Process for the production of commercial grade silicon

Abstract

A process for preparing a highly pure silicon by reduction of a calcium silicate slag using a source of aluminum is disclosed. The process involves forming a molten calcium silicate slag, reducing the calcium silicate slag to Si metal and forming a calcium aluminate slag, and separating the Si metal from the calcium aluminate slag.

Claims

1. A process for a preparation of a silicon (Si) metal, the process comprising: (I) combining a silicon dioxide and a calcium oxide (CaO) in a vessel at a temperature of 1500-2000° C. to form a molten calcium silicate slag; transferring the molten calcium silicate slag to a first furnace in a series of reduction furnaces; (III) introducing an aluminum (Al) metal to a last furnace in the series of reduction furnaces, wherein in the series of reduction furnaces, the molten calcium silicate slag is reduced to the Si metal and forms a calcium aluminate slag, wherein the molten calcium silicate slag moves from the first furnace to the last furnace in the series of reduction furnaces, and wherein the Si metal moves from the last furnace to the first furnace in the series of reduction furnaces; and (IV) separating the Si metal from the calcium silicate slag in the first furnace.

2. The process as claimed in claim 1, wherein the Si metal recovered is solar grade silicon, high purity silicon particles, or silicon-metal.

3. The process as claimed in claim 1 in which the reduction in the step (III) is performed at a temperature of from 1500 to 1800° C.

4. The process as claimed in claim 1 in which the Al metal used in the reduction in the step (III) has a purity of 99.99% or more.

5. The process as claimed in claim 1 in which a content of boron (B) in the silicon dioxide and the CaO in the step (I) is less than 1.0 ppm.

6. The process as claimed in claim 1 in which a content of phosphorous (P) in the silicon dioxide and the CaO in the step (I) is less than 1.0 ppm.

7. The process as claimed in claim 1 in which the temperature within the vessel is 1500 to 1800° C.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a conventional SAF process.

(2) FIG. 2 is a summary of current technology for making solar grade silicon.

(3) FIG. 3 is a flow diagram of a highly preferred process for making solar grade Si.

(4) FIG. 4 shows a more simple process for making solar grade Si.

(5) FIGS. 5 and 6 show more simple processes for achieving different purity of Si.

(6) FIG. 7 shows an arrangement for a counter current reduction-refining step.

(7) FIG. 8 shows a graphite crucible containing Si as prepared in the examples.

(8) FIG. 9 shows a more complex multiple furnace reduction-refining process.

(9) FIG. 10 shows a process for obtaining Al from alumina to enable Al recycling.

(10) FIG. 11 is a general overview of the most preferred process of the invention for making solar grade Si.

(11) FIG. 12 is a general overview of another preferred process of the invention for making solar grade Si.

EXAMPLES

(12) Materials

(13) Pure oxides of CaO (99%), SiO.sub.2 (99.7%) were used to make a calcium-silicate slag. Pure aluminum 99.99% was used as the reductant material. Graphite crucibles with cylindrical shape were used for smelting of materials and reduction-refining.
Aluminothermic Reduction of Slag

(14) A mixture of CaO+SiO.sub.2 powders with (molar ratio of CaO/SiO.sub.2=1) was prepared (150 g mixture). The mixture was then heated up and smelted in the graphite crucible. The temperature in the crucible was continuously measured by a thermocouple. The smelting was performed using induction furnace in a closed chamber under controlled continuous argon (+99.999%) gas flow.

(15) The mixture became molten at a temperature between 1600° C. and 1650° C., and then the temperature of the molten slag became stabilized to around 1600° C.

(16) Aluminum metal was added to the liquid slag for the reduction of silicon oxide. The amount of Al added was in stoichiometric ratio to reduce all SiO.sub.2 of the slag. The reaction started immediately through the contact of Al with slag, chemical reaction (3).

(17) The temperature of the melt increased rapidly to higher temperatures up to 1760° C., and then it dropped again to lower temperatures. The crucible containing metal and slag phases was cooled down after 30 minutes holding at elevated temperatures from the time Al was added. The solidified metal and slag phases were separated and their chemical compositions were determined using ICP-MS.

(18) The measured chemical compositions of the two phases are: Metal: 80% Si, 13% Ca, 7% Al Slag: 42.5% CaO, 46.5% Al.sub.2O.sub.3, 11% SiO.sub.2

(19) In this bench scale proof of concept experiment, we obtain a calcium-aluminate slag which is relatively low in SiO.sub.2, and hence the majority of SiO.sub.2 has been reduced to Si metal so that the metal phase became silicon containing Ca and Al elements.

Example 2

(20) Example 2 shows the benefit of the counter current reduction process. The above produced metal was combined with a molten CaO—SiO.sub.2 slag (CaO/SiO.sub.2=0.67). The slag/metal mass ratio was 2/1. Following the protocol of example 1, the process was heated and stabilised at 1600° C. in around 30 minutes. The chemical analysis of the slag and metal after the test indicated the production of highly pure silicon and a calcium-silicate slag containing small amount of Al.sub.2O.sub.3: Metal: 99.4% Si, 0.4% Ca, 0.2% Al Slag: 47% CaO, 47% SiO.sub.2, 6% Al.sub.2O.sub.3

(21) As can be seen, when the impure Si metal contacts the calcium silicate slag, purity is increased. This is exactly the process that occurs using a counter current reduction step as herein defined.

Example 3

(22) A mixture of CaO+SiO.sub.2 powders with molar ratio of CaO/SiO.sub.2=0.67 was prepared (4 kg mixture). The mixture was heated up and smelted in a graphite crucible, while the temperature in crucible was continuously measured by a thermocouple. The smelting was performed using induction furnace in a closed chamber under controlled continuous argon (+99.999%) gas flow.

(23) The mixture became molten at temperatures between 1600° C. and 1650° C., and then the temperature of the molten slag became stabilized to around 1600° C.

(24) Aluminum metal was added to the liquid slag for the reduction of silicon oxide of the slag. The amount of Al was 90% of the stoichiometric ratio to reduce the majority of SiO.sub.2 of the slag. The reaction started immediately through the contact of Al with slag through reaction (3).

(25) The temperature of the melt increased rapidly to higher temperatures (1700° C.-1800° C.) during the Al addition and for a period after Al addition, before dropping to lower temperatures. The crucible containing metal and slag phases was cooled down after 30 minutes holding at elevated temperatures from the time Al addition was completed. The solidified metal and slag phases were separated and their chemical compositions were determined using ICP-MS.

(26) The chemical compositions of the two phases are presented as follows: Metal: 88.2% Si, 7.5% Ca, 4.1% Al Slag: 39.1% CaO, 56.0% Al.sub.2O.sub.3, 4.9% SiO.sub.2

(27) As we see above, we obtain a calcium-aluminate slag which is relatively low in SiO.sub.2, and the majority of SiO.sub.2 has been reduced to Si metal so that the metal phase is silicon containing Ca and Al elements. Compared to Example 1, there is less Ca and Al in the metal phase due to the use of less Al than the stoichiometric amount for complete SiO.sub.2 reduction, and also using a different slag composition with higher SiO.sub.2 concentration.

Example 4

(28) Example 4 shows the benefit of the counter current reduction process. The above produced metal (around 1 kg) was combined with a molten CaO—SiO.sub.2 slag (CaO/SiO.sub.2=0.67) in the same approach described in example 3, with slag/metal mass ratio as 2/1. The process was again carried out at 1600° C. for 30 minutes, and the melts were cooled down and solidified in the crucible. FIG. 8 shows the solidified slag and silicon in the crucible after the test and breaking the top part of the crucible.

(29) The measured chemical analysis of the metal after the test indicated the production of highly pure silicon as seen in table below.

(30) TABLE-US-00001 Si Ca Al Fe Ti Mg Mn B P (wt %) (wt %) (wt %) (ppmw) (ppmw) (ppmw) (ppmw) (ppmw) (ppmw) 99.2 0.6 0.2 310 51 270 60 0.2 8

(31) The applied materials in the examples were high purity with regard to B concentration. However, there was some P present, in particular in the lime (CaO), and therefore we see some P in the produced silicon. In the industrial integrated solar grade silicon process taught herein, P is removed before the reduction step, and moreover, if the CaO is recycled, there is very small P flow in the whole process.

(32) It would then be possible to maintain P concentration below 0.5 ppm in the process. The metallic impurities Ca, Al, Fe, Ti, Mg, Mn are easily removed in the final process step as they segregate in solidification. It is worth noting that the amounts of Fe, Ti, Mn in an industrial process would be significantly lower due to the removal of these impurities in the slag-making step. Ca and Al content would also be lower due to prior directional solidification.

(33) Calcium-silicate slag containing small amount of Al.sub.2O.sub.3 was produced as its composition presented below. Slag: 39.2% CaO, 57.5% SiO.sub.2, 3.3% Al.sub.2O.sub.3

(34) The metal and chemical compositions show that Al and Ca are adsorbed into the slag phase from the primary Si—Ca—Al alloy produced in experiment 3. As observed above, the production of silicon from CaO—SiO.sub.2 slags by aluminothermic reduction is possible. The whole reduction-refining process can be carried out in a counter current approach in which Al and slag are introduced into furnaces in series as schematically illustrated in FIG. 9 when four furnaces are used in series.

Example 5

(35) Pure oxides of CaO (99%), SiO2 (99.7%) were used to make a calcium-silicate slag.

(36) High purity silicon scrap from the solar industry was used for dephosphorization of slag.

(37) Pure aluminum 99.99% was used as the reductant material.

(38) Graphite crucibles with cylindrical shape were used for smelting of materials and reduction-refining.

(39) Slag Making and its Dephosphorization

(40) A two-step slag making-dephosphorization was performed to obtain a low P-containing calcium silicate slag:

(41) Step 1:

(42) A mixture of CaO+SiO.sub.2 high purity powders with molar ratio of CaO/SiO.sub.2=0.67 was prepared as described in example 2. The mixture was then heated up and smelted in the graphite crucible, while the temperature in crucible was continuously measured by a thermocouple.

(43) The smelting was performed using induction furnace in a closed chamber under controlled continuous argon (+99.999%) gas flow at 1450-1600° C.

(44) The mixture became molten with good fluidity at temperatures between 1550° C. and 1650° C., and then the temperature of the molten slag stabilized to around 1600° C.

(45) High purity silicon scrap from solar silicon crystallization process was added into the slag, which melted rapidly. The silicon:slag mass ratio was 1:5. The scrap contained around 0.2 ppm P.

(46) The melt (molten slag and silicon on top) was held for around 1 hour at 1600° C., and then it was cooled down to the room temperature.

(47) Slag and silicon were completely separated. The concentrations of P in the silicon was measured later as 5.3 ppmw, which shows the removal of P from the slag as that the P content in silicon has been increased from 0.2 ppm to 5.3 ppm.

(48) Step 2

(49) The above dephosphorization process was repeated through fresh Si scrap and its addition to the above partially dephosphorized slag at 1600° C. (molten slag) under Ar flow. The silicon:slag mass ratio was 1:5.

(50) The melt (molten slag and silicon on top) was held for around 1 hour at 1600° C., and then it was cooled for solidification and cooling to the room temperature under Ar flow.

(51) Slag and silicon were completely separated. The concentrations of P in the silicon was measured as 2.1 ppmw, which shows further removal of further P from the slag.

Example 6

(52) Aluminothermic Reduction of the Dephosphorized Slag

(53) A two-step reduction-refining experiment was carried out, in which pure Al metal was introduced into the dephosphorized slag and it reduced the slag components CaO and SiO.sub.2. A Si—Ca—Al alloy was initially produced and a slag.

(54) The described procedure for example 2 was repeated for the aluminothermic reduction of the above dephosphorized slag, where 90% of stoichiometric required Al was used and reaction duration was about 45 min. The chemical compositions of the two phases after the first step were obtained as: Metal alloy: 89.1% Si, 7.1% Ca, 3.8% Al Slag: 39.3% CaO, 56.5% Al2O3, 4.2% SiO2

Example 7

(55) Refining of Silicon Alloy

(56) This Si—Ca—Al alloy was contacted with a new dephosphorized silicate slag and Ca and Al elements in the Si—Al—Ca alloy were redistributed into the slag through reduction of the SiO.sub.2 of the slag. Therefore a high purity silicon was produced.

(57) Alternatively, the produced silicon alloy containing Ca and Al (around 1 kg) was contacted with a molten dephosphorized CaO—SiO.sub.2 slag (CaO/SiO.sub.2=0.67), with slag:metal mass ratio as 5:1. The refining process was again at 1600° C. for one hour duration, and the melts were cooled down and solidified in the crucible.

(58) The measured chemical analysis of the metal after the test indicated the production of highly pure silicon as seen in table below:

(59) TABLE-US-00002 Si Ca Al Fe Ti Mg Mn B P (wt %) (wt %) (wt %) (ppmw) (ppmw) (ppmw) (ppmw) (ppmw) (ppmw) 99.4 0.4 0.2 60 29 180 25 0.2 0.9

(60) Compared to the example 4, a more highly pure slag was consumed in the aluminothermic reduction process through an innovative approach for removing metallic impurities i.e. Fe, Mn and Ti, and most importantly P impurity.

(61) The obtained P concentration and also the other levels of metallic impurities Ca, Al, Fe, Ti, Mg, Mn can be removed in a final process step by their segregation in directional solidification, and the concentrations are therefore acceptable for solar silicon ingot casting. It is worth noting that a calcium-silicate slag containing small amount of Al2O3 was also produced. The overall composition of slag in this step is given below:

(62) Slag: 39.9% CaO, 57.0% SiO2, 3.1% Al2O3

(63) The metal and chemical compositions show that Al and Ca are adsorbed into the slag phase from the primary Si—Ca—Al alloy produced in the main reduction step.

(64) According to this experiment, the production of high purity silicon for solar applications with concentrations of B and P below 1 ppmw is possible through the invented process. In particular the process can be more flexible with regard to the use of raw materials, as effective dephosphorization is possible using silicon scrap from the process (final solidification step) or even the silicon scrap from the solar market. Around 40% of silicon in very high purity is lost in the production of solar cell silicon in the form of lump, particles, and fines. This kind of scrap can be used in the invented process.

(65) In the above experiment, the dephosphorization of slag prior the reduction and refining steps was performed discontinuously. In practice, however, the dephosphorization step can be done through a two vessel counter current process in which one is the slag making furnace and the other is a ladle furnace as illustrated schematically in FIG. 12.