CA1122415A

CA1122415A - Synthetic flux for steel slag

Info

Publication number: CA1122415A
Application number: CA332,889A
Authority: CA
Inventors: Giovanni Crespi
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-08-22
Filing date: 1979-07-31
Publication date: 1982-04-27
Also published as: IT7826910A0; BR7905220A; IT1098133B

Abstract

ABSTRACT OF THE DISCLOSURE

Synthetic flux for steel slag consisting of a mixture of aluminous or alumina residues with one or more of the compounds of the group:
clay, marl, sodium carbonate, borax, nepheline, bentonite.

Description

~2~5 The object of the present :~lvention i5 a ~1~ or fluidifier ~or steel slag.
The importc~nt -fuction o~ slag in steel ~orking and ~melting pro-cesses is well kn~n: the slag~ reacting t~ h the molten me~al, absorbs various impurities, as silicates7 ~ides~ sulphur~ phosphorous and others.
The reaction between slag and molten metal is Yery important for skeel production both in arc ~urnaces and in Martin Siemens furna-ces and in o~ygen converters - wherein, over large quantities of ma-teriil, a continuous and diffused contaot between the slag ~nd t~e metal being smelted has to be produced and this is why the forming of the slag is studied ~nth particular care.
For the process to start regularly~ it is first of all necessary for the slag to be formed as rapidly as pvssible and ~ith the right degree of ~iscosit~. This rapidity in the form m g o~ the slag is far more felt in modern technique, where ~ne tends to accelerate to the UtDtost all the stages o~ the steel ~-~orking process.
On ~he other hand, the slag is formed by introducing on the metal bath lime or limestone~ ~ich talce a certain time to reach the molten state and to start the slagging action. To shorten this melting time~
it is known to add to the lime and limestone some fluic~fying agents or ~l~ces. As a general rule, the in~luence of the flu~ in the slag lowers the de~sity of the same and the adhesion between steel and slag, favouring the rise of the material to be eliminated.
A very rell known ~nd widespread fluid~^fier used nowadays is essentially fluorite~ which ho~lever contains various imI*Irities~ as for example silicates, silica and sulphur, ~hich produce delc~ys and inconveniences in the production process~ ~loreover, the melting fluo-rite normaIly causes the developmen~ of harm~ul gases, as silicon -1 ~

tetrafluoride .
I~ l~s also been proposed to use fl.uidifiers based on premixed mixtures of -fluorite .~ith other naturll nnnerals, as dolomite, lime-stone c~ld bau~ite, however essentially for economical reasons but ~ith no improvement of the teohnical characteristics of the actual fluidifier.
The chemical composition of ~he slag :un ~ygen converters is on the average the foIlo~ing:
CaO 30-41%
SiO22S~27%
~1203 2-3~
Fe oxides24~9%
MgO 5-~%
MnO ~ 47~
P205 1-1.5%
S 0~2-0~3~o while, ;n arc fu m aces, the slag contains a smaller ~lantity of Fe oxides. From the chemical point of view, it is fundamental for ~his composition to keep a pre-established degree of basicity~ which is normally expressed by the formula:
basicity index - (CaO ~ ~IgO)/(Si~2 ~ P205) = 3.5~3.7 ~ le object of the present i~vention is to propose a new type of flux for ste~l slag, belng apt to answer the aforespecified re-quir~nents of basicity but allowin~, on the other hand~ to eliminate the cited drawbacks of the l~nown fluidi~iers.
'~his result is obtained with the fluidifier according to the yresent invention, which is essentially characterized m that it con-sists of a synthetic mixture of c~lumInous and/or alum m a residues with ~ne or more of th~ compcunds forming part of ~he group: clay,

2~

marl (calcareous clay)~ sodil~l carb~late~ borclx (sodlum barate)~ ne~
pheline (s~lieate and orthosilicate of Al~ Na, ~, and ~entonite.
It should ~Imediately be poin~ed out that, by the t~rm ~alumi-nous or ~lumina residues'l are meant scrap products coming ~rom the production of alwminum, the aver~ge chemical an~l~sis of which is the following:
70-90% al~nina (Al203) ~ -15% metallic alw mnum 10-12% mixtures of al~ninum hydroxide, of alumin~n trihydrate and of calcium-aluminum silicates The various constituents may enter the formation of the mixture accordmg to the present invention in the following proportions:
fr~n 40 to 70% alum m ous and alumil~ residues from lO to 20% clay fron S to 10% borax from 5 to 10~ sodi~ carbonate from 10 to 20% marl from S to 30% nephel~ne ~rom S to 10% bentoDite Som0 examples of preferred compositions according to thc present inv~ntion are given in the follo~nng table:

- ~ Oo--I IIIII IV~
Aluminous and alumina residues 70% 70% 66% 65%
Marl 10% 10% - -Clay 10% - 109~ -Sodiwn carb~nate 10% 1û% 10,' Borax - S% - --- 3~

:~ZZ~

Nephe.line ~ 5% 7% 27%

Bentonite - - 7%

"----o 0~
The essential object of the aforedescribed ~lu~ mixtt~e i~ to favouu and accelerate ~le fo~mation of reacti.ve slag with desulphu-rizing and dephosphori~.ing qual:i~:ies~
An additional object of the present i~vention is however to also favour a slag reducing reactîon. To this end, a ~urther object of the present invention consists in the use vf sodium hydro~ide (Wa~l~
and of calcium carbide (Ca~2).
The addition of NaO~I to the flux is ho~ever not so simple~ as the flu~ is thereby made highly hygroscopic and its storage3 in the large quantities required for s~eelworks3 becomes extremely proble-matic. On the other hand, also the addition of CaC2 is in turn very delicate, since this produc~ ~y give rise~ with h~midity, to the form m g of explosive mix~ures~
This problem is howe~er overcome, according to the present in-vention~ by providing to mix 1 part of CaC2 with 1 or 2 parts of MaOH
and furthe~nore, preferably, ~ith 2 parts of sodium carbonate In this way~ it has been possible to establish that the mi~ture - dried and reduced into grains - is suf~iciently stableO A better result is ob~ained by melting said mixture (taking advantage, for this pur-pose, of the low meltir~ po m t of ~aO~I).
An eve~ fu~her stabili~y - anda above all, resistance to humi-di.ty - is conferred by introducing in this mixture also 2 parts of clay and 3 pQrts of limestone~
This reduc.ing mixture~ essentially based on NaO~I ~nd C~C2, is introduced in ~he alUmin~lS ~1~LY according to the invention in the proportion of from 2 to ~o3 according ts the type o~ steel produced~

~ Ioreover it has been surp~isingl~ fo~nd ~ut that the addition of alIsaline salts, specifically N~O~-I and/or sodiwn carbona*e to the al~uNinous flu~, carries on a supplemen~ary function of puri~icationO
As a matter of fact a.luminous flux may easily contain ~npurities which, reacting ~ith the cill~li.ne salts~ create compounds being after ~rard c~bsorbed in ~he lime slag of the furnace.
Such alkaline salts are added to the aluminous ~lux in a percen-tage detern~ned ~ h respect to the cluantlty of ~he impurities in the M ux, and in any case inferior to lO~. They may be added in sol~tion state or in a dry state, The flu~ mi~ture according to the present invention is prefera-bly prepQred in pieces, grains or granules~ for example through pel-leting te~miques Imo~rn per se. For thi.s purpose, binders can be added to the mixture, in proportions varying between 0~3 and 2%~ con-sisting for example of the known binding resins and hardeners normal-ly eMpl~yed in the pelleting process, in the r~fracto~y mixtures and in the mold~ng sands~ such as ~he silicone resins, ~he epoxy resins, the silic~n-organic resins, polyethylene o~ide, ethylcellulose hydro-xide~ and ~he like~ which volatilize at the melting temperature of the steel and do no* affect, ~1erefore, ~he correct for~ing of the slag.
Fur*hermore, ~he use of molasses as a binder has been found par-tlcularly interesting. This product~ which may be utili~ed at the pure state, o~ring to its intrinsic composition transforms itself -at the high temperatures to which the flux is subjec~ed - leading to the fonmation of nascent carbon7 this in t~n favours the redu-cing reaction of the slag.
Considering tha* the modern metallurgical technique involves large quantities of o~ygen ~both in ~ygen converters and in arc urnaces), the ~ 5 _ ~ZZ41~

reducing feature of the flu~r, and h~nce of the slag, becomes hi~hly important for reali~ing a cert~in neutrali~ation of the excess of oxygen present in the actual Iw lten slag.
Finally, there iæ also the possibility to agglomerate ~he flux miYture by making use of pitch~ in a ~a~ l~no~n per seO
The ~luidifier according to the present invelltion has been tested in arc furnaces of 40, 60 ~nd 70 tons, in carbon steel castings, with excellent results~ particularly for what concerns the prompt formation of the slag. The t~sts have been carried out also m castings prepa-red with different types of sLags~ c~lwc~ys 1rith excellent results.
An essenticilly positive side effect has reover been fo~ld in a re~uction of the erosion effect of ~he furnace refractory lining - at slag level - which can clearly be ascribed to the s.lightly basic characteri~tics ~f the flux according to ~he inventîon~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Synthetic flux for steel slag, characterized in that it consists of a mixture of from 40 to 70% of aluminous and/or alumina production residues with one or more of the compounds of the group: 10 to 20% clay, 10 to 20% marl, 5 to 20% sodium carbonate, 5 to 10% borax, 5 to 30% nepheline, and 5 to 10% bentonite.

2. Flux as in claim l, wherein the mixture comprises from 10 to 20% of clay.

3. Flux as in claim 1, wherein the mixture comprises from 5 to 10% of borax.

4. Flux as in claim 1, wherein the mixture comprises from 5 to 20% of sodium carbonate.

5. Flux as in claim 1, wherein the mixture comprises from 10 to 20% of marl.

6. Flux as in claim 1, wherein the mixture comprises from 5 to 30% of nepheline.

7. Flux as in claim 1, wherein the mixture comprises from 5 to 10% of bentonite.

8. Flux as in claim 1, further comprising from 2 to 8% of a reducing mixture, essentially based on NaOH and CaC2.

9. Flux as in claim 8, wherein said reducing mixture is formed by 1 part of CaC2 and by 1 to 2 parts of NaOH, and further comprises 2 parts of sodium carbonate.

10. Flux as in claim 9, wherein said reducing mixture also comprises 2 parts of clay and 3 parts of limestone.

11. Flux as in claim 1 or 9, wherein the mixture is prepared as an agglomerate, in form of granules or pieces, by means of a binder of the group: pitch, tar or molasses.

12. Fluidifying process of a steel slag, characterized in that it makes use of a flux according to any one of claims 1, 8 and 9, in the proportion of 5 to 8% calculated in respect of the weight of the lime and/or limestone introduced into the furnace for the forming of the slag.

13. Fluidifying process of a steel slag, characterized in that it makes use of a flux according to claim 10, in the proportion of 5 to 8% calculated in respect of the weight of the lime and/or limestone introduced into the furance for the forming of the slag.