Process for making raw material bodies especially for the production of silicon or silicon alloys

Raw material bodies such as briquettes, for use in the production of silicon or silicon alloys, are formed by mixing a pitch and caking coal at a temperature above 100.degree. C. and up to 200.degree. C. to form a pitch/coal alloy. This hot binder composition is mixed with sand and a noncaking carbon carrier at a temperature in this range to form the starting composition from which preforms are pressed. The preforms are subjected to a heat treatment which involves raising the temperature to above 450.degree. C., preferably in a sand filled rotary furnace to harden the preforms into the bodies.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to my copending application Ser. No. 07/551,517 
filed July 11, 1990 entitled METHOD OF MAKING FUEL BRIQUETTES AND THE 
BRIQUETTES SO MADE. It is also related to my copending application Ser. 
No. 07/220,616 filed July 18, 1988, now U.S. Pat. No. 4,975,226 which, in 
turn, relates to subject matter found in my earlier U.S. Pat. Nos. 
4,389,493, 4,364,974, 4,366,137 and 4,820,341. 
FIELD OF THE INVENTION 
My present invention relates to a process for producing raw-material bodies 
for use in the production of silicon or silicon alloys in a low-shaft 
electric arc furnace, the bodies being generally in the form of briquettes 
or other pressure-shaped elements The invention also relates to the bodies 
made. 
BACKGROUND OF THE INVENTION 
In the production of silicon or silicon alloys in a low-shaft electric arc 
furnace, it is known, as the discussion below will demonstrate, to utilize 
raw material bodies, blanks or elements which are composed of quartz sand, 
a noncaking carbon carrier, preferably petroleum coke, and a 
pitch-containing binder. 
A composition of this type is formed by mixing the quartz sand, the 
noncaking carbon carrier and the pitch-containing binder and subjecting 
the mixture to briquetting thereby forming green briquettes or preforms. 
These briquettes then subjected to a hardening heat treatment to produce 
the raw material bodies with sufficient structural stability that they 
will retain integrity over at least part of the silicon or silicon alloy 
producing process occurring in the low-shaft electric arc furnace. 
In the production of such raw material bodies, quartz sand in the particle 
size or grain range of 0.05 to 0.2 millimeters is usually employed and 
indeed with various grain-size bands within this range. It will be 
understood that the raw material bodies for the production of ferrosilicon 
can also contain finely divided iron or finely divided iron oxide. The 
iron or the iron oxide can also be added directly to the burden of the 
low-shaft electric arc furnace, either in the form of pieces or pellets. 
The burden predominantly consists of the raw material bodies and quartz. 
Depending on the specific use to which the raw materials are to be put, 
they just have certain chemical and physical properties. 
Thus for the production of silicon, in chemical terms the raw material 
bodies must have a composition (see the Great Britain Patent 2 084 122) 
such that, in their passage through the low-shaft electric arc furnace, in 
the upper part, they sustain a reaction according to the equation 
EQU SiO.sub.2 +3c=SiC+2CO 
leading toward the formation of silicon carbide. For that purpose, the 
total carbon content of each individual raw material body must be 
correspondingly selected. In general, however, a superstoichiometric 
operation is preferred (see Great Britain Patent 2,150,128). In physical 
terms the raw material bodies must have sufficient strength and integrity 
that they will not deteriorate during passage at least through the upper 
portion of the low-shaft electric arc furnace and especially in the 
reaction which forms the silicon carbide so that the bodies can reach the 
lower part of the electric arc furnace where they can react further with 
the molten quartz in accordance 
EQU SiO.sub.2 +SiC=3Si+2CO. 
It will be understood that in the low-shaft electrode arc furnace further 
reactions can be carried out. Among these are: 
EQU SiO.sub.2 +C=SiO+CO 
which occurs at high furnace temperatures to generate volatile silicon 
monoxide which can be lost and can have a detrimental effect on the output 
of the process and the heat balance. 
In an earlier process from which the present invention has developed (see 
German Open Application 37 24 541), the raw material bodies free from 
caking coal are formed by a binder briquetting. The hardening heat 
treatment is effected in a heated rotary kiln or rotary tube furnace which 
has its lower portion so filled with quartz sand that the heat treatment 
takes place by immersion of the briquettes in the quartz sand bed. This 
has been found to be highly satisfactory. The hardened raw material bodies 
are able to withstand all of the stresses arising in the process during 
the transiting of the low-shaft electrode arc furnace and hence the 
chemical processes which are carried out are particularly well defined. 
However, the bodies do not have satisfactory long term storage capabilities 
in many respects and as to such long term storage properties, therefore, 
improvements are possible. For example, the mechanical strength of such 
bodies decreases with time to the point that the bodies may deteriorate. 
It has been found that such deterioration may be a result of diffusion of 
air and moisture into the bodies when they are stored in the open and that 
such diffusion may reduce the binding force of the binder which is used. 
This is not the case with raw material bodies for the same purpose which 
may be fabricated by hot briquetting techniques (see German Patent 
Document DE-OS 30 09 808), but the hot briquetting process is relatively 
more expensive. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide an improved process for the 
production of raw material bodies, especially for producing silicon or 
silicon alloys in low-shaft electric arc furnaces, so that these bodies 
not only have all of the physical and chemical requirements necessary to 
satisfy the needs for the process in the travel through the electric arc 
furnaces, but also have improved long term storage properties, especially 
for transport and storage in the open without mechanical deterioration. 
Another object of the invention is to provide an improved raw material body 
which can be stored and transported in the open, while being exposed to 
ambient air and moisture without mechanical deterioration. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the invention by a method in which the pitch 
containing binder is made from a binder mixture of pitch and caking coal 
and which is brought to a temperature in excess of 100.degree. C. and up 
to 200.degree. C. The binder is combined with the quartz sand and the 
noncaking carbon carrier at a mixing temperature which is in the same 
temperature range as that at which the binder was formed and the blanks or 
green briquettes are briquetted at this latter mixing temperature, the 
green briquettes being subjected to a heat treatment such that the 
briquettes at the final portion of this heat treatment at least have a 
terminal temperature in excess of 450.degree. C. 
In the process of the invention, the briquetting which forms the raw 
material bodies or preforms is a binder briquetting of the type common in 
bituminous coal briquetting. Indeed, in spite of the high content preform 
of quartz sand, the known binder briquetting technology can be used. In 
general, one operates with conventional briquetting press and press 
pressures of 1 to 2 t/cm.sup.2. 
Surprisingly, the use of the aforedescribed binder composition and 
maintaining the given parameters, yields raw material bodies which satisfy 
all of the requirements for the low-shaft electric arc process but also 
has a long term storage capability in the sense that the briquettes can be 
stored for long periods in the open air and even in the presence of 
moisture without mechanical degradation. This is especially the case when 
the binder is formed by a pitch/coal "alloy". A pitch-coal alloy is a 
composition in which the pitch and the caking coal are mutually soluble 
and, following the formation of the binder it is practically impossible to 
discern separate pitch and caking coal phases. This is easily achievable 
when a sufficiently fine caking coal is used. The pitch/coal alloy in the 
raw material bodies fabricated according to the invention are surprisingly 
insensitive to air and moisture diffusing into the briquettes. 
Especially effective results are obtained when the quartz sand, the 
noncaking carbon carrier and the binder mixture all have the same 
temperature on mixing and the mixing is carried out with a temperature of 
about 160.degree. C. 
While the process of DE-OS 37 24 541 is carried out in a rotary tube 
furnace for the hardening heat treatment of the bodies, the heat treatment 
according to the invention can be carried out also in other ways. For 
example, the process can be a traveling grate process in which the 
traveling grate passes through a corresponding furnace or a corresponding 
chamber and the briquettes to be treated are provided in a single layer or 
a plurality of layers on the travel grate. 
In a preferred embodiment of the invention, however, the preforms are 
hardened in a rotary kiln or rotary tube furnace. It has been found that 
best results in the heat treatment are obtained and the preforms are 
hardened to very high final compressive strengths, when the preform has a 
specific gravity or weight as controlled by the ratio of components of the 
briquetting mixture and the compaction pressure, which is greater than the 
piled weight of the quartz sand, i.e. the bulk weight, bulk density or 
apparent density thereof. Hence, when the preform is introduced into the 
heated rotary kiln or rotary tube furnace where the latter is sufficiently 
filled with quartz sand, an immersion bed is formed and the heat treatment 
is carried out by contact with the hot sand as the preforms are fully 
surrounded thereby. 
The quartz sand has a preferred temperature of 500.degree. to 530.degree. 
C. at least at the outlet end of the rotary kiln or furnace. 
It has been found to be advantageous, moreover, to fill the rotary furnace 
with quartz sand so that the quartz sand volume is at least equal to twice 
the pore volume in a loose pile of the preforms in an amount of the latter 
intended to be treated at one time in the furnace. In general, however, 
the techniques set out in DE-OS 37 24 541 can be used. 
As to the binder itself, with respect to the starting composition adapted 
to be formed into the briquettes, i.e. the preforms, the binder should 
include at least 7% by weight of the starting mixture of pitch in the form 
of coal or crude oil pitch, especially electrode pitch, and at least 12% 
by weight of fine grained caking coal. Preferably the binder is 
constituted of 7 to 12% by weight of the aforementioned starting 
composition of pitch and 12 to 14% by weight of the fine grained caking 
coal. 
The starting mixture can also include, in addition to this binder, 20 to 
40% by weight of quartz sand and the balance petroleum coke. 
Advantageously, the quartz sand has a particle size of 0.05 to 0.2 
millimeters and the petroleum coke has a particle size less than two 
millimeters and at least 60% below 0.5 millimeters. The caking coal used 
in the binder may have a particle size of 0.01 to 0.25 millimeters. 
The raw material bodies produced in accordance with he invention have been 
found to be well capable of withstanding long term storage in open air and 
then used in the low-shaft electrode arc furnace. It is possible to add 
finely divided mineral substances to the starting mixture or activation 
purposes which can enable them to tolerate temperatures above 1500.degree. 
C. The raw material bodies can also be used in cupola furnaces for the 
production of cast iron and as both silicon and carbon carriers in the 
latter application.

The graph of the drawing has compressive strength plotted along the 
ordinate and time plotted along the abscissa for the raw material bodies 
made by the present invention and the comparative example. 
SPECIFIC DESCRIPTION AND EXAMPLE 
In a first test (comparative example), raw material bodies for producing 
silicon or silicon alloys in a low-shaft electrode arc furnace are 
produced of the following composition: 
38% by weight quartz sand of a grain size 0.08 to 0.25 millimeters 
50% by weight petroleum coke of a particle size of about 2 millimeters and 
12% by weight of electrode pitch. The components are mixed intimately at a 
temperature of 160.degree. C. 
The softening point of the electrode pitch was 90.degree. C. according to 
Kaines. After termination of mixing, the so-produced starting mixture had 
a temperature of 145.degree. C. After the mixture cooled to 105.degree. 
C., it was briquetted in a briquette press into substantially spherical 
preforms of a diameter of about 2 centimeters with a press pressure of 1.5 
t/cm.sup.2. 
The preforms were heat treated for hardening in a rotary furnace as 
described in DE-OS 37 24 541. The raw material bodies thus produced had, 
after storage in open air, a compressive strength curve a with long term 
storage. Before the fall off in physical properties, the bodies had all of 
the physical and chemical properties required for the silicon or silicon 
alloy producing process in the low-shaft electrode arc furnace. 
2. In a second test (this invention, best mode), one part by weight of the 
electrode pitch of Test 1 and 1.8 parts by weight of caking coal, at least 
80% of which had a particle size between 0.1 and 0.2 millimeters were 
heated at 160.degree. C. with melting to form a pitch/coal alloy. 18% by 
weight of this pitch/coal alloy was substituted for the electrode pitch in 
Test 1 while all other parameters were retained. The test was repeated 
under these conditions and the raw material bodies produced. After storage 
in open air for a prolonged period, no deterioration was observed. The 
comparative results are shown in curve b of the graph. 
3. In a third test, (this invention), Test 2 was reproduced except that the 
preforms were not hardened in the sand bed in a rotary furnace but rather 
hardened at the same temperature and for the same time on a traveling 
grate the long term storage results are shown in curve c.