Method for the treatment of phosphuretted slag

Process for the treatment of phosphuretted slag obtained by refining a molten bath, such as steel. The process according to the invention involves hot oxidizing the slag into which is blended, after refining, a pulverized solid adjuvant, strongly oxidizing and easily fusible upon contact with the slag. Among utilizable adjuvants, the carbonates of alkaline metals or alkaline earth metals are preferred, in particular sodium carbonate, which additionally, is a product currently utilized in the domain of the elaboration of metal, notably in iron metallurgy. The invention ensures a rapid passivation of the slag in stabilizing the phosphorus by forming phosphate and allowing in this manner the avoidance of the liberation of very toxic phosphenes which are produced naturally by cold hydrolysis.

BACKGROUND OF THE INVENTION 
The invention concerns the treatment of phosphuretted slag obtained by 
refining a metallic bath. 
It is known that the refining operation for melted metal, such as steel, 
consists notably in the elimination from the metallic bath of a determined 
quantity of dissolved elements such as phosphorus, sulfur, among others. 
One succeeds customarily by forming on the bath a slag for which the 
essential role is precisely to grow rich in these elements. It is likewise 
known that certain slag can be formed in this manner, if appropriate 
measures are not engaged in at the right time, which contain contaminating 
agents that are sometimes very harmful to man. 
This is particularly the case with phosphuretted slag obtained by 
dephosphorylation of a metallic bath in reducing phase by means of the 
alkaline earths, such as calcium (see e.g. French Pat. No. 
2,428,672-IRSID) or by which is compounded halogenated (see e.g. French 
Pat. No. 2,356,732-Nippon Steel). 
It has been found, in effect, that such slag, rich in phosphorus, cold 
hydrolyzes upon simple contact with air to produce phosphines according to 
the reaction 
##STR1## 
where M represents the alkaline earth utilized as dephosphorylating agent. 
The phosphine being a very toxic gas, it is indispensable to stabilize the 
slag before it becomes hydrolyzed. 
The known solution (French Pat. No. 2,356,732) involves hot oxidizing of 
the slag in an atmosphere enriched in oxygen. This is then carried by the 
phosphorus for formation of a very stable phosphate. 
However, the reaction of a gas such as oxygen on a liquid refining slag 
always presents rather slow kinetics. Moreover, the practicality of the 
method is notably limited by the necessity of installing particular means 
in order to assure the blowing onto the slag of a gaseous oxygenated 
current. Additionally, it is imperative that the slag should be 
preliminarily isolated from the metallic bath in order to avoid its 
becoming oxidized or the taking on of phosphorus. In other respects, this 
is done by cooling the slag for a time. A temperature is then maintained 
by means of an oxidizing flame burner. However, it is advisable then to 
utilize a combustible such as gaseous CO in order to avoid that the 
combustion be accompanied by the formation of water vapor. 
SUMMARY OF THE INVENTION 
The present invention therefore is based upon the object of providing a 
simple and efficient solution to the problem of the passivation of a 
phosphuretted refining slag, which solution necessitates neither any 
particular installation nor preliminary metal-slag separation, and which 
rapidly brings about an inhibiting of noxious constituents. 
To this end, the invention is based upon the object of providing a process 
for the treatment of a phosphuretted refining slag by hot oxidation, 
characterized by the incorporation therein of an easily fusible and 
strongly oxidizing solid adjuvant. 
By "easily fusible" it is meant to be designated that the adjuvant be 
capable of easily melting upon contact with the slag in thermal 
equilibrium with the metallic bath at the bottom of the refining mass. In 
this regard it is recommended to employ adjuvants having a melting point 
below about 900.degree. C. 
Among the products satisfying the necessary specification it is preferred 
to utilize carbonates of alkaline metal or alkaline earths which are 
excellent oxidizers (such as Na.sub.2 CO.sub.3, MgCO.sub.3, K.sub.2 
CO.sub.3). 
Among these, sodium carbonate (Na.sub.2 CO.sub.3) is found to be 
particularly appropriate by reason of its low melting point (850.degree. 
C. In other respects, the product is not expensive and is commercially 
available in great quantities. Moreover, it does not necessitate any 
particular precuation either with regard to the conditioning or the 
handling, and its chemical properties upon melting lead to a 
siderurgically utilizable product. 
Apart from this, the carbonates present, in relation to other compounds 
capable of being utilized, the advantage of forming carbon oxide (CO) upon 
hot decomposition. This gas, which then emerges from the slag, burns in 
the atmosphere, releasing calories which tend to reduce the cooling effect 
due to the provision of cold matter into the slag. 
It should be well understood that the essential problem behind the 
invention was to attain realization of an intimate contact--as intimate as 
possible--between the oxygen and the phosphorus in the slag. To this 
effect, the inventors have come to the idea of utilizing a fixing agent 
for oxygen, capable of instantly freeing itself, that is to say, capable 
of forming at low temperature a liquid phase of low viscosity which 
infiltrates into the anfractuosities of the phosphuretted slag.

It should also be understood that the carbonates of the alkaline metals or 
the alkaline earths, preferably Na.sub.2 CO.sub.3, respond to such 
exigencies as are shown by the following elementary reactions: 
##STR2## 
The produced oxygen becomes carried on the phosphorus according to 
EQU M.sub.3 P.sub.2 +4O.sub.2 .fwdarw.3MO +P.sub.2 O.sub.5 
The passivation of the slag in the reaction mass may be set forth thus: 
##STR3## 
Implementation of the invention poses no particular difficulties. 
The sodium carbonate is presented preferably in pulverized form so as to 
facilitate the fusion. It can be applied delivered loose or in the 
condition of small packets. Whatever the mode of delivery, according to 
customary practice, it is introduced onto the slag at the end of the 
refining. 
The slag may or may not have been preliminarily separated from the metallic 
bath. 
In the affirmative case, the carbonate can be cast in small packets onto 
the slag without any particular precaution. 
When there has been no preliminary separation, it is preferable, for the 
reasons set forth above, to sprinkle the carbonate onto the slag in such 
manner as to not overly expose the metallic bath or provoke an agitation 
thereof. 
The quantities to be brought into play depend, as should be well 
understood, upon the quantity of phosphorus transferred to the slag during 
the course of the refining. The inventors have been able to show that the 
quantity Q of sodium carbonate to be employed is a function of the 
diminution D of the extent of phosphorus in the bath during the course of 
the refining, according to the relation: 
EQU 7.5.times.10.sup.3 .times.D.ltoreq.Q.ltoreq.15.times.10.sup.3 .times.D 
wherein Q is expressed in kg per 100 tons of metal (kg/100 tm) and D is in 
percent. 
The experience gleaned from a steel bath, the phosphorus content of which 
has gone, in the course of the refining by injection of calcium, from an 
initial value of 0.025% to a final value of 0.015% (D+0.010%), has shown, 
in accordance with the above relationship, that the quantity of Na.sub.2 
CO.sub.3 to be utilized runs from between about 75 and 150 kg/100 tm. In 
other words, the weight relationship between the quantity of Na.sub.2 
CO.sub.3 to be added to the slag and the quantity of initially contained 
calcium/-phosphorus compound (Ca.sub.3 P.sub.2) in the slag, is situated 
between about 2.5 and 5. 
Other metallic baths, refinable as appropriate, can be likewise treated in 
analogous manner with likewise determined adjuvant/refining agent 
relationship. 
The difference, represented by a coefficient of 2 between the extreme 
values, is explained by the fact that the atmospheric oxygen also 
participates in the passivation of the slag. Its contribution, which to be 
sure remains limited, is augmented in dependence on its partial pressure 
in the atmosphere above the slag and upon the time which is prescribed for 
effecting the treatment of the slag. 
Between the above-mentioned limiting values, the quantity of adjuvant to be 
utilized thus becomes determined as a function of these conditions. Aside 
from this, it has been observed to start with a quantity of adjuvant on 
the order of about 150 kg/100 tm, to rapidly stabilize the slag, the same 
in the presence of a neutral atmosphere. 
On the other hand, outside of the lower limit of 75 kg/100 tm, the 
passivation no longer appears to be capable of being correctly guaranteed 
with regard to the acceptable delay, this too even under an atmosphere 
strongly enriched in oxygen. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of metal 
treatments differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
method for the treatment of phosphuretted slag, it is not intended to be 
limited to the details shown, since various modifications and structural 
changes may be made without departing in any way from the spirit of the 
present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention. 
What is claimed as new and desired to be protected by Letters Patent is set 
forth in the appended claims.