AU616181B2

AU616181B2 - A method and apparatus for continuously producing liquid silicon from carbon and silica

Info

Publication number: AU616181B2
Application number: AU16991/88A
Authority: AU
Inventors: Jerome Feinman
Original assignee: SKF Plasma Technologies AB
Current assignee: SKF Plasma Technologies AB
Priority date: 1987-10-09
Filing date: 1988-06-02
Publication date: 1991-10-24
Anticipated expiration: 2008-06-02
Also published as: SE8703895D0; NO884496L; BR8804291A; FR2621575B1; AU1699188A; SE8703895L; SE461037B; NO884496D0; JPH01115811A; FR2621575A1

Description

4. The basic application referred to in paragraph 2 of this Declaration the first application m ade in a Convention country in respect of the invention the subject of the application.

DECLARED at ,!...Hofors Swede.n this h thi s.2 Sth day f 1.8A3.

SKF PLASMA TECHNOLOG .ES A f Signalurc, v-rw "M I I I I IN 11- 1 1 0 ~ij 1 1

I

r COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 616181 FORM COMPLETE SPECIFICATION (Original) Application Number: Lodged: Complete specification Lodged: Accepted: Published: 'lass: Int. Class 0 *eoo~fl 6 0 0 0 0 S* 0 0 006 0 o0 09 0 O 0 Priority: Related Art: 0: 0o Name of Applicant: 0 6 0 0 fit Address of Applicant: a 90 a0 B o>0 SKF PLASMA TECHNOLOGIES AB P, 0. Box 202, S-,813 00 Hofors, Sweden.

00 0 0 00c 03 0f Actual InVentor/X: Address for Service: Complete Specification JEROME FEINMAN.

E. F. WELLINGTON CO., Patent and Trade Mark AttOrneys', 457 St. Kilda Road, Melbourne, 3004, Vic.

for the invention entitled: "A METHOD AND APPARATUS FOR CONTINUOUSLY PRODUCING LIQUID SILICON FROM CARBON AND SILICA" The following statement is a full description of this invention including the best method of performing it known to meI/us: -1- 4-.

1A The- present invention relates to a method of continuously producing liquid silicon from carbon and silica with the addition of thermal energy. The invention also relates to apparatus for performing the method.

In a conventional electrothermal process for producing silicon from silica, the quantity of silicon recovered is limited to about 80% due to the formation of silicon oxide in gaseous form, SiO which leaves the reaction zone and the furnace before it can be reduced to liquid silicon Si and also because SiC collects as a ;;ate product in the cooler parts of the furnace. The mechanism and theory behind these phenomena are discussed in "Heat and Mass Transfer in the Fer'osilicon Process", by M B Mller, S E Olsen and J K Tuset in The Scandinavian 15 Journal of Metallurgy I (1972) 145 155.

An object of the present invention is thus to improve the silicon recovery percentage, thus reducing the consumption of material and energy, SThi, bject is achieved according to the present invention *too 20 in a method of continuously producing liquid siliron from carbon and silica which comprises: feeding carbon into a first upper reaction zone; introducing silica into a first 4 *4 plasma gas; feeding said first plasma gas containing silica into said first reaction zone containing carbon; maintaining conditions in said first reaction zone, including supplying theriul e'nergy thereto, sufficient to cause said silica to react with said carbon to produce silicon carbide; feeding said silicon carbide downwardly into a second, lower reaction zone; introducing silica into a second plasma gas; feeding said second plasma gas containing silica into said second reaction zone substantially simultaneously to feeding said first plasma gas into said first reaction zone; supplying thermal energy thereto, sufficient to cause siliua to react with silicon carbide; and recovering silicon.

t Itts /SI A N/ r 7 a j 2 In preferred embodiments, the method of the present invention is characterized: in that the quantity of silica fed to said first reaction zone is r.,,,out 2/3 and the quantity of silica fed to said sec' %J reaction zone is about 1/3 of the (whole) total silica utilized in said method; or (ii) in that the temperature during the second step is maintained at about the same temperature as said first step at at least about 1900 0 C; or (iii) by including: initially providing preformed, lump form silicon carbide in said second reaction zone; initially providing preformed, lump form carbon in said first. reaction zone; injecting inert plasma gas 4 into said reaction zones f or a time and at a temperature suf ficient to heat said zones to a level f 0 sufficient to sustain said reactions; and then o to injecting silica into said plasma gases.

The present invention also provides an apparatus for a. performing the said method for continuously producing 6 6 620 liquid silicon from carbon and silica with the addition of heat, comprising a reactor vessel adapted to conta.tn coal.

in lump form, means connected to the reactor vessel for the supply of coal in lump f orm, a pipe connected to the reactor vessel for the cemoval of exhaust gases therefrom, 2$ a f irst plasma generator connected to the upper part of the 3 reactor vessel for introducing plas:ita gas into the upper part of the coal bed in the reactor vessel to iu-,rm a f irst reaction zone in the bed, and 'means for injecting silica with plasma gas fArom the first plasma generator into the first reaction zone, said apparatus further comprising a second plasma generator located at a lower level in the reactor vessel, with means connected to the lower part of the reactor vessel for introducing silica with plasma gas from the second plasma~ generator inito a second reaction zone formed by the second plasma generator at the lower level, in a bed primarily of silicon carbide which has been produced in the first reaction zone, said reactor vessel also having an Qi2-.let in the lower part of the reactor for tapping off the silicon produced.

the present invention prtqvides a method of continuously producing liquid silicon from carbon and silica, wherein in a first step a first quantity of the silica is converted with carbon to form silicon carbide, and wherein in a second, consecutive step the remaining, second quantity of the silica is converted 4 with the sillcon carbide produced in the first step, to form liquid silicon, thecmal energy being supplied during both steps of the process. The first quantity of silica is chosen to be approximately two thirds and the second quantity is thus approximately one third of the whole.

The temperature during the first step is preferably at least 1900 0 C and the temperature du~ring the second step yy> 4 is preferably also at least 1900 0

C.

The method according to the invention thus offers a process wherein the above mentioned losses of SiO g are reduced or eliminated and the proportion of SiC remaining in the product is minimized.

The method according to the invention can be illustrated in the following way.

In the first step a carbothermic reduction of SiO 2 is performed in a first zone containing a surplus of carbon.

SiC is thus produced as follows: SS2 9 t 2 SiO 4C 2 Si 2CO0 or g s totally 2 SiO 2 6C 2 SiC 4C0 In the second step, the SiC produced in the first step is utilized to reduce additional SiO 2 as follows: SiO SiC Si SiO CO and 2 S 1 g SiO SiC 2 Si CO or g s 1 totally Sio 2 2SiC s 3 Si 2CO0 The total reaction for the two combined steps (equation plus equation is 3Si02 6C 3Si 6CO (7) 9.499 O *4 *4t Q 9* 9 *u °044 p49 The method according to the invention is advantageously performed in a shaft furnace kept filled with carbonaceous material in lump form. This reactor is suitably lrovided with supply means for the carbonaceous material in lump form, and a pipe for withdrawing gases produced and introduced into the system. Means are provided at an upper level in the reactor for supplying silica and thermal energy in a first reaction zone. At a second, lower level in the reactor means are provided for the supply of silica and thermal energy for performing the second reaction step. The lower part of the reactor is designed in conventional manner to permit tapping; off the silicon produced. It may advantageously be designed to permit temporary collection of liquid silicon which is then tapped off batchwise from the reactor.

The means for supplying thermal energy may preferably be means for the supply of energy independent of combustion, such as plasma generators, SiO 2 in particle form being injected at said means.

20 The process according to the invention, in accordance with the stoichiometric equations 9ven above will now be described in more detail with rFference to the single drawing showing schematically an apparatus for performing the method according to the invention.

The apparatus shown schematically in the drawing comprises a shaft furnlace 1 provided with at least one plasma g-knerator 2 at an upper level, and at least one plasma generator 3 at a lower level, The furnace is also provided with a conventional sluice 4 in the upper part 9 for the supply of coal in lump form, and a pipe 7 for the removal of gases produced and introduced into the furnace. Injection means 5 aad 6, respectively, for injecting SiO 2 4' 6 in particle form at each plasma generator, are also shown.

Tapping means 8 are also shown at the bottom of the furnace for periodic withdrawal of the product, liquid silicon.

The first step of the process is performed in the upper part of the shaft furnace I. which comprises the plasma generators 2 at the upper level and the furnace volume above this upper level. This furnace volume is filled with coal in lump form which is fed through the upper part 9 of the shaft. Plasma gas, which may be recirculated exhaust gas or some other suitable gas, together with Sio 2 injected in particle form into the plasma gas, enters the carbon bed and reacts in accordance with equations and to form SIC and CO. The temperature in this reaction zone after substantially complete conversion will be at least 1)00"C, preferably higher, and this temperature will decrease to z:bout 1300 0 C at the top of the shaft furnace as the coal supplied in lump form moves down and is, heated to reaction temperature by the rising gases. it is obvious that a considerable part of this pre-heating of the coal will be effected in the upper part of the reactor volume so that Mnost of the volume above the level o14 the upper plasma g3enerator 2 will be at or close to the reaction-zone temperature, This provides favourable conditions for complete reduction of all Sio in accordance with reaction The SIC produced in this first step will move downwardly, together with any unconverted coal, Into tha lower part of the shaft furnace 1, said part containing the plasma generators at the lower level 3 and means for collecting and withdrawing Sil And this lower part is utilized to perform the second step of the process.

Additional SiO2, together with plasma gas from the plasma 7 generators at the lower level, is supplied here in the second step and will react with the downwardly moving SiC and unconverted coal to produce Fi 1 and CO in accordance with reactions and possibly SiOg C> Si1 CO Si 1 produced will be collected at the bottom of the furnace, and is periodically withdrawn by tapping in conventional manner. In a preferred embodiment of the invention, the furnace is started by filling its lower part with SiC in lump form and filling its upper part with carbon in lump form, which may be graphite or coke.

9 °The plasma generators at levels 2 and 3 are operated °o0 by a suitable inert plasma gas to heat the system to reaction temperature. Continuous production of Si can 15 tYhen be started by injecting SiO 2 at the plasma generatcrs 2 and 3 to obtain the reactions described earlier.

o0400: An example of th, process according to the invention, o00:0, using relatively pure reactants, is given in the following table for a production flow of one ton silicon per hour.

The reaction-zone temperatures were maintained at about 1900°C in the example.

o 9 Examples of operation at a production rate of 1 ton Si per hour, per ton SiO 2 Carbon consumption, kg 855 855 Silica, kg 2139 2139 for the first step 1426 1426 fo, the second step 713 713 electricity, kWh 7924 12915 for the first step 4546 7590 for the second step 3378 5325 ,cont'd T, i 8 cont'd Plasma gas, kmol for the first step for t'he second step Off-gas, kmol* CO, mol fraction

N

2 mol fraction Off-gas temperature, °C 49 34 15 125 0.938 0.062 -~1250 58 34 24 134 0.938 0.062 -1310 The plasma gas in these operating examples is recycled off-gas.

*p.

J o I a p t t Ip P *P Pa rt r t r P P f t s a P ae m e

PP

P* 4 a Ip *4*1 ap a p 4 4 .4p N: C

Claims

1. A method of continuously producing liquid silicon from carbon and silica which comprises:t feeding carbon into a first J upper reaction zone; introducing silica into a first plasma gas; feeding said first plasma gas containing silica into said f irst rection zone containing carbon; maintaining conditions in said f irst reaction zone, including supplying thermal energy thereto, sufficient to cause said silica to react with said carbon to produce silicon carbide; feeding said silicon carbide downwardly into a second, lower reaction zone; introducing silica into a second plasma gas; feeding said second plasma gas containing silica into said second reaction zone substantially simultaneously to feeding z'aid first plasma gas into said first reaction zone; supplying therinal energy 4 t. thereto, sufficient to cause silica to react with silicon carbide; and recovering silicon,

2, A method as claimed in claim 1, wherein the quantity of silica fed to said first reaction zone is about 2/3 and the quantity of silica fed to said second reaction zone is about 1/3 of the (whole) total silica utilized in said method,

3. A method as claimed in claim I. or 2 wherein the 0 0 temperature during tho second step is maintained at about the same temperature as said first step at at least about 19000C,

4. A method as claimed in anyone of claims I to 3 includingt Initially providing preformed, lump form silicon carbide in said second reaction zone; initially providing preformed, lump form carbon in said first reaction zone; injecting Inert plasma gas into said reaction zones for a time and at a temperature sufficient to heat said zone~s to a level sufficient to sustain said reactions; and then injecting silica into said plasma gases.

An apparatus for performing the method as claimed in claim 1 for continuously producing liquid silicon from carbon and silica with the addition of heat, comprising a reactor vessel adapted to contain coal in lump form, means connected to the reactor vessel for the supply of coal in lump form, a pipe connected to the reactor vessel for the removal of exhaust gases therefrom, a first plasma generator connected to the upper part of the reactor vessel for introducing plasma gas in to the upper part of the coal bed in the reactor vessel to form a first reaction zone in the bed, and means for injecting silica with plasma gas from the first plasma generator, into the first reaction zone, said apparatus further comprising a second plasma generator located at a lower level in the reactor vessel, with means connected to the lower part of the 4 44 15 reactor vessel for introducing silica with plasma gas from the second plasma generator into a second reaction zone formed by the second plasma generator at the lower level, in a bed primarily of silicon carbide which has been produced in the first reaction zone, said reactor vessel also having an outlet 20 in the lower part of the reactor vessel for tapping off the S•silicon produced,

6, An apparatus as claimed in claim 5, substantially as described herein with reference to the accompanying drawing. V 4

7. Silicon when obtained by the method as claimed in any one of claims 1 to 4, or when obtained by use of the apparatus ts claimed in claim 5 or 6. DATED this 13th day of August, 1991 SKF PLASMA TECHNOLOGIES AB, By its Patont Attorneys, E F, WELLINGTON C. SBy: CE S 1 WE-,LI BBRUCE s WEUWLIC1014