Original Article

, Volume: 15( 3)

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Upgrading of Metallurgical Grade Silicon (MG-Si) Using Wet Milling Process as a Pretreatment Step before Acid Leaching

*Correspondence:

Ali HHM, Central Metallurgical Research and Development Institute (CMRDI), Cairo, Egypt, Tel: +963-11-5123819; E-mail: hebahma@yahoo.com

Abstract

Locally produced metallurgical grade silicon (MG-Si ~ 97%) was upgraded by using wet-milling process. This is including milling of MG-Si in presence of 2 M HCl solution for different time periods up to 60 min. The effect of milling time in HCl solution was studied. Soaking of the milled products (at optimum milling time 15 min) with continuous stirring in the same solution of 2 M HCl for interval time up to 240 min also were investigated. Finally, the effect of addition of 2 M HF to the milled product was discussed. The leached samples were investigated by X-ray fluorescence (XRF) and scanning electron microscope (SEM). It was found that, 99.92% purity of MG-Si was achieved after 15 min milling time with 2 M HCl followed by addition 2 M HF for 240 min with continuous stirring. About 97% removal efficiency of the impurities was obtained.

Keywords

MG-Si; Upgrading of Mg-silicon; Purification of silicon; Acid leaching

Introduction

In recent years, solar grade silicon (SOG-Si), purified from metallurgical grade silicon (MG-Si) as the feedstock of the PV industry, has been in great demand, with a rapid development of more than 30% annual growth rate [1-13]. The dominant method for purification of silicon is Siemens process. But it is a highly complicated and considered as of high cost process. The development of a cheap process for the production of solar grade silicon (SOG-Si) of 99.9999% purity is a challenging task and the subject of recent worldwide research. These developed methods involving separation of impurities from silicon by acid leaching, slag refining, vacuum distillation, directional solidification [1-4]. Among the proposed alternative purification methods is the acid leaching of pulverized MG-Si. The principle of the acid leaching process is that most of the metallic elements present as impurities in MG-Si have a high segregation coefficient in silicon. Thus, in spite of the high solubilities of the impurities in the molten silicon, they have small solubilities in the solid and remain concentrated at the grain boundaries. Upon grinding the MG-Si, fracture occurs mainly at grain boundaries exposing the impurities to the action of the acids. The acid leaching enables to purify silicon, with the advantages of simple equipment, low cost, low energy consumption and dealing with a large quantity. The past studies are almost focused on the effect of impurity removal of a variety of specific methods and the optimum process parameters. Due to use different materials, the results of different studies are quite different. Recently, Sahu and Asselin [7] investigated the effects of using two different oxidizing agents, such as ferric chloride and ammonium persulfate, on the purification of MG-Si by leaching with hydrochloric acid. The addition of an oxidizing agent improved the extraction of impurities from the MG-Si. Also, the pretreatment of MG-Si to remove certain metallic impurities has a significant effect on the refining process. The purification of MG-Si can be achieved, also by calcination and quenching before leaching, in addition to complexation with glycerin as a ligand [10]. The aim of this paper is utilization of MG-Si produced by reduction smelting technique in EAF at Central Metallurgical Research & Development Institute (CMRDI) to obtain upgrade the purity value of MG-Si. This is applied through studying the factors affecting the wet milling in presence of hydrochloric acid solution and the effect of hydrofluoric acid addition.

Experimental

The chemical analysis of the starting MG-silicon ( ~ 97%) is shown in TABLE 1, the main impurities are Fe, Ca, Al, Ti and Cu with minor amounts of P, Na and S, also are present. The silicon lumps were crushed by a jaw crusher and subjected to sieve analysis. The milling process was carried out using planetary ball mill (P400 Germany) and ball powder ratio (BPR) of 10:1. The leaching process was initiated during the milling process by milling -0.5 mm+2.5 mm fraction with 100 ml of 2 M HCl solution which called wet-milling process. Also, all experiments were carried out with liquid-solid weight ratio of 10: 1. After each experiment, the sample was filtered and washed with de-ionized water before drying. The metallic impurities were chemically analyzed by XRF. The flow sheet of the wet milling process of MG-Si is shown in FIG. 1.

TABLE 1. Chemical analysis of MG-Si.

The removal efficiency of impurities (RM) can be calculated according to the following equation:

Where,

M Raw Si=Content of total impurities in raw MG-Si

M Purified Si=Content of total impurities in purified Si.

Results and Discussion

Effect of milling time

To study the effect of milling periods, about 10 g of MG-Si sample was subjected to milling with 100 ml of 2 M HCl solution in planetary vertical ball mill for different periods 15 min, 30 min and 60 min. FIG. 2 shows the effect of milling period with 2 M HCl solution on the concentrations of the impurities which present in MG-Si as Al, Fe and Ca. It is noticed that, the concentrations of the metallic impurities decreased up to milling time 15 min and no significant result was obtained after this period (FIG. 3). It is also observed that the removal of impurities was better for calcium, aluminum and worst for iron. This result may be due to, by increasing the milling time, the particle size of MG-Si was reduced to less than 45 μm after milling time 60 min, where some impurities tend to segregate to grain boundaries or to interstitial positions. These impurities are very friable so can be removed more effectively. Moreover, some impurity phases (as Fe) adsorbed at the silicon surface which became more difficult to be removed from this finer fraction by further washing during the leaching process.

Effect of soaking time in HCl solution after milling step

The MG-Si samples which milled in 2 M HCl solution for 15 min (wet milling step) were soaked in the same solution for different periods up to 240 min with continues stirring at room temperature. FIG. 4 shows the effect of soaking time in 2 M HCl for different periods on the metallic impurities concentrations as Al, Fe and Ca which present in the milled MG-Si. It was observed that the impurities concentrations decreased by increasing the soaking period in HCl solution. Also, the removal efficiency for Fe, Al and Ca was increased.

The SEM analysis of the milled MG-Si after soaking in 2 M HCl for 240 min was shown in FIG. 5. From SEM analysis there are many grooves, scrapes and agglomeration of the impurities at the surface of silicon after milling and soaking processes. These agglomerations of impurities in separated forms caused during the milling process. From XRF analysis, the purity of obtained silicon was increased from 98.3% to 99.0% after soaking in the same solution for 240 min. it was concluded that the milling assisted leaching process with further soaking in the same solution which is better for calcium and aluminum than iron. This phenomenon may be due to less solubility intermetallic of iron phases. It was mentioned that aluminum-containing intermetallic compounds often contains calcium, so consistence of the removing trend between aluminum and calcium is better than between aluminum and iron. Therefore the removal of impurities by HCl was better for aluminum, calcium and worst for iron.

Effect of HF addition on the milled MG-Si

The wet milled samples by 2 M HCl at optimum milling time (15 min) were subjected to leaching process with 2 M HF solution. HF was added directly to the samples after milling step. The effect of HF addition directly to the milled samples for different periods 60 min, 120 min and 240 min at room temperature was investigated. FIG. 6 shows the impurities concentration of Al, Fe and Ca in MG-Si samples after milling step followed by adding HF for different periods up to 240 min. It can be seen a sharp decrease in the impurities concentration after adding HF to the solution, also the removal efficiency increased to about 97% for iron, aluminum and calcium respectively at room temperature. It is worthy to notice that, 99.92% purity of the silicon was achieved after addition of 2 M HF and this is in agreement with many authors. The addition of HF directly to the sample after milling accelerates the removal of the metallic impurities of MG-Si than using HCl only.

FIG. 7 illustrates the morphologies of the MG-Si after milling with 2 M HCL for 15 min then adding 2 M HF for 240 min. It was clear that the surface of the MG-Si appears smooth and clean and there are many grooves and scrapes on the surface of the silicon so the crystal boundary not obvious. This change may be attributed to the milling process which enhances the removal of metallic impurities by acid leaching process.

Conclusions

1. The wet milling with 2 M HCl is a pretreatment step and a novel method for upgrading of MG-Si and used to increase the efficiency of the leaching process.

2. This method proved that the optimum liberation size of silicon is ranged from +90 μm to +45 μm.

3. This study concluded that the wet milling with acid improves the leaching and upgrading process of MG-Si. This is because the wet milling causes groves, scraps and micro inclusions. Also the phases of unstable impurities are dissociated and exposed at the surface of silicon particle which enhance its removal by acid leaching.

4. The addition of HF improves the effect of HCl on the removal of the impurities.

5. The addition of hydrofluoric acid (2 M HF) to hydrochloric acid (2 M HCl) resulted in refining of MG-Si (99.92%) at room temperature for 240 min after milled with 2 M HCl solution for 15 min as a pretreatment step for the leaching process. The removal efficiency of impurities also enhanced to about 97%.

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