GB1564921A

GB1564921A - Introduction of magnesium to molten iron

Info

Publication number: GB1564921A
Application number: GB676676A
Authority: GB
Original assignee: Materials and Methods Ltd
Current assignee: Materials and Methods Ltd
Priority date: 1977-01-24
Filing date: 1977-01-24
Publication date: 1980-04-16

Description

(54) INTRODUCTION OF MAGNESIUM TO MOLTEN IRON (71) We, MATERIALS AND METHODS LIMITED, a British Company of Meerion House, 38 Albert Road North, Reigate, Surrey, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a magnesium metal or alloy composition and to the use thereof in the treatment of molten cast iron.

In treating molten cast iron in which the graphite is in flake form, in order to convert the flake graphite to spherical or nodular graphite; there is added to the iron a nodularizing agent, a magnesium alloy being commonly used for this purpose.

In the case of magnesium alloys, a problem arises due to the very high volatility of the magnesium at the temperatures of operation which are necessary when dealing with molten iron. Thus, not only does the considerable volatilization of the metal lead to production of fumes and pyrotechnics when contacted with the molten iron, but it also leads to a loss of that magnesium which is not absorbed into the molten iron, and can, in extreme cases, lead to the build-up of such pressure that explosions may occur, the consequences of which, in cases of handling molten iron, are manifestly serious.

The problem is particularly severe in the case of treatment methods involving the repeated use of the same vessel at frequent intervals for the addition of a nodularizing alloy containing magnesium to successive batches of molten iron. In such cases the vessel rapidly heats up until its temperature may reach almost the boiling point of magnesium even in the absence of a charge of molten iron and hence if the treatment method is such that the nodularizer is introduced into the vessel before the metal charge to be treated, there is inevitably a large losss of magnesium from the vessel.

This is the case, for example, where the nodularizer is placed in the bottom of a ladle before pouring in molten metal, or is placed in a reaction chamber through which the molten metal passes during treatment.

As an example of how the increase in temperature of such a vessel or chamber affects the magnesium vapour pressure, the following data is presented. This shows how the flow time increases as the temperature of the vessel rises, this increased flow time being caused by the increasing back pressure built up in the vessel as its tempera- ture increases.As indicated, in the last treatment run the reaction was violent:-

Molten Metal Vessel Flow Flow Alloy temperature temperature Time Rate Reaction 5% Mg.Fe.Si 14800 C 2000 C 30 10 kg./sec Quiet ,, 14800 C 6000 C 35 8.5 kg./sec Increased reactivity 14800 C 9000 C 40 7 kg."sec Violent As stated previously the vapour pressure of magnesium is high at the temperatures which are involved in the nodularization treatment of molten iron.Thus, for example, at 1300"C, magnesium has a vapour pressure of 4 atmospheres, at 1400"C, of 7.5 atmospheres and at 15000C of more than 13 atm. Temperatures conventionally used in carrying out nodularizing treatment are nowadays of the order of 1500"C.

Although these vapour pressure figures relate to pure magnesium, the same problem does in fact arise with the magnesium alloys which use ferrosilicon as a carrier and which have been conventionally widely used as a nodularizing agent.

The magnesium-ferrosilicon alloys used contain usually from 5 to 15% of magnesium, though the amount of this which is in alloyed form as the metal is generally somewhat less, as some of the total is in fact in the form of magnesium oxide. Typically in a nominal 6 ó Mg-ferrosilicon alloy, from l to 3% of the magnesium may in fact be in the form of the oxide leaving from 3 to 5% available for introduction to the molten iron.

The use of ferrosilicon-magnesium alloys leads to improved dissolution of the magnesium in the molten metal, and therefore an improved recovery of the available magnesium. In these alloys it is the silicon which is effective in providing an improved solubility of the magnesium since it forms a compound Mg2Si and this has the effect of reducing the magnesium partial vapour pressure after addition of the alloy to molten metal, or in other words the solubility of magnesium in the metal is increased. It has accordingly been suggested previously to increase the amount of silicon in the alloy, so as to extend the favourable effect of this element on magnesium solubility. However it is found that when a ferrosilicon/magnesium alloy is added to molten iron, the silicon is quickly taken up by the iron and most of the magnesium is left behind due to its slower dissolution.This refects the fact that in the three component system of the alloy (Fe-Si-Mg) it is the interaction between the iron and the silicon which is strongest due to the stability of the FeSi compound, and so the Si is taken up very readily into the iron. As a result the moderating influence of silicon on the activity and vapour pressure of the magnesium is only temporary and is found only during the initial stages of the treatment being lost thereafter, the magnesium partial vapour pressure returning to a figure which is that of pure magnesium metal. In other' words at this stage the remaining magnesium merely burns off and is lost from the treatment and furthermore this gives rise to production of flare and fumes such as always occur when no silicon at all is present.

It is therefore desirable and is an object of this invention to provide an improved means of controlling the vapour pressure of magnesium during its dissolution into molten iron, and particularly to provide a magnesium alloy or pure magnesium metal composition which will allow of a controlled dissolution of magnesium extending over a longer period of time than has heretofore been attained. Such a composition is achieved, in its broadest aspect, by providing a conventional magnesium alloy with a coating of a material which has itself a limited solubility in iron but which is effective to lower the vapour pressure of the magnesium. By this means the magnesium partial vapour pressure is controlled throughout its addition to the molten iron and a nodularising or innoculation treatment is achieved which is quiet (i.e.

non-violent) and which proceeds smoothly with the generation of minimum flare and fume. By using such a material in combination for example with the conventional magnesium-ferrosilicon alloy, the advantageous effect of the silicon in increasing the solubility of magnesium in the iron is retained or alternatively, if the magnesium for treatment is in the form of the pure metal, than an amount of silicon is also provided in the magnesium treatment com pbsition to ensure this advantageous effect.

Accordingly the present invention provides a nodularising agent or an innoculant comprising particles of magnesium in elemental form or in the form of an alloy, the particles being provided with at least one coating which is self-adhereing or fixed to the particles and which coating comprises one or more solid inorganic compounds in finely divided form having a limited solubility in iron and being effective to lower the vapour pressure of magnesium.

According to the present invention the coating may be hard or soft, it can be selfdrying or dried by external means and it can be applied in one or more layers, preferably not more than two layers. The coating may be self-adhesive with respect to the magnesium particles or it can be fixed e.g.

bonded, for example by mixing the finely divided inorganic compound with a bonding agent, for example a resin such as shellac.

When two coatings are applied to the magnesium particles the second coating can be applied together with or subsequent to application of the first coating. Thus, the second coating may be adhered or bonded to the magnesium particles and/or to the first coating.

The solid inorganic compound may be, for example, a compound of a metal of Group IIA of the mendeleev Periodic Table such as calcium fluoride (fluorspar).

In a particular embodiment of the invention finely divided fluorspar is applied to the particles of magnesium and is self adhered thereto.

In another embodiment of the invention a finely divided inorganic compound such as talc is mixed with a bonding agent e.g. a resin such as shellac or a drying oil and applied to the particles of magnesium to provide a composition in pellet form com prising a bonded coating. After drying a second coating for example of FeSi and/or graphite may be applied to the first coating.

Alternatively, the second coating may be applied, in admixture with the first coating, to the magnesium particles.

This invention is also concerned with a method of innoculating or nodularising iron by treating molten cast iron with the nodu larising agent or innoculant.

The invention is particularly concerned with a process of nodularising grey iron, in which the graphite is in flake form, to provide iron castings in which the graphite Is in spheroidal form, said process comprising treating said grey iron with a sufficient quantity of a nodularising agent to effect nodularisation of the graphite in the ,metal, the nodularising agent comprising particles of magnesium in metal or alloy form, said particles being provided with at least one coating comprising a finely divided inorganic compound, self-adhering of fixed to the magnesium particles.

The magnesium particles coated accord ing to the invention allow the magnesium or magnesium alloy to be used for the treat ment of molten cast iron without a violent reaction. The coating serves as a protective coating which serves to control the vapour pressure of the magnesium during its dissolution into molten iron. We have also found that the coating materials act as a nucleant in the metal, which is important in producing nodular graphite. In the case of an innoculant or nodulariser which is introduced into molten iron, the coating allows the magnesium to be pulled a satisfactory distance (i.e. at least 12 inches) below the surface of the molten iron before the magnesium begins to be dissolved. Thus, treatment of molten iron is achieved smoothly in a non-violent manner, without turbulence and with generation of minimum flare and fume.

Preferably the coating of the finely divided, solid inorganic compound comprises from 1 to 5% by weight of the total composition. Where a further coating e.g. a graphite, a silicide or a calciumcontaining compound is applied, this coating is preferably present in an amount comprising from 2 to 5% of the total composition. Also it is advantageous that the amount of magnesium in the nodularising agent or innoculant be from 5 to 15% by weight thereof, and that there should be from 40 to 65% by weight of silicon present.

This is provided either by the silicon component in the magnesium-ferro-silicon alloy used as the basis of the nodularising agent or innoculant, or by addition of ferrosilicon to the magnesium metal when the magnesium is used in this form. The balance of the nodulariser is made up by the iron provided in the alloy or in the added ferrosilicon.

Specific magnesium alloys which can be used in the present invention comprise 5% Mg, 45% Si with the balance being iron or 9% Mg, 45% Si with the balance being iron.

The finely divided. solid inorganic compound may be a Group IIA metal compound preferably a compound of calcium, and in particular a halide thereof.

Most preferably the calcium compound forming the adhering powder is calcium fluoride ground to a fineness of not more then 55 microns (commercially available as W300 grade CaF2). As the further coating material, a calcium-containing substance is desirably used, and calcium silicide of grading not more than 100 L has been found particularly useful.

Calcium compounds are preferred in the compositions described since the calcium metal forms with magnesium a stable intermetallic compound (Mg2Ca) which provides a minimum in the Mg-Ca alloy vapour pressure curve so low as to be below the vapour pressure of either of the pure components at the relevant temperature. Also the calcium is not highly soluble in the molten iron, and most particularly its solubility is roughly of the same order as that of the magnesium so that there is no problem of differential solution such as arises with silicon and the moderating influence therefore extends throughout the time that the magnesium is being added.

According to another aspect of this inven- tion, there is provided a method of producing a nodularising agent or an innoculant in which magnesium particles in elemental or in alloy form are provided with at least one coating of a finely divided solid, inorganic compound which coating is self-adhesive or fixed to the magnesium particles.

The magnesium-ferrosilicon alloy or the pure metal. whichever is used, is in granular form and, in the case of double-coated compositions, the compositions are preferentially prepared by putting the granular starting material into a rotary barrel mixer and adding thereto the first coating material, for example finely-ground fluorspar. The mixer is then run to coat the magnesium material, which may take about 34 minutes, and the second coating material, for example calcium silicide, is then added to the mixer. After further operation thereof until the outer coating has been applied to the inner adhesive layer formed on the metal or alloy grains, the material is discharged from the rotary mixture.

According to one particular embodiment the magnesium is used in the form of an alloy comprising 5% or 9% Mg, 45% Si, with the balance being iron. The grading of this material is 2 mm down. The magnesium alloy is placed in a pelletising machine to which is added fine fluorspar and a bonding agent such as a resin e.g. shellac or a drying oil. The alloy is coated with these materials in the pelletising machine after which the pellet is dried with hot air. A second material in fine form e.g. ferrosilicon, calcium silicide or graphite is added again with a bonding agent and the alloy is coated and dried as before. The resulting pellet has two fixed coatings approximately 0.5 to 1.0 mm thick.

An alternative method of preparing such compositions, which is not as effective as that just described, involves applying both coatings to the magnesium particles simultaneously, whereby a mixed coating results.

According to another embodiment of this method, magnesium particles or nodules, either in the form of an alloy such as magnesium-ferrosilicon alloy or in the form of the pure metal, are sprayed with a heatresistant material e.g. a resin such as shellac.

Fluorspar (CaF) is then blown onto the particles and allowed to dry. A further coating of graphite or a fine silicide such as FeSi or CaSi is then applied e.g. by spraying.

If necessary a further coating of a bonding agent such as shellac can be applied between the two coatings. The resulting coated particles may have a diameter of approximately + inch.

It is to be understood that in the case of a coating such as a coating of fine fluorspar, material not only adheres to the metal or alloy 'core' of the composition, but also provides an adhesive surface to which the second coating material attaches, so forming coherent particles of the composition. Conveniently these particles of the composition have a grading of from less than 50 microns to 4 mm.

Where the coating material is not selfadhesive then a bonding agent is generally employed in order to bind the coating material on to the magnesium particles or, in the case of a second coating, to the first applied coating.

In the case where only the finely divided inorganic compound is applied to the magnesium particles only the first stage of the above described preparation will of course be carried out.

The present invention is now further described with reference to the following actual Examples of the preparation of compositions according to the invention.

Example 1.

A composition was prepared using 99 parts by weight of MgFeSi of I to 5 mm grading and 1 part by weight of Fluorspar W300, of grading not greater than 55 microns, by mixing the two materials thoroughly together.

Example 2.

A composition was prepared using 96 parts by weight of MgFeSi of 1 to 5 mm grading, I part by weight of Fluorspar W300 (grading not greater than 55 microns), and 3 parts by weight of calcium silicide (grading not greater than 100 microns). The MgFeSi and fluorspar were first mixed thoroughly together and then the CaSi was added and the whole composition thoroughly mixed.

Using this material, the following tests were carried out in a nodularising treatment apparatus of the type described in British Patent No. 1,311,093, to observe the reactivity of the alloy addition; and the following results were obtained:

Molten Metal Chamber Flow Flow Nature of the temperature temperature time rate reaction 14800 C 2000 C 27 10 quiet 14800 C 6000 C 26 11.0 quiet 14800 C 1 9000 C 25 12.0 quiet WHAT WE CLAIM IS:- elemental form or in the form of an alloy, 1. A nodularising agent or an innoculant the particles being provided with at least comprising particles of magnesium in one coating which is self-adhering or fixed

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

for example calcium silicide, is then added to the mixer. After further operation thereof until the outer coating has been applied to the inner adhesive layer formed on the metal or alloy grains, the material is discharged from the rotary mixture.

According to one particular embodiment the magnesium is used in the form of an alloy comprising 5% or 9% Mg, 45% Si, with the balance being iron. The grading of this material is 2 mm down. The magnesium alloy is placed in a pelletising machine to which is added fine fluorspar and a bonding agent such as a resin e.g. shellac or a drying oil. The alloy is coated with these materials in the pelletising machine after which the pellet is dried with hot air. A second material in fine form e.g. ferrosilicon, calcium silicide or graphite is added again with a bonding agent and the alloy is coated and dried as before. The resulting pellet has two fixed coatings approximately 0.5 to 1.0 mm thick.

An alternative method of preparing such compositions, which is not as effective as that just described, involves applying both coatings to the magnesium particles simultaneously, whereby a mixed coating results.

According to another embodiment of this method, magnesium particles or nodules, either in the form of an alloy such as magnesium-ferrosilicon alloy or in the form of the pure metal, are sprayed with a heatresistant material e.g. a resin such as shellac.

Fluorspar (CaF) is then blown onto the particles and allowed to dry. A further coating of graphite or a fine silicide such as FeSi or CaSi is then applied e.g. by spraying.

If necessary a further coating of a bonding agent such as shellac can be applied between the two coatings. The resulting coated particles may have a diameter of approximately + inch.

It is to be understood that in the case of a coating such as a coating of fine fluorspar, material not only adheres to the metal or alloy 'core' of the composition, but also provides an adhesive surface to which the second coating material attaches, so forming coherent particles of the composition. Conveniently these particles of the composition have a grading of from less than 50 microns to 4 mm.

Where the coating material is not selfadhesive then a bonding agent is generally employed in order to bind the coating material on to the magnesium particles or, in the case of a second coating, to the first applied coating.

In the case where only the finely divided inorganic compound is applied to the magnesium particles only the first stage of the above described preparation will of course be carried out.

The present invention is now further described with reference to the following actual Examples of the preparation of compositions according to the invention.

Example 1.

A composition was prepared using 99 parts by weight of MgFeSi of I to 5 mm grading and 1 part by weight of Fluorspar W300, of grading not greater than 55 microns, by mixing the two materials thoroughly together.

Example 2.

A composition was prepared using 96 parts by weight of MgFeSi of 1 to 5 mm grading, I part by weight of Fluorspar W300 (grading not greater than 55 microns), and 3 parts by weight of calcium silicide (grading not greater than 100 microns). The MgFeSi and fluorspar were first mixed thoroughly together and then the CaSi was added and the whole composition thoroughly mixed.

Using this material, the following tests were carried out in a nodularising treatment apparatus of the type described in British Patent No. 1,311,093, to observe the reactivity of the alloy addition; and the following results were obtained:

Molten Metal Chamber Flow Flow Nature of the temperature temperature time rate reaction 14800 C 2000 C 27 10 quiet 14800 C 6000 C 26 11.0 quiet 14800 C 1 9000 C 25 12.0 quiet WHAT WE CLAIM IS:- elemental form or in the form of an alloy, 1.A nodularising agent or an innoculant the particles being provided with at least comprising particles of magnesium in one coating which is self-adhering or fixed

to the particles and which coating comprises one or more solid inorganic compounds in finely divided form having a limited solubility in iron and being effective to lower the vapour pressure of magnesium.
2. A nodularising agent or an innoculant as claimed in claim 1, which comprises one coating.
3. A nodularising agent or an innoculant as claimed in claim 2, wherein the coating comprises finely divided fluorspar, selfadhering to the particles.
4. A nodularising agent or an innoculant as claimed in claim 1, which comprises two coatings.
5. A nodularising agent or an innoculant as claimed in claim 4, in which the particles of magnesium or magnesium alloy are provided with a second coating of a material different from a first coating, the second coating being applied to said first coating or applied in admixture with said first coating.
6. A nodularising agent or an innoculant as claimed in any of claims 1 to 5, wherein the solid inorganic compound is a compound of a metal of Group IIA of the Mendeleev Periodic Table.
7. A nodularising agent or an innoculant as claimed in claim 6, in which the solid inorganic compound is a compound of calcium.
8. A nodularising agent or an innoculant as claimed in claim 7 in which the compound is calcium fluoride.
9. A nodularising agent or an innoculant as claimed in claim 8 in which the calcium fluoride is ground to a fineness of not more than 55 microns.
10. A nodularising agent or an innoculant as claimed in any of claims 4 to 9 wherein the second coating comprises a calciumcontaining substance and/or graphite.
I I. A nodularising agent or an innoculant as claimed in any of claims 4 to 9, in which the second coating is formed of calcium silicide or ferrosilicon.
12. A nodularising agent or an innoculant as claimed in claim 1, which comprises a first coating of talc or CaF2 bonded with a resin and a second coating of FeSi or graphite.
13. A nodularising agent or an innoculant as claimed in any of claims I to 12, in which said coating or said first coating constitutes I to 5% by weight of the total composition.
14. A nodularising agent or an innoculant as claimed in any of claims I and 4 to 13, in which said second coating constitutes from 2 to 5% by weight of the total composition.
15. A nodularising agent or an innoculant as claimed in any of claims 1 to 14, containing from 5 to 15% of magnesium based on the total weight of the composition.
16. A nodularising agent or an innoculant as claimed in any of claims 1 to 15, wherein the magnesium is in the form of an Mg-Fe-Si alloy.
17. A nodularising agent or an innoculant as claimed in any of claims 1 to 16, containing from 40 to 65/ó by weight of silicon based on the total weight of the composition.
18. A nodularising agent or an innoculant as claimed in claim 17 in which the silicon is provided either in combination with the magensium when this is used in the form of a magnesium ferrosilicon alloy or as added ferrosilicon when the magnesium is used in elemental form.
19. A nodularising agent or an innoculant as claimed in claim 1 substantially as herein described with reference to the Examples.
20. A method of making a nodularising agent or an innoculant as claimed in claim 1 which comprises applying at least one coating of a solid inorganic compound in finely divided form to particles of magnesium or magnesium-containing alloys.
21. A method as claimed in claim 20, wherein the coating is self-adhered to the particles.
22. A method as claimed in claim 20, including the step of applying a second parti culate material different from that forming the first coating either simultaneously or subsequently thereto.
23. A method as claimed in claim 22 which comprises applying a first coating comprising a finely divided inorganic com pound and a bonding agent, drying the resulting coating and subsequently applying a second coating.
24. A method as claimed in claim 20, substantially as herein described with refer ence to the Examples.
25. A nodularising agent or an innoculant when prepared by a method as claimed in any of claims 20 to 24.
26. A method of nodularising or in noculating iron in which a nodularising agent or an innoculant as claimed in any of claims 1 to 19 and 25 is introduced into molten cast iron.
27. A method as claimed in claim 26 for nodularising grey iron, in which the graphite is in flake form, to provide iron castings in which the graphite is in nodular form, which comprises treating said grey iron with a sufficient quantity of a nodularising agent as claimed in any of claims 1 to 19 and 25, to effect conversion of the grey iron to nodular iron.
28. A method as claimed in claim 26 or 27 substantially as herein described with reference to the Examples.
29. Cast iron treated by a method as claimed in any of claims 26 to 28.
30. Nodular iron made by a method as claimed in claim 27 or 28.