric titration, a 150-g catalyst sample was crushed in an agate mortar to a particle size of 2310 mm, placed in a 500-ml flask, poured over with 300 ml of a 20% sulfuric acid solution, and the mixture was boiled for 20 324 h. Preliminary studies demonstrated that nickel is completely recovered from the catalyst under these conditions. The thus obtained leaching solution was titrated with Na2EDTA at pH 9, maintained with an ammonia buffer mixture. Murexide served as indicator. The content of nickel in the catalyst sample was 7.05% according to the results of complexometric ti-

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tration, and 7.04% as indicated by X-ray fluorescence analysis. Nickel was leached-out from the catalyst under static and dynamic conditions. Nickel was leached-out under static conditions as follows. The reactor having the form of a cylindrical glass vessel (height 15325 cm, diameter 338 cm, volume 200 31000 ml) with a discharge cock was charged with a weighed portion of a mixture of spent catalysts. Then a prescribed volume of a sulfuric acid solution was poured into the reactor. Every hour, the acid solution was discharged from the reactor, a sample for analysis was taken, and the solution was again poured into the reactor. The experiment was performed until a constant nickel concentration was reached in the leaching solution. In the case of leaching under dynamic conditions, a weighed portion of a catalyst was placed in a conical vessel, poured over with a prescribed volume of a sulfuric acid solution, and agitated with a mechanical stirrer at a rate of 40 350 rpm. Samples for analysis were taken from the leaching solution at regular intervals of time. As a pH value of 2 was reached in the leaching solution, concentrated sulfuric acid was added to pH 0.7. The experiment was terminated as a constant nickel concentration was reached in the leaching solution at pH 2. The sulfuric acid solutions were prepared by dilution of concentrated (91392%) sulfuric acid. The concentration of sulfuric acid in the solutions prepared was determined by acid3base titration. The influence exerted by the concentration of sulfuric acid on the rate and extent of nickel leaching-out from the catalyst under static conditions was studied. Irrespective of the acid concentration, complete recovery of nickel from the catalyst could not be achieved. Therefore, leaching was performed in several stages in order to obtain the maximum possible degree of nickel recovery. For this purpose, the leaching solution was discharged after a steady state was attained. After that the catalyst was poured over with the same volume of a fresh sulfuric acid and leaching was continued until a new steady state was attained. The results obtained are listed in Table 1. The studies demonstrated that equilibrium is attained in the second and third stages of leaching at a lower nickel concentration in solution: about 0.13 M in the second stage, and 0.07 M in the third. In all cases, the concentration of sulfuric acid has no effect on the degree of nickel recovery (nickel concentration in leaching solutions). These facts indicate that the atRUSSIAN JOURNAL OF APPLIED CHEMISTRY

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Table 1. Ni recovery from catalysts with sulfuric acid solutions of various concentrations under static conditions (weight of catalyst sample 50 g)

ÄÄÄÄÄÄÂÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ Acid ³ Acid ³Leach- ³ Ni concentration in ³ Degree concen- ³ vol- ³ ing ³the leaching solution ³ of retration, ³ ume, ³ time, ÃÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ´ ³ ³ covery, M ³ % M ³ ml ³ h ³ g l!1 ³ ÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄ First stage 0.5 1.0 2.4 3.7

³ ³ ³ ³

75 75 60 67

³ ³ ³ ³

59 59 59 59

³ ³ ³ ³

21.2 23.1 29.5 23.6

³ ³ ³ ³

0.36 0.39 0.50 0.40

³ ³ ³ ³

55 61 58 56

0.12 0.13 0.14 0.13

³ ³ ³ ³

70* 77* 72* 69*

Second stage 0.5 1.0 2.4 3.7

³ ³ ³ ³

75 75 60 67

³ ³ ³ ³

43 43 43 43

³ ³ ³ ³

7.1 7.7 8.3 7.4

³ ³ ³ ³

Third stage

³ 75 ³ 6.5 ³ 3.0 ³ 0.05 ³ 76** ³ 75 ³ 6.5 ³ 3.0 ³ 0.05 ³ 83** ³ 60 ³ 6.5 ³ 5.3 ³ 0.09 ³ 81** ³ 67 ³ 6.5 ³ 5.3 ³ 0.09 ³ 79** ÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄ 0.5 1.0 2.4 3.7

* Total degree of recovery in the first and second stages. ** Total degree of recovery in the first, second, and third stages.

tainment of equilibrium by the leaching process is not related to formation of nickel-saturated leaching solutions in the reactor volume. The solubility of nickel sulfate in sulfuric acid solutions was evaluated. It was found that the solubility of nickel sulfate at 25oC is 59 g l!1 in a 30% sulfuric acid solution and 103 g l!1 in a 5% solution. It was assumed that the equilibrium position is determined in this process mode by saturation with nickel of leaching solutions that fill pores in catalyst particles and by blocking of these pores by nickel sulfate precipitated from saturated solutions. To confirm the blocking of pores in catalyst particles by nickel sulfate, an attempt was made to perform a microscopic study of the cross-section of catalyst samples before and after leaching. However, no reliable and unambiguous results confirming that crystalline nickel sulfate is present in the pores of catalyst particles could be obtained. Therefore, to verify the assumption made above and to examine the possibility of preventing the blocking of the pores in the No. 4

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KOLOSNITSYN et al.

Table 2. Ni recovery from catalysts with a 1% sulfuric acid solution under static conditions, with the leaching solution strengthened at regular intervals of time (initial H2SO4 concentration in the leaching solution 0.1 M)

ÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ Weighed ³ Acid ³ ³ Leaching ³ Final Ni concentration in the leaching solution ³ pH of the ³ Degree portion of ³ volume, ³ T, C ³ time, ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ ³ leaching ³of recovery, ³ M ³ solution ³ % a catalyst, g ³ ml ³ ³ h ³ g l!1 ÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÅÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄ 250 ³ 500 ³ 25 ³ 384 ³ 23.16 ³ 0.40 ³ 0.75 ³ 82.3 5000 ³ 10 000 ³ 15 ³ 233 ³ 28.47 ³ 0.49 ³ 1.65 ³ 81.3 100 ³ 150 ³ 20 ³ 233 ³ 20.65 ³ 0.36 ³ 1.13 ³ 88.4 ÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ

o

o

Table 3. Ni recovery from catalysts with 1% sulfuric acid solutions under dynamic conditions (total leaching time 240 h, s : l = 1 : 2, temperature 23 C, pH of the leaching solution 1)

ÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÒÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ Cat- ³Ni concentration in the leaching solution ³ Degree º Cat- ³Ni concentration in the leaching solution ³ Degree alyst ³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´³ of recov- º alyst ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´ ³ ³of recov³ M ³ ery, % º sample³ g l!1 ³ M ³ ery, % sample³ g l !1 ÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ×ÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄ 1 ³ 8.1 ³ 0.14 ³ 23 º 3 ³ 12.7 ³ 0.22 ³ 36 2 ³ 10.0 ³ 0.17 ³ 28 º 4 ³ 19.5 ³ 0.34 ³ 55 ÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÐÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄ catalyst by particles of nickel sulfate, leaching under static conditions with a dilute (1%) sulfuric acid solution, strengthened at regular intervals of time, was studied. The leaching solution was strengthened with a 20% sulfuric acid solution after the solution pH reached values of 233. The volume of the 20% sulfuric acid solution added to the leaching solution was chosen so as to restore the initial concentration of sulfuric acid in the leaching solution. The leaching-out of nickel in this mode was also performed until the leaching solution was saturated with nickel ions. The results obtained in these experiments are listed in Table 2. In all cases, leaching solutions with a high concentration of nickel sulfate and greater degree of nickel leachingout from the catalyst were obtained. The results of these experiments confirmed the validity of the assumption that the catalyst pores are blocked by nickel sulfate when concentrated sulfuric acid solutions are used as a leaching agent. A study of the leaching process under static conditions demonstrated that its rate is low. The possible reason may be the appearance of an outer-diffusion control over the leaching process. Therefore, leaching was studied under dynamic conditions, because the outer-diffusion limitations are lifted in this mode. Nickel was leached from four catalyst samples differing in the outward appearance with a 1% sulfuric acid solution at 20 325oC, with permanent

strengthening of the leaching solution. The results of these experiments are listed in Table 3. It can be seen that the degree of nickel recovery does rather strongly depend on the type of a sample, but does not exceed 55% at the very best. As demonstrated by the results of the experiments, the concentration of the sulfuric acid solution has no effect on the extent of nickel leaching-out from catalyst particles. In leaching-out of nickel with both concentrated and dilute sulfuric acid solutions, the degree of nickel recovery is virtually the same (553 60%). The experimental study also revealed that, in leaching-out of nickel with a dilute (1%) sulfuric acid solution under dynamic conditions, the degree of nickel recovery from the catalysts is lower than that in leaching with the same solution under static conditions, with the leaching solution periodically discharged from the reactor and the same solution poured in. It follows from the experimental results that the leaching-out of nickel from the catalyst is limited by its diffusion from catalyst pores into the bulk of the leaching solution. In those cases when the rate of nickel diffusion from the catalyst pores is lower than the nickel dissolution rate, nickel sulfate is deposited in the catalyst pores, pore mouths are completely blocked by solid products, and leaching terminates. Agitation of the leaching solution (dynamic mode) does not lift the inner-diffusion limitations.

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The leaching rate can be markedly raised if the catalyst pores are periodically freed of the leaching solution. CONCLUSIONS

5.

(1) Nickel recovery from spent alumina3nickel catalysts for methane conversion mainly occurs in the inner-diffusion region and is controlled by the rate of nickel diffusion from catalyst pores into the bulk of the leaching solution.

6.

(2) The maximum possible degree of nickel recovery from catalyst particles (85390%) and the minimum expenditure of the reagents are achieved in leaching with a dilute (1%) sulfuric acid solution, with permanent strengthening of this solution in the course of leaching and catalyst pores periodically freed of the leaching solution.

7. 8. 9.

10.

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