DE2165595B2 - Sintered pellet and process for its manufacture - Google Patents

Description

The invention relates to a sintered pellet which is oxidized at 45 °. In addition, because of the crime Manufacture of a chromium-containing ferrous thing that the pellet has an excess of free alloy can be used, and a process contains carbon, its mechanical strength and to make this sintered pellet. thermal shock resistance low. Consequently

When chromite ore is reduced with carbon it can be as a result of mechanical; ii during impacts, the oxides of chromium and iron contained in the ore in the transport and thermal shock are reduced via an intermediate stage in which an electric furnace crumbles to powder, resulting in carbides being formed, and finally - The consequence is that the charge layer in which the metal state is reached is inhibited under the electric furnace passing gas or formation of a chromium-containing ferroalloy. is hindered. In addition, the pellet melts if one tries to use the chromium-containing ferrous it is introduced in the form of a layer on top of the coke bed alloy by direct reduction of the chromite ore layer, which is layered as a reduction zone in with carbon in an electric furnace a large amount of expensive electrical energy is consumed easily and passes through said coke bed layer, with the result that the production cost therethrough without being completely reduced becomes extremely high. 60 and falls to the bottom of the molten slag- However, for the production of the sintered layer below the coke bed layer, ie on the pellets, in which a substantial part of the ore surface of the metal layer produced, leading to the containing oxides of chromium and iron, such as The above consequence has that the equilibrium of the furnace conditions is indicated, up to the carbide form intermediate stage redu- is lost and the continuous smooth operation has been liert, cheap fuels, such as. B. heavy fuel oil, 65 of the furnace cannot be maintained,
Powdered coal or the like. Can be used. If in order to avoid the disadvantages described,

Now the pellets obtained in this way in one has already been proposed, the surface of the electric furnace subjected to final reduction pellets with glue, starch, water glass, pitch or

to be coated with a slagging component. However, this has the disadvantage that extra costs for this coating required that unnecessary Impurities increase and that the coating in the passage due to the different Properties of the coating and the pellet material located inside can be stripped off.

From the German patent specification 1 143 Ü29 pellets for metallurgical reduction processes are with a core made of a mixture of a metal oxide with a reducing agent and an outer one Coating known as the core of a compacted mixture of a finely divided oxide of iron. Titans and / or chromium with such amounts of a solid carbonaceous reducing agent that Sufficient for complete reduction of the oxide and, moreover, the metal formed wholly or can partially convert into a carbide, and as a coating a mixture of an acidic slag-forming Substance with a basic slag-forming substance contained in the subsequent reduction melt at least partially before the end of the reduction, and in this case such a viscous melt form that it practically does not penetrate into the interior of the pellet.

The use of pellets with a slag-containing coating is undesirable because it does so uakonlrolliert foreign substances are introduced into the melt for the production of the ferroalloy and, moreover, no dense and tough skin is obtained.

The object of the invention is to create sintered Pellets covered with a thick, tough skin, highly resistant to breakage during transport and possibly thermal shock and have a special strength.

This object is achieved by a sintered pellet with a core which consists essentially of carbides and oxides of chromium and iron and incidental impurities and is surrounded by a skin which is characterized in that the skin consists of metal oxides, and both the Skin and core contain the same metallic elements in practically the same proportions, the ratio of Cr / Fe about 1.5 to 4.0, the sum of the total Cr content and the total Fe content about 45 to 60 ° Total weight of the pellet, the bound carbon content is less than the total Cr multiplied by 0.099 plus the total Fe content multiplied by 0.092.

A particular advantage of the sintered pellet according to the invention is the fact that it is dense and has tough skin and that it contains no free carbon.

The invention also includes a method of making a sintered pellet, which is characterized in that powdered chromite ore having a Cr / Fe weight ratio of about 1.5 to 4.0 and powdered carbonaceous material are mixed in such a ratio that the carbon coefficient is within the range from about 0.5 to 1.0, the mixture thus obtained is pelletized, and the green pellets obtained after drying and preheating in the presence of excess oxygen in a high temperature zone at about 1200 to 150O ⁰ C, substantially reduced with rotation .

One of the essential features of the. According to the invention sintered pellet is that it has a skin of metal oxides. As this skin during When the sinter stage is formed by sintering the chromite ore itself, it is dense and tough. Also contains they have the same metallic elements in practically the same proportions as they are in that Core part are included. Since the pellet has a dense and tough skin of metal oxides, it will

to by mechanical or thermal shock during transport or within the electrical Oven it is put into, not broken or powdered. As a result, those in the Chromium and iron contained in the core after reduction to the carbides are not reoxidized, and the gas permeation is also not affected when the pellets are introduced into the electric furnace. In this way, the pellet according to the invention, if it is still immediately after its production is in a state of elevated temperature, transported or for the production of the alloy can be inserted directly into the electric furnace. When the pellets are in a condition still elevated Temperature introduced into the electric furnace, electrical energy can be 500 to 800 kWh per ton produced. Chromium-containing ferroalloy can be saved compared to the case that the agglomerates are introduced only after cooling to room temperature. Also is the metal oxide skin of the pellet according to the invention is less fusible than the core part. Hence melts the core part of the pellet in the lowest part of the introduced layer, d. H. the core parts of the pellets in the Contact with the coke bed layer, first, and only then does the skin gradually melt. In this way the pellets do not flow through the coke bed layer and do not fall in an incomplete manner reduced state to the bottom of the furnace. Therefore the internal order of the electrical Maintain the furnace, and that not only makes it possible the continuous operation of the electric furnace, but this also makes the yield more desirable Metal alloy greatly increased.

The pellet of the invention is usually a nearly spherical body with a diameter from about 10 to 30 mm. The metal oxide skin should be from the standpoint of ensuring the mechanical Strength and tightness, which is necessary to prevent the coke content from reoxidation, as viewed, preferably be about 0.2 to about 2 mm thick.

The above-defined Cr / Fe ratio and the sum the total Cr and Fe contents are proportions, which can easily be obtained by sintering the pellets if you have a single variety or a Mixture of a number of grades of natural chromite ores used. In addition, these proportions are within the range required for the manufacture of a chromium-containing ferro-alloy is suitable, which contains the chromium in the desired amount.

The amount of bound carbon contained in the pellets, ie the carbon present in the form of carbides, is, as defined above, less than (0.099 * total Cr + 0.092 * total Fe). Although there are different classes of carbides of chromium and iron, those of the molecular formula Cr ₇ C ₃ and Fe ₇ C _{3 represent} practically all

those. The amount of the above-mentioned bound carbon contained therein is an amount that below that resulting from these molecular formulas is calculated. The fact that it has been found to contain no free carbon together with the fact that it has the skin described above, contributes to the high mechanical Strength of the pellet according to the invention. In addition, since there can be no free carbon in the pellet included is the amount of additional carbonaceous material necessary for the final reduction of the Oxides of Cr and Fe in the pellet in the electric furnace is required to make the alloy, can be increased, with the result that the coke bed layer can be made thicker. This will ensures the stability of continuous furnace operation.

The chromium and iron are contained in the pellet according to the invention in an acid-soluble and in an acid-insoluble form. The terms "acid soluble" and "acid insoluble" as used herein refer to the degree of solubility of the material in aqueous sulfuric acid. The acid-soluble forms are primarily carbides and the acid-insoluble forms are primarily oxides. The above-defined range of the amounts of these forms of chromium and iron present in the pellet is subject to the change in the composition of the chromite ore used as the starting material and the amount of the carbonaceous material admixed to produce the pellets. The incidental impurities contained are those metal oxides other than those of chromium and iron contained in the starting chromite ore, e.g. B. SiO ₂ , ΑΙ-, Ο, Γ MgO and CaO. The amounts of these oxides vary depending on the type of starting chromite ore.

An essential condition in the preparation of the pellets is that the amount of the ^{pulverförrr:} gene chromite ore to be mixed carbonaceous material is far smaller than in the conventional methods. The invention differs fundamentally from the prior art in this respect. The chromite ore and carbonaceous material which are suitably used are those which are reduced to a grain size of e.g. B. have been finely comminuted below a mesh size of 0.147 mm. Coke is conveniently used as a carbonaceous material because of its purity, but anthracite or other carbonaceous materials having a low ash content and volatiles can be used singly or as a mixture of two or more of them.

The amount of carbonaceous material used is limited to a carbon coefficient of 0.5 to 1.0. The term "carbon coefficient" as used herein means a number representing the ratio of the amount of carbon used to the stoichiometric amount of chromium and iron _{oxides required to reduce the total amount of chromium and iron oxides contained in the chromium ore in the form of So Cr 7} O ₃ and Fe ₇ O ₃ Represents carbon. By blending the carbonaceous material in a smaller proportion than the stoichiometric amount, the pelletized agglomerates become so strong that they are hardly pulverized during the drying, preheating and sintering steps, thereby not only causing abnormal ring formation of the molten pellets which are to be shut down of the rotary kiln used for sintering is prevented, but also minimizing the pulverization of the pellets to a size of less than 5 mm during the subsequent operation of the electric furnace, a process which must be avoided in the last-mentioned mode of operation in order to stabilize the furnace operating conditions.

On the other hand, the reason is that, despite the smaller amount of carbonaceous material, there is a reduction in the amount can be achieved, which compared with that in the usual method or cheap is even higher that a suitable combination of sintering temperature and oxidizing atmosphere is applied. When the carbon coefficient is below 0.5, the reduction rate becomes the Chromium and iron oxides are low and the economic value of the sintered pellet suffers.

As the binder Z to eiv.a 6 ",, bentonite, for example, about added to the starting material in the above amount ratio LMn _n is _set to the mixture a small amount of water.. Iufgesprüht thereafter the mixture to pellet ·, spherical with virtually. shape with a? m diameter of about 10 to 30 mm with a granulator ceformt. Ah binder usable are Beiuonit, water glass and Pulpenabfallflüssigkeit. These may be used singly or in combinations of two or more thereof.

The green pellets thus obtained are then dried with, for example, a dryer with a grate: this is followed by preheating of the dried pellets. The heat consumed in the sintering stage is advantageously used in the preceding drying and preheating stages. If the preheating is carried out with the aid of a shaft furnace, in which the waste heat from the rotary kiln used for sintering is used, there is a possibility of oxidation loss of the carbonaceous material present in the pellets, which is caused by water vapor and carbon dioxide. In order to keep this loss minimal, therefore the temperature should not be limited than 600 ⁰ C, preferably to a value of. If the preheating is carried out with a gas whose water vapor and carbon dioxide content is low, the preheating can be carried out at a temperature of up to about 900.degree.

Subsequently, the preheated pellets are introduction of excess oxygen (it can also be used air) introduced into a zone of a temperature of about 1200-1500 ⁰ C, ^d: e is generated by the Verbrcnnungsfiamme a fuel, whereby they are sintered while they be circulated. The usual rotary kiln is used as the furnace. Heavy oil, propellant gas, powdered coal, etc., can be used as fuels. The reason why excess oxygen should be present when editing the pellets is as follows:

In the method according to the invention that is during the reduction of the metal oxides with the Carbon monoxide formed from carbonaceous material mixed with oxygen or air necessary for conversion the carbon monoxide in carbon dioxide is required by further combustion of the same

are, creating an oxidizing atmosphere with increased Temperature is formed. In this way the pellets are exposed to the influence of these oxidizing agents Subject to atmosphere, and an oxidation takes place on its surface with the formation of an oxide skin on the same, even before they reach the zone with an elevated temperature of, for example, 130 (TC. As a result, this oxide skin, together with its compression during sintering, becomes even more dense and tougher in the zone with an elevated temperature above 1300'C. Since this self-forming Oxydhaiit is dense and tough, the inside of the pellets is hardly affected by the oxidizing atmosphere of the furnace influenced. This will remove the chromium and iron components within the pellets that once too Carbides have been reduced without reoxidation subject. Therefore, the pellets according to the invention have compared to the usual pellets without such a skin, there is a great improvement in the unit consumption of additive carbonaceous material to achieve the desired rate of reduction. In addition, occurs also has the effect that the pulverization phenomenon that occurs when a large amount of carbonaceous Material can be determined, can be controlled. In addition, as already mentioned, a highly stable Furnace operation in the production of the alloy by completing the reduction in an electric Sintered pellets introduced into the furnace are maintained. In addition, the inventive Notable economic advantages since the sintered pellets are in the electrical process Furnaces can be introduced while they are still in the elevated temperature state.

Since the pellets have been thoroughly preheated, they are very resistant to thermal shock and are hardly pulverized as a result, even if they are suddenly introduced into the interior of the rotary kiln in ^{high temperature zones of about 1200 to about 1500 ° C.}

According to the method according to the invention, the pellets, after they have been preheated, are successively passed through the zones of increased temperature from about 1200 to about 1500 ° C., the reduction reaction taking place, and from about 1300 to about 1500 ° C., within the rotary kiln, the reduction taking place particularly rapidly, migrate, or they are introduced directly into these zones. Since the pellets are exposed to a high temperature under an oxidizing atmosphere, the surface of the pellets is rapidly compacted and the pellets are covered by a self-forming, dense and tough oxide skin, therefore the pellets obtained have a remarkable strength so that their inclination for pulverization when they are vigorously agitated (shaken) is extremely low. The substantial reduction in Metalio.xyde to the carbides takes place in the temperature zone 1200 to 1500 ⁰ C. The term "as used herein substantial reduction" means that particular chromium is rapidly and almost completely to its carbide reduced. The significant reduction of this chromium is best quickly in the zone of elevated temperature of 1300 to 150ü ^"J C. Ferric oxide is easily reduced than chromium.

When the sintered pellets obtained in this way are removed from the rotary kiln and collected in a container desired sintered pellets are obtained. These can be used as starting material for making the chromium-containing alloy immediately while they are still hot, or after Cooling can be introduced into the electric furnace. Since in the case of the process according to the invention sintered agglomerates obtained, the preheated pellets suddenly exposed to an oxidizing atmosphere at elevated temperature, as indicated above,

ίο be exposed, a strong compaction occurs by sintering with the formation of a dense and tough oxide skin. It therefore practically does not occur Reoxidation of the carbides of chromium and iron in the sintered pellets mentioned above formed reduction products, even if they are free after removal from the rotary kiln Exposed to air.

ϊο B e i s ρ i e I 1

Manufacture of sintered pellets

Chromite ores of various types of the grades given in Table I below were each finely crushed until more than 90% of the particles were less than 100 microns in size, uri.i 448 kg of chromite ore from the Philippines, 70 kg of Indian chromite and 482 kg of South African Chromite ore, a total of one ton, was thoroughly mixed with 189 kg (corresponding to a Carbon coefficients, as defined above, of 0.85) powdered coke and 39.2 kg of powdered coke Bentonite. After spraying about 7% water onto this mixture, the mixture became with pelletized under a pan-type pelletizer with further water spraying Formation of about 1400 kg pellets with a spherical shape with a diameter of about 10 to 30 mm.

The chromium-iron ratio in the pellets was 1.97. The above pellets were 40 minutes long dried in a mesh-type dryer, with a stream of air at a maximum temperature of 250 ° C was passed through to reduce the residual moisture to about 0.5 ", '.

The dried pellets were immediately transferred to a preheater, the preheating was carried out in such a way that the exhaust gas from the rotary kiln was passed through the preheater after the gas temperature had been reduced to about 600 ^: C by means of a heat exchanger.

Heavy oil was used as fuel in the rotary kiln, the zone of the highest temperature of the same was kept at 1470 ° C while excess air was introduced to prevent the combustion of fuel and the formation of carbon monoxide.

The above preheated pellets were introduced into the feed end of the rotary kiln.

and during the rotation it was caused to pass through the high temperature zone with the above Temperature passed to effect their substantial reduction, after which they were allowed to reach the discharge end of the oven where they are in a container for the sintered pellets were withdrawn. The composition of the thus obtained sintered Pellets were analyzed to give the same results as shown in Table II below.

Table I Grades of Chromite Ores and Coke

Art

Won in the Philippines

Chromite ore

Ir; Chromite mined in India / Chromite ore mined in South Africa

O.O,

Components ("")
ΛΙ..Ο,! M _S ()

49, 30th 17th .88 53. 73 13th .57 44, 16 24.43

11.9X
11.90

15.79

15.14
9.7S

10.31

SiO,

3.04
5.09

2.95

CiO

0.3
0.3

0.27

Carbonaceous
material

coke

Components ('; ") the following facts that are of a special Mrwähnuni are worth receiving.

carbon

87.0

volatiles

ash

10.8

Table II Composition of the sintered agglomerate

Components

Total cr

Acid Soluble Cr

Total Fe

Acid soluble Fe
Bound
Overall-C

MgO

Al ₂ O ₃

SiO ₂

Composition ("/")

total

34.6
20.0
17.6
15.2

2.8
12.8
15.3

6.7

Skin core

30.0 0

15.5 0

0.04 11.3 13.8

5.9

35.0 22.8 17.9 17.3

3.14 13.2 16.5

7.4

Molar ratio

the

metallic elements

Skin I core

L.00 0.48

0.49 0.47 0.17

Total Cr / total Fe 1.97 ⁿ _u

Total Cr + total Fe 52.2 ° _o

Soluble Cr / Total Cr 58.0%

Soluble Fe / Total Fe 86.0 " ₀

Other characteristics:

Skin thickness: average 0.8 mm (0.5 to

1.1 mm),
Compressive strength: average 65 kg / pellet

(43 to 86 kg / pellet).

The sintered pellets described above were, while still at an elevated temperature (about 1000 ⁰ C) were mixed with lump coke and Sinciumdioxyd and limestone as a flux, the mixture was introduced into an electric furnace, and the melting operation was carried out, wherein the specified below Results obtained were:

1000 kg of the sintered pellets described above, 84 kg of silica, 115 kg of limestone and 95 kg Lump coke was fed into a 4000 kW Siemens Martin electric furnace as raw materials for the production of high carbon ferrochrome, and the furnace conditions became so monitors that the required data is received during operation -: rden. As a result, were

. The sintered pellets in the Maieri;
retained their original shape
at a high temperature. without d; ii <.
pulverizes the thermal shock \ un
them in the melted responsiveness

. despite the fact that the sintered Γ
the material layer of a high "li-i;
were exposed, there was almost no back
of metallic chrome and iron. , Ii ,:
Core of the pellets were present, dent;:
the pellets fell from that one by one
Part of the material layer in the ge-cl
Reaction layer and none occurred.
Melting or splashing of the Schluo. '·
Electrodes around on. The electric
consumption was in this case! ■ ·
Ton. (If the operation is under W;
from common sintered pellets to c · -.
Way was done. Iv
electrical energy consumption about 3
J " ^onn , ^e , ·) ^Dai > for this oven treatment:>
Metal is listed in Table III below,
The chromium yield in this furnace
was about 93 ° :.

Table III

Composition of chromium included-:
Ferro alloy

Components (° _o )
Si

schicli Ibst be ic through (lon. bi: icleii: Vllcts it lpcratui ydatior in derr ■ htet: lower 'üol / ene: it iellei m dk • icrsie- "kWh- ■. • ndung ■ corpse.! <_' clei

i!> iltenf ■. icben,

,! aliting

57.4

31.2

Example 1

Manufacture of sintered pellets
Chromite ores of the types given in Table I of Example I were each finely comminuted,

S Tn μf ⁹³ "" of ^{the particles had a} size of less than 10 M.kron, and 603 ke of a chromite ore extracted in the Philippines and 397 kg of chromite ore extracted from I ^nes ' ^{n Indlen} , a total of one lonne with 211 kg (corresponding to ^ einera carbon coefficient of 0.95) of powdered coke, and 43 kg bentonite mixed and then κ p; nV ⁿ RTT ^{el l tO nearly} spherical pellets with W5H ° ^urchm eater of about 12 to 26 mm granulated, n «A ^{a dle Mix} 8 water sprayed The total weight of the pellets obtained was ""' ^rhr oni-iron ratio:> der?: üe! 5

Components

Total cr

Acid Soluble Cr

Total Fe

Acid soluble Fe
Bound
Overall-C

MgO

Al ₂ O ₃

SiO,

Composition ( ° ",

Ge
velvet skin 36.3 33.2 24.0 0 13.2 12.1 10.0 0 4.4 0.07 13.7 12.4 13.8 12.7 7.9 7.0

Interior

37.2 31.2 13.5 13.0

4.4 14.1 14.1

8.0

Molar ratio

the

metallic elements

skin

1.00 0.34

0.48 0.39 0.18

Interior

1.00 0.34

0.49 0.38 0.19

Total Cr / total Fe 2.75?;,

Total Cr -4- Total Fe 49.5 ',' "

Soluble Cr / total Cr 66.2 ° ₀

Soluble Fe / Total Fe 75.8 ";

Other characteristics:

in this case was 2.75. As in Example 1 were these pellets are then dried, preheated and then placed in a to a maximum temperature of 1455 ° C held rotary kiln introduced, in which the reduction of these pellets is carried out with circulation became. The composition of the sintered pellets thus obtained is shown in Table IV below specified.

Table IV
Composition of the sintered pellet

Skin thickness: average l.l mm (0.8 to

4.4 mm)
Compressive strength: average 74 kg / pellet

(60 to 88 kg / pellet).
5

The sintered pellets thus obtained were as in Example I. while they were still at a high temperature mixed with lump coke and silicon dioxide, then the mixture was placed in a clean oven and the melting process commenced carried out, whereby the following results were obtained.

82 kg of slag coke and 83 kg of silicon dioxide were mixed with 1000 kg of the above-mentioned sintered pellets of elevated temperature, and the mixture was placed in a 4000 kW Siemens Martin electric furnace introduced for the production of high carbon ferrochrome.

stable furnace operation was maintained, without gas or slag spraying on the surface of the material layer. The composition of the The alloy obtained is given in Table V below. The electrical energy consumption at this way of working was 1920 kWh per ton of alloy, a value about half that of the usual Represents consumption. In addition, the yield of chromium in the electric furnace was about 94.3 ".

Table V
Composition of the alloy

Cr

Components (%)
Fe 'Si

65.9

25.2

1.4

6.4

Claims (3)

electrical energy consumption is lower, and a cheaper alloy product can then be obtained. 1. Sintered pellet with a core that is ultimately made during manufacture The practice consists essentially of carbides and oxides of chromium s a chromium-containing ferroalloy and iron and incidental impurities therein, as described above, as an intermediate stands and is surrounded by a skin, d a- produced pellets either as starting material characterized in that the skin or to use as an additive. These pellets can consists of metal oxides and both the skin and are made in the following ways: also the core the same metallic elements io a powdery mixture of chromite ore and in practically the same proportions a carbonaceous material becomes briquettes included, with the ratio of Cr / Fe roughly or pellets processed, after which these are processed by 1.5 to 4.0, the sum of the total Cr content sintering with the help of the heat of combustion of a combustion and the total Fe content about 45 to 60% of the fabric sintered. Total weight of the pellet, the content of the 15 A typical example of previously known sintered bound carbon is less than the total Cr pellets, as in U.S. Patent 2,883,27S content multiplied by 0.099 plus the total Fe-described, such a product that practical salary multiplied by 0.092. is uniform and consists essentially of Oxyds-n 2. A method for the production of a sintered and carbide of iron and chromium and free pellets according to claim 1, characterized in that there is 20 carbon, such a composition :::. ' that one has powdered chromite ore with a that about 15 to 40 percent by weight, Lt Cr / Fe weight ratio of about 1.5 to 4.0 and total composition of bound chromium represent powdery carbonaceous material, of which about 15 to 80 percent by weight acid such Ratio mixes with each other that the are soluble and the remainder is insoluble in acid. et. \ ;; The carbon coefficient is within the range 25 12 to 60 percent by weight of the total composition: - from about 0.5 to 1.0, the composition thus obtained represent bound iron, of which the mixture is pelletized, and the resulting green at least 85 percent by weight are acid-soluble and < irr pellets after drying and preheating in counter- rest is insoluble in acid, which is sufficiently free. wart of excess oxygen in a carbon, so that the Gesamtkohlenstoii high-temperature zone at about 120 o to 1500 ^c C, content at least about 5 percent by weight of excess, with rotation substantially reduced. compared to the one to achieve a 1: 1 gram 3. The method according to claim 2, characterized in the atomic ratio of carbon to oxygen draws that the preheating is done by exhaust gases in the unit, the remainder being carbon from the high temperature zone at one temperature and oxygen combined with iron and chromium in that of not more than 600 ° C. 35 pellet and incidental impurities are. From the above The description shows that the pellet is essentially uniform and not tough Has protective coating. When the thus obtained briquettes or pellets that are still on a high 40 temperature, stacked on top of one another | will, Difficulties arise because the briquettes or pellets fuse together or the chrome or the iron, which has been reduced to carbides, by contact with the air again