US3326273A - Exothermic hot top - Google Patents

Description

United States Patent 3,326,273 EXOTHERMIC HOT TOP Edward John Jago, Ber-ea, and Ronald W. Ruddle, Rocky River, Ghio, assignors to Foseco International Limited, Birmingham, England, a British company No Drawing. Filed Dec. 28, 1965, Ser. No. 517,132 Claims. (Cl. 164349) This invention relates to the provision of linings for moulds used to make ingots or castings from molten metal, and for hot tops, risers and the like used with such moulds. It further relates to the new compositions which are employed in the production of such linings.

In the production of ingots and castings from molten metal it is necessary to provide that molten metal may feed to the body of the ingot or casting to compensate from the shrinkage which occurs on cooling since otherwise the ingot or casting may be formed with internal cavities or fissures. The usual method is to provide that the solidification of the head metal in an ingot mould, or in a hot top provided thereon, or in the risers and feeder heads of a casting mould is delayed, so providing a reservoir of molten metal which may feed to the ingot or casting proper. This delay may be achieved by setting up a barrier to the loss of heat from the head metal by lining the head of the ingot mould or the hot top, risers, feeder heads and the like with a refractory heat-insulating composition, or by using a composition of which the ingredients are ignited by the heat of the molten metal to react exothermically.

In recent years there have come into use, for the production of linings for the inner molten-metal-contacting surfaces of metal casting moulds, or of a hot top for such a mould, shaped bodies or linings made of castable compositions which contain predominantly a refractory filler material, usually with minor amounts of an organic fibrous material and of a binding medium. The organic fibrous material is usually a paper pulp, e.g. rep-ulped old newsprint. The binding medium may be based on any of a wide variety of materials, e.g. a natural or synthetic resin or glue, elg. a silicone resin, ureaor phenol-formaldehyde resin, a cellulose glue, sulphite lye, or sodium-silicate.

The refractory material used in the said compositions is generally a siliceous material such as sand, quartz, quartzite, inorganic silicate, or may be a material such as dolomite. The refractory material may also include a fibrous refractory, e.g. asbestos, glass fibre, or rock wool.

It has now been discovered that valuable advantages can be obtained by including in compositions as aforesaid, in replacement of the whole or part of the refractory material, ballmill dust as hereinafter defined.

According to the present invention, therefore, there is provided a composition suitable for lining the inner surface of a casting mould for casting metal or of a hot top for such a mould which comprises a predominant amount (i.e. at least about 50%) of ballmill dust, advantageously together with about 2 to 30% by weight of organic fibrous material, about 1 to by weight of a binding medium and optionally from about 1% up to 10% fibrous refractory material. Preferred compositions are those containing 78 to 94% ballmill dust, 3 to 9% of organic fibrous material and 1 to 8% of binding agent. The organic material and the binding agent may be any of those referred to above or mixtures thereof.

It is found that important advantages flow from the use of ballmill dust. It is very cheap material and since it contains a relatively large percentage of alumina it is substantially as effective a refractory as silica or silicate, but presents no silicosis hazard.

Moreover, since it contains finely divided aluminum metal this tends to burn exothermically when linings of the composition are contacted by the molten mould metal. The compositions being thus self-heating, they tend to improve the feed characteristics of the molten metal with which they are used. It is to be noted in this connection that various ingredients present in ballmill dust, e.g. halide salts, may tend to facilitate the combustion of the aluminum.

A particular advantage of the hot tops or risers of the invention is that they are highly exothermic without requiring the presence of chemical oxidizing agents. Heretofore it has been the practice when formulating aluminumcontaining exothermic compositions to include substantial amounts (e.g. 5 to 30%) of an oxidizing agent such as iron oxide, manganese dioxide, or an inorganic nitrate or chlorate. The present invention, by eliminating the need for such agents, avoids the possibility of iron or manganese altering the molten metal composition, and reduces moisture absorption caused by deliquescent nitrates or chlorates.

In the method, products and compositions of this invention, use is made of the material known commercially and industrially as ballmill dust.

Ballmill dust is obtained from the skimming and drosses formed during the metling of aluminum and aluminum alloys in an oxygen-containing atmosphere. Usually the skimmings and drosses pass to the secondary melters for pulverizing by ballmilling or grinding. In some cases the dross may need to be reduced in size in a jawcrusher but generally it is sufliciently fine for ballmilling Without any pretreatment. After ballmilling it is usual to screen the residue. The coarse material (normally +10 or +16 mesh) contains most of the metallic aluminum and is removed for remelting. The fine material, which is called ballmill dust, may be washed by the producer in order to remove water-soluble salts.

The dross usually is composed mainly of aluminum oxide (resulting from the oxidation of the molten metal) and particles of aluminum or aluminum alloy, together with a few percent each of metallic contaminants such as copper, silicon, iron, zinc, magnesium, and/ or their compounds. Some silica is generally present, as are fluorides and chlorides of sodium, potassium, and/or other metals (from fiuxing ingredients and their various reaction products). Aluminum nitride is also usually present, resulting from the reaction between aluminum and atmospheric nitrogen.

Generally the fluxes used with aluminum or mixtures containing one or more of the following components: sodium fluoride, sodium chloride, sodium sulphate, potassium chloride and cryolite.

The ballmill dust may contain up to 50% sodium chloride and values of 10 to 15% total fluorides (Water-soluble and water-insoluble) have been noted.

Sodium aluminate, sodium carbonate and the oxides of the alloying elements are also often found.

Generally speaking the less developed the aluminum industry in a particular country, the higher quality the ballmill dust available in it, e.g. there is a considerable quantity of ballmill dust containing up to 40% aluminum available in Spain. This is due to both the limited use of fluxes, leading to higher aluminum contents and low chloride and fluoride con-tent of the dusts, and to restricted refining capacity. "In the United Kingdom any ballmill dust is refined to extract aluminum metal if its metal content exceeds 30%. By comparison, ballmill dusts containing over 60% metallic aluminum are by no means uncommon in other European countries.

The residual aluminum content of ballmill dust depends therefore on the source and on the type of processing it receives but normally is between 10 and 30%. It may however contain as little as 5 or as much as 60 or 70% metallic aluminum. For optimal exothermic performance when pouring ferrous metals, it is preferred that the ballmill dust contain from about 5 to about 45 weight percent aluminum metal (e.g. about to and accordingly it may in some instances be desirable to fortify aluminumlean dust with blown or ground aluminum metal. With non-ferrous metal casting, a higher aluminum content may be desirable.

It is to be understood that the term ballmill dust used herein means a product as thus defined.

The composition of the ballmill dust preferably used in the practice of the invention may vary widely, e.g. within the ranges shown below:

Percent Aluminum 5 to 70 Aluminum oxide 15 to 60 Zinc 0 to 5 Chloride 0 to lead 0 to 1 Most preferably, however, the composition contains approximately 15-45% aluminum, about 1% chloride and not much more than 0.1% each of zinc and lead. A minimum aluminum content of 15% seems to provide a suificiently intensive reaction to make feeding of steel castings efficient in riser sizes down to about a 3 inch diameter. However, improved efficiency can beobtained by using higher aluminum contents in the feeding of both ferrous and non-ferrous alloy castings. Generally speaking, the advantage of higher aluminum contents is most marked with the smaller diameter risers, but advantage is to be expected with all sizes.

Impurities such as zinc, chloride, and lead are detrimental in that they cause the evolution of excessive or toxic fumes during combustion of the product. They do not eifect the resultant castings however. The presence of some fluoride is beneficial since it increases the sensitivity of the mixt-ure; for example, the addition of 2% sodium fluoride /2% in the finished product) results in improved burning especially on small diameter risers.

The 'ballmill dust should preferably not contain large particles of dross since, if included, these tend to increase the density of the final product, and reduce the efficiency of the aluminum combustion.

Typical component and sieve analyses of ballmill dust are given in the table below. The samples are identified as follows:

Available in United Kingdom.

TAB LE I.TYPICAL ANALYSIS OF BALLMILL DUSTS Sample A B O D E Composition, wt.

percent:

A 22. 34 10. 00 31. 00 9. 40 0. 75 0.50 1. 00 0.50 0. 10 0. 10 0. 10 0. 10 1.30 0. 10 1. 70 0. 45. 25 24. 35. 28 41. 43 5. 50 2. 0O 2. 62 8. 96 0. 0. 21 0.98 0. 35 1. 15 1. 89 4. 28 0. 89 0. 13 Nil 2 86 2. 83 15. 40 3. 01 0.70 0.80 1. 4. 00 0.75 0.75 0.77 0. 98 0. 55 O. 50 0. 39 0. 64 1. 73 48. 9 2. 34 14. 76 0. 10 2. 5O 0. 30 1. G6 0. 02 0. 10 0. 63 3. 54 1. 16 7.25 Trace Truce TABLE II.-TYPICAL SIEVE ANALYSIS OF BALLMILL DUSTS Sample A l B l o Sieve Analysis:

Ballmill dust particle size distribution depends both on the extent of grinding and on the screens used to recover the aluminum. It is preferred however that more than half, optimally more than 95%, of the dust pass a 10 mesh screen, and optimally more than pass a 20 mesh screen.

The compositions of this invention are preferably preformed as slabs or sleeves for use in lining the head of an ingot or casting mould or in lining a hot top for a metal casting mould.

An especially useful shape in the practice of this invention is that of a slightly coned sleeve having at its narrower end a wall across it provided with a vent, substantially the same shape as, for example, an inverted flowerpot. In normal foundry practice it is customary, when using exothermic riser sleeves or sleeves of refractory insulating material, to cover the top layer of molten metal, after pouring, with exothermic or refractory heat-insulating powder. This method has several disadvantages, such as inhomogeneity and inconvenience to the user, and heat loss due to the splitting of the powder layer during cooling. However, fabrication of sleeves in the shape indicated above eliminates the need for powder, increases the insulation of the top metal and makes the casting operation easier. In addition, sleeves thus formed are stronger and easier to store.

The slabs or sleeves may conveniently be formed by a slurry technique as follows: The ingredients of the composition are made up to an aqueous slurry. Advantageously, a small quantity of a surfactant known per se is included in the composition to facilitate this. The slurry is charged into a vessel having a perforate wall or walls and pressure or vacuum is applied to cause the slurry to be urged against the perforate walls. The liquid me dium of the slurry passes through the perforate walls as eflluent and the solid constituents are compacted against the perforate walls as layers of desired thickness.

One of the most convenient methods of effecting this process when making sleeves is to spin a porous mould containing the slurry at high speed, thereby to drive the Water from the slurry; the mode of action is that of a normal domestic spin-dryer. By this method, the internal diameter of the sleeve formed may only be controlled by the amount of slurry added to the mould, and the solids content thereof. In this connection the solids content of the slurry is preferably to 50%, most preferably 30 to 35%. Typical spinning times and speeds are 1200 to 1400 r.p.m. for 1.5 to 3 minutes. The compacted sleeves may be withdrawn from the mould, and are fairly easily handleable.

As indicated above, vacuum forming processes for slabs and sleeves are also practical. However, economic considerations usually limit the use of this method to small slabs or sleeves. Water extraction times tend to be longer than with spin-forming. It is preferred to use a slurry of to solids content, lower concentrations of solids requiring longer water extraction times. This method is of great value where, for example, the internal diameter of a sleeve must be formed to close dimensional tolerances.

Before use the slabs or sleeves must be dried. This is generally effected by drying on vented core plates in normal ovens through which air is passed.

It must be appreciated that where the foregoing technique is employed any binder which is soluble in the liquid medium will be largely lost in the effluent (from which if desired it may be recovered) and it is therefore necessary to employ sufiicient binder to ensure that enough is present in the liquid which is retained by the compacted solid contsit-uents to provide the necessary composition as earlier described. However, a water-insoluble, thermo-softening binder may be employed in solid powder form (e.g. phenolic resins in a suitable state of polymerization). In such case losses with the efiluent will be negligible. A mixture of insoluble and soluble binders may be employed.

Further, it is desirable that the ballmill dust used contain not more than 50 to 60% of material of particle size 200 mesh (Tyler) since slurries made using dust with these or even higher fine dust contents tend to be nonporous and tend to require longer spinning or compacting to extract the water. Where long water extraction times are acceptable, however, materials containing up to 85%, or over, of 200 mesh particles may be used.

Permeability in formulations containing unwashed ballmill dust may be increased by incorporating a small amount, e.g. 1 to 5% of Wood flour in the mix. This amount brings the permeability to at least about 4 or 5 A.F.S. units in order to avoid blowing during metal pouring. The preferred range of permeabilities is about 812 A.F.S. units.

The following specific example will serve to illustrate the invention:

Example The following Were mixed:

Parts by wt.

Ballmill dust 87.70 Urea formaldehyde resin solids 1.25 Phenol formaldehyde resin solids 2.50 Asbestos fibre 1.75 Cellulose fibre (scrap newsprint) 6.50 Surfactant (Arquad 2C-75, a dialkyl quaternary ammonium salt) 0.30

and water added to give a slurry of solids content approximately 33%. This slurry was formed into slabs and sleeves of varying sizes, and these were then dried in ovens. The sleeves were used to line riser cavities in the manufacture of said castings and gave excellent feed down to 2 inch internal diameter risers, with the formation of much reduced pipe, as compared with the use of prior refractory heat-insulating materials.

A similar formulation which included 0.5 by weight of sodium fluoride (replacing 0.5% of ballmill dust) gave adequate feed in risers down to 3 inch internal diameter. In like manner, calcium fluoride or cryolite may be used, each in amounts of 0.1 to 2%.

A major advantage of the invention is that the exothermic refractory of the invention becomes a relatively soft, frangible material after completion of the exothermic reaction. It thus may easily be removed from the solidified metal in the mould cavity, allowing facile recovery of the riser metal and clean rolling of hot topped ingots.

Additionally, the compositions are essentially nonsmoking, and may be used without discomfort to near-by workers.

We claim as our invention:

1. As a new article of manufacture, an exothermic hot top, riser, or the like having a molten-metal-contacting surface comprising a shaped body composed dross obtained by the atmospheric oxidation of molten aluminum metal, said dross having a composition comprising from about 5 to about weight percent aluminum metal and from about 15 to about 60 weight percent aluminum oxide and being of a particle size which will pass through a 10 mesh screen, said shaped body being substantially free of oxidizing agents and from about 2 to about 30 weight percent of an organic fibrous material.

2. The article of claim 1 wherein said shaped body contains from about 1 to about 10 Weight percent of an inorganic fibrous material.

3. The article of claim 1 wherein said shaped body contains from about 1 to about 10 Weight percent of a binding agent.

4. The article of claim 1 wherein said shaped body contains from about 84 to about 92 weight percent of said finely pulverized dross.

5. The article of claim 1 wherein said shaped body comprises about 78 to 94 weight percent of said dross, about 3 to 9% of organic fibrous material, and about 1 to 8% of binding agent, wherein all of said dross passes through a 20 mesh screen and not more than about 60% passes through a 200 mesh screen, and wherein said shaped body has an A.F.S. porosity of at least about 4.

References Cited UNITED STATES PATENTS 2,390,500 12/ 1945 Charman et a1. 22-1 2,500,097 3/1950 Soifel 221 2,891,293 6/1959 Forsythe 221 3,039,158 6/1962 Mueller 221 3,171,173 3/1965 Ingala 24962 3,212,749 10/1965 Labate 249- 200 I. SPENCER OVERHOLSER, Primary Examiner. E. MAR, Assistant Examiner.

Claims (1)

1. AS A NEW ARTICLE OF MANUFACTURE, AN EXOTHERMIC HOT TOP, RISER, OR THE LIKE HAVING A MOLTEN-METAL-CONTACTING SURFACE COMPRISING A SHAPED BODY COMPOSED DROSS OBTAINED BY THE ATMOSPHERIC OXIDATION OF MOLTEN ALUMINUM METAL, SAID DROSS HAVING A COMPOSITION COMPRISING FROM ABOUT 5 TO ABOUT 70 WEIGHT PERCENT ALUMINUM OXIDE AND BEING OF A PARTICLE SIZE WHICH WILL PASS THROUGH A 10 MESH SCREEN, SAID SHAPED BODY BEING SUBSTANTIALLY FREE OF OXIDIZING AGENTS AND FROM ABOUT 2 TO ABOUT 30 WEIGHT PERCENT OF AN ORGANIC FIBROUS MATERIAL.