염산 용액 중에서 ZnS를 환원제로 한 망간단괴의 침출특성

구성은, 김동수, 최경수*

이화여자대학교 환경공학과, 대한광업진흥공사*

Leaching Behavior of Manganese Nodule with Zinc Sulfide for a Reducing Agent Using Hydrochloric Acid as a Lixiviant

Sung-Eun Kuh, Dong-Su Kim, Kyung-Soo Choi*

Dept. of Environmental Engineering, Ewha Womans University, Seoul 120-750, Korea

Korea Mining Promotion Corporation, Seoul 156-703, Korea*

Introduction

Among various hydrometallurgical treatment methods developed for the extraction of variable metals from deep-sea nodules, dilute acid leaching is considered to be a desirable method, since conventional concentrated acid leaching processes have many drawbacks such as waste acid solution treatment, equipment corrosion and large acid consumption etc. In dilute acid leaching process, it often needs some reductants to destroy mineral structure of nodules and dissolve the metal values. In recent years, attempts have been made to leach manganese nodules in dilute HCl and H2SO4 with reductants, such as NaSO3, carbon, pyrite and copper matte.

The authors have been attempting to take nonferrous metal sulfides as reductants to leach the nodule in dilute acid solutions. It has been proved the copper matte is an efficient reductant for the leaching of the nodule in dilute HCl solution. Jena et al. (1993, 1988, and 1989) also reported pyrite was a very effective reductant for the extraction of Mn and Co from Indian Ocean manganese nodules. Chen et al. (1992) developed a process for extraction of Mn, Co, and Cu using the nickel matte as reductants. These processes are proved to be very suitable for extraction of manganese nodule. This work reports the reduction leaching behavior of the Pacific Ocean nodule with zinc sulfide as a reductant in dilute HCl solution.

Experimental

The manganese nodule sample was obtained from the Pacific Ocean at 12 03′north latitude and 139 26′west longtitude with depth of 4,980m. The zinc sulfide was obtained from China Tianshui smelter. Before leaching experiments, manganese nodule and zinc sulfide were crushed and ground in jaw crusher and porcelain mortar and sized to obtain -0.104mm and -0.173mm, respectively. The analysis grade of HCl was used in leach experiment. Distilled water was also used in all of experiments.

The leaching experiments were carried out in a three-necked round bottom glass flask with a condenser. The agitator and thermostat were attached in this reactor. The reactor contains 300ml leachant which was heated and stirred. Requisite powered nodule, zinc sulfide and volume of acid solution were introduced into the flask. The temperature of solution was controlled to ±1℃. The 20ml of leached solution samples were withdrawn after different periods of leaching, and the contents were filtered, washed with distilled water. The elemental analysis was carried out using the atomic absorption spectrophotometer(Varoan model 1475). The effects of time, acid concentration, temperature and liquid/solid ratio on the extraction of various metals have been studied.

Results and Discussion

In the HCl solution, the reactions between MnO2 and zinc sulfide(ZnS) are as follows:

MnO2 + 2Fe2+ + 4H+ = 2Fe3+ + Mn2+ + 2H2O                   (1)

ZnS + 2Fe3+ = Zn2+ + 2Fe2+ + S0                      (2)

1. Effect of leaching time

Considering the effect of the leaching time of manganese nodules, the  extraction of Cu, Co, Mn and Ni reach their equilibrium in about 90minutes. More than 99% of Mn can be leached in 2M HCl at 70℃ for 120 minutes. The extraction of Ni, Co and Mn are observed to be above 70% after leaching time 30minutes.

2. Effect of the amount of ZnS

Fig. 1 shows the effect of the ratio of manganese nodule and zinc sulfide on the extraction of metal values in 2M HCl at 70℃ for 90 minutes. The term , N/R, means the weight(g) of manganese nodules per the weight(g) of ZnS. Extraction of Cu, Ni, Co and Mn increased slightly with the decrease in the added amount of the zinc sulfide at the N/R ratios 2 and 3, whereas the extracted amount decrease beyond this N/R ratio. As a large amount of zinc sulfide was added, i.e. when N/R ratio is 2, the extraction of Cu, Ni, Co and Mn were slightly decreased. The decrease in the extraction of metal values is probably due to the precipitation of metal oxide form, Me(OH)2 as the pH is increased on consumption of H+ ion in the leaching process. The other reason may be the formation of the elemental sulfur film(S0) on the surface of the unreacted zinc sulfide. A relatively large amount of ZnS, N/R=3, results in the extraction of Cu, Ni, Co and Mn to be 73.2%, 83.9%, 82.6% and 90.2%, respectively. The optimal N/R ratio for extraction of manganese nodules is around 3 except the case of Fe. The extraction amounts of Fe increased from 20.1% to 56.3% with the N/R ratio increase from 2 to 5. Therefore, it is important to control the amount of zinc sulfide to get the high extraction of the metal values.

3. Initial HCl concentration

Fig. 2 shows the effect of HCl concentration at 70℃ after 90minutes of leaching. The results indicate that the metal values in the nodule are easily leached and affected by the initial acid concentration. A relatively concentrated HCl, 3M HCl, results in the extraction of Cu, Ni, Co and Mn to be 78.3%, 93.6%, 94.2% and 96.3%, respectively. From this figure, it can be inferred that extraction of Ni, Co and Mn are highly dependent, and extraction of Cu and Fe are moderately dependent upon acid concentration. This suggests that strong acidity seems to impede the formation of Me(OH)2 and elemental sulfur film.

4. Temperature effect

Fig. 3 shows the effect of temperature on the extraction of metal values. The extraction of the Mn, Co, Ni and Cu increase with the increase of temperature, whereas the Fe extraction shows a different behavior in comparison with that of other metals. The extraction of Mn, Co and Ni is not so sensitive in the experimental temperature range. It can be observed from this figure that even at 30℃ considerable amounts of metal values are extracted. At 80℃, the extracted amount of Mn, Co and Ni reach 90∼98%. The extraction of Cu is around 78% at this temperature. The iron content of leach liquor changes little with the temperature below at 50℃, but decrease with the increase in temperature higher than 50℃, due to the precipitation of ferric ion. Therefore, temperature around 70℃ is considered to be suitable for acid leaching.

The manganese nodule leaching system may be represented by shrinking core model (Levenspiel et al., 1972). All extracted metal values, Mn, Co, Ni, Cu and Fe, would be fitted by the following equations.

1 - (1-□)1/3  =  k1t                                (3)

1 - 2/3□ - (1-□)2/3  =  k2t                                (4)

1 - (1-□)1/3 + k3{1 - 2/3□ - (1-□)2/3}  =  k4t                   (5)

In these equations, □ means the fraction of metal dissolved at leaching time t and k1, k2, k3 and k4 are the rate constants, respectively. Equation (3) explains that, if a certain reaction can be described by this equation, that reaction has a characteristic of surface chemical reaction.  Equation (4) explains pore diffusion process. There are some processes which have surface chemical reaction and pore diffusion process as the rate determining step, and they are represented by equation (5), which is combined form of equation (3) and (4).

The plots of 1 - (1-□)1/3/t versus 1 - 2/3□ - (1-□)2/3/t are described by a series of straight lines in the temperatures from 30℃ to 80℃. Therefore, the leaching reaction of manganese nodule is probably controlled by both the surface chemical reaction and the pore diffusion process in the each case of Mn, Co, Ni, and Cu(The plots not shown here).

5. Liquid/Solid ratio

Fig. 4 shows the effect of liquid/solid ratio on the extraction of metal values. The ratio has a considerable effect on the extraction. The extraction of Mn, Co, Ni and Cu increase sharply as the ratio changes from 3 to 6. The Cu extraction changes more rapidly with an increase in the L/S ratio in comparison with that of Mn, Ni and Co. In the Fe extraction, L/S ratio has little effect from 3 to 5 range, whereas the Fe extraction sharply increase beyond this range. At the L/S ratio 5, the extraction of Mn, Co, Ni and Cu are 77.8%, 91.3%, 89.3% and 53.6%, respectively, whereas only 0.15% Fe leached. Considering this different characteristics, selective leaching can be attained from this experimental result.

References

1. Choi, K. S., Han, O. H., Shin, B. S., "Reduction Leaching of Manganese Nodules with Copper Matte," Shigen-to-Sozai, 112, 113 (1996).

2. Jena, R. K., Singh, D. D. N., Roy, S. K., sircar, S. C., "Electrochlorination of Sea Nodules to Recover Copper, Nickel and Cobalt," Hydrometallurgy. 32, 21 (1993).

3. Chen Hsiaohong, Fu, Chongyue, Zheng, Diji, □Reduction Leaching of Manganese Nodules by Nickel Matte in Hydrochloric Acid Solution," Hydrometallurgy, 28, 269 (1992).

4. Saha, A. K., Khan, Z. H., Akerkar, D. D., □Studies on Lignite Reduction of          Manganese Sea Nodules for Extraction of Nickel, Copper and Cobalt. Trans.         Indian Inst. Met., 44, 137 (1991).

5. Levenspiel, O., "Chemical Reaction Engineering," John Wiley & Sons, Inc.,  New York, 366 (1972).