The invention relates to steel slag.

It is generally known to convert iron products obtained from a blast furnace into steel in a converter, for example by blowing through oxygen. During this treatment a steel slag is formed which, however, has various disadvantages for use in practice.

Firstly, mention may be made of the particularly high content of free calcium oxide, as a result of which, when said slag is processed in road building materials, cracks can form in a road surface when the calcium oxide is converted into calcium hydroxide. This consequently leads to accelerated deterioration of a road surface formed using such a slag.

Another disadvantage is the high weight per unit volume of 2,000 kg/m³. Finally, this slag is difficult to process after solidification since said slag must first be broken and screened in order to obtain particles of from 0 to 25 mm in size.

For all of these reasons there has been insufficient interest in steel slag hitherto and the latter can be processed virtually only by mixing with blast furnace slag and granulated blast furnace slag, usually in a composition of 70 % blast furnace slag, 20 % steel slag and 10 % granulated blast furnace slag.

However, as a result of increasing processing of blast furnace slag in the cement industry, a much smaller amount of blast furnace slag is available for processing to a road building composition, as a result of which it is imperative to search for other applications for very large amounts of steel slag.

By way of illustration it may be mentioned that in the case of a relatively large processing unit for the conversion of pig iron into steel, about 450.000 tonnes steelslag comprising about 300,000 tonnes of steel slag having a particle size of less than 25 mm becomes available. The coarser fraction mainly measuring 40-180 mm is used in the hydraulic architecture. From the finer fraction nevertheless, only a minor amount of which can be processed to road building material products by mixing with blast furnace slag and granulated blast furnace slag, or as a gravel replacement in concrete and asphalt.

It is remarked that the finer fraction was used in the past as a fertiliser in the agriculture because of its high phosphor content. Because of the use of richer iron ores, the phosphor content has nevertheless been lowered. Further, the dispersion of the present heavy metals must be restricted in view of environmental measures.

As yet there are no uses for the remainder.

It has now been found that the abovementioned disadvantages of too high concentrations of free calcium oxide in steel slag can be overcome by granulating the steel slag. In this way, a porous granulated steel slag is obtained which has a weight per unit volume of less than 1 kg per dm³ in the loosely dumped state; in particular values of 0.77 kg/dm³ can be achieved, whereas the weight per unit volume in the compacted dumped state is 0.99 kg per dm³.

The free calcium oxide content in a porous granulated steel slag is at most 1/10, preferably 1/50, of the content in the non-granulated slag, more particularly less than 1 % and especially less than 0.2 %.

In particular, in a steel slag the free calcium oxide content is reduced on granulating to a porous granulated steel slag from about 5 to 6 % to 0.1 %.

Moreover, the porous granulated steel slag obtained can be made more valuable by magnetic removal of the iron from the granulated slags. This technique is known per se.

The slags obtained after removal of iron can then, surprisingly, easily be separated into two fractions after fine grinding: a first fraction with a higher ferrite content and a second fraction with a lower ferrite content. This separation is also carried out magnetically. The fraction with the higher ferrite content can be used again in this form in the blast furnace for the production of pig iron. The second fraction with the lower ferrite content can particularly advantageously be used for complete or partial replacement of cement since it has a Ca/Si ratio which is advantageous for this purpose.

Granulating steel slag therefore leads to the following advantages:

• a) the weight per unit volume of the granulated porous steel slag product can be made much lower than 1.
• b) the chemical composition of the granulated porous steel slag is greatly improved by a much lower free calcium oxide content.
• c) Moreover, by further removal of iron from the granulated steel slag it is possible, on the one hand, to obtain a fraction which is lower in ferrites but has a Ca/Si ratio advantageous for use as an inorganic binder and, on the other hand, a fraction which is richer in ferrites and can be used as such for steel production.

By converting steel slag into a porous granulated steel slag it is therefore possible to use this type of slag as embankment materials, in which context particular mention may then be made of the characteristic of a much lower density than that of water, which can be obtained by perfect granulation, as a result of which the embankment material can float on water, and the low weight per unit volume. This porous granulated steel slag is also very suitable as road building material and as binder for partial or complete replacement of cement.

The porous granulated steel slag according to the invention can be obtained, in particular, by spraying a molten stream of steel slag with a sprayed pressurized atomised stream of water as a result of which the slag is hit apart. The amount of water is determined empirically and is usually about 4-8 tonnes of water per tonne of molten steel slag composition. The same effect can be obtained by means of a rotating drum wherefrom water is quirt to the outside.

It is pointed out that granulation of blast furnace slags obtained in iron production in blast furnaces is known per se, but in this case the density of the blast furnace slag falls from 1,650 to 1,000 kg/m³, whereas in the case of steel slag the density surprisingly can fall from 2,100 to as low as 770 kg/m³.

It is also pointed out that quenching liquid steel slags to form granules is known per se from DE-A-3,609,568. However, in this case quenching is carried out by feeding the slag stream into an amount of water and not by spraying the liquid slags with a pressurized atomised stream of water. By spraying with an atomised pressurized stream of water, on the other hand, an appreciable lowering in the density of the steel slags is obtained, which effect cannot be obtained by quenching the slags in an amount of water. Moreover, according to this known method it is necessary, for use as cement, finely to grind the slag granules together with an amount of gypsum and/or anhydrite. The material obtained in this way acts as an activator in, for example, blast furnace cement.

It is also known that the presence of gypsum in cement leads to the formation of ettringite. However, set concrete which contains ettringite can show cracking if it comes into contact with sulphate-containing water, as a result of expansion of the ettringite, so that it is highly desirable to restrict the amount of gypsum present in a cement mixture. The invention meets this aim.

Particularly advantageously, the invention relates to a steel slag composition which is characterised in that a porous granulated steel slag is converted into comminuted form, for example by grinding, as a result of which the steel slag can easily be separated into three fractions which are valuable per se, as explained above.

It is pointed out that, according to the invention, grinding is preferably carried out in the absence of substances which modify the lime content. Because of the Ca/Si ratio which exists in the treated steel slags, extra addition of lime-containing substances in order to express the latent binding properties appears completely superfluous.

It has been found that when steel slags such as are obtained according to the invention are used as hydraulically setting binders, the conventional addition of gypsum to the mixture to be set can be dispensed with, or at least can be appreciably lower than is usual. The reason is that this addition of gypsum was made in practice in order to obtain a slower onset of setting and a harder end product. However, this effect is also obtained with the steel slags according to the invention, without modification of the composition thereof.

Ground, porous granulated steel slags can be processed easily in building materials such as sandlime brick, cellular concrete and normal concrete, as gravel-replacement material, and are also suitable as raw material for embankment materials because of the large volume and the low density, and, in particular because of the favourable Ca/Si ratio, as a constituent for cement. Moreover, when they are used as a constituent of cement savings can be made in respect of the required amount of Portland cement clinker or blast furnace granules. Granulated steel slags also have the advantage that the amount of grinding energy required for cement preparation is appreciably lower, as normal air cooled steel slag.

The invention therefore also relates to the use of porous granulated steel slag, optionally in comminuted form, as an aggregate in building materials, as a raw material for embankment materials and as a raw material in an inorganic binder such as cement.

A particularly advantageous application is the use of granulated porous steel slag, optionally in comminuted form, as raw material for road building materials.

Particularly advantageously a porous granulated steel slag according to the invention, optionally in comminuted form, is suitable as raw material for road building materials. In this case the ground porous granulated steel slag products serve as finely gradated aggregate for asphalt and concrete. Aggregates are, for example, indispensable in an asphalt mixture in connection with good matrix structure in the fine particle size range and in order to obtain good solidification of the bitumen in order to ensure good adhesion. Lime-like substances, fly ashes or dust removal residues are frequently used as aggregates, but the problem of secondary raw materials is, however, that the quality is not constant and in particular fly ashes are less suitable because of the relatively high temperature in the electric power plants, as a result of which they have become more spherical and glassy.

Porous granulated steel slag in comminuted form does not have these disadvantages, as a result of which products which have a continuous particle size distribution, and are therefore of constant quality, can be obtained from these slags.

Moreover, as a result of the porous characteristics of granulated steel slag there is very good adhesion between bitumen and the said steel slag particles. This, of course, also applies in the case of the use of other binders, such as in the building materials.

The invention therefore also relates to comminuted, in particular ground, steel slags, the comminution having been carried out in the absence of substances which modify the amount of lime. The Ca/Si ratio inherently present in the material is therefore kept essentially constant during comminution.

Finally, the invention relates to building material products obtained using a porous granulated steel slag, optionally in comminuted form, which is incorporated as aggregate in the building materials.

The invention will now be illustrated with the aid of a few illustrative embodiments.

EXAMPLE I

Steel slag originating from a steel converter is ground and the iron is removed with the aid of a magnet. After removal of the iron, a composition having the following screen analysis is obtained

0.063 mm

51.1%

0.063-0.125 mm

29.1%

0.125-0.25 mm

14.7%

0.25-0.5 mm

2.3%

0.5-1 mm

0.9%

1-2 mm

0.5%

2-4 mm

1.4%

The composition of the slag can be seen from the analysis figures shown in the table.

The slag is melted and then granulated by spraying with a pressurized water mist obtained by means of nozzles.

The amount of pressurized water sprayed on is about 7 tonnes per tonne of liquid steel slag composition.

To remove water adhering to the porous granulated slag thus obtained, the composition is subjected to rotation in a perforated drum.

In this operation a porous granulated slag is obtained which has a weight per unit volume of 0.77 kg/dm³ in the loosely dumped state and of 0.99 kg/dm³ in the firm compacted state.

This granulated slag is found to have a much lower free CaO content than the non-granulated slag, as can be seen from the table.

EXAMPLE II

Porous granulated steel slag as obtained according to Example I is processed, after crushing, in a composition for forming a bitumen road surface.

As a result of the low free calcium oxide content in the porous granulated steel slag, the road surface obtained has a particularly long life since no cracks form as a result of absorption of water by calcium oxide with the formation of calcium hydroxide.

EXAMPLE III

Porous granulated steel slag according to Example I is finely ground to a particle size of about 63 µ. The iron present in this finely ground product is separated off magnetically and the finely ground product is then incorporated as aggregate in a bitumen composition for forming a road surface.

Very good adhesion between bitumen and ground steel slag particles is obtained as a result of the porous characteristics of said steel slag particles.

When a road surface of this type is used, no cracks occur as a reaction between water and free calcium oxide because of the low content of the said compound in porous granulated steel slag according to the invention.

EXAMPLE IV

The porous granulated steel slag according to Example I is ground to a particle size of about 63 µ. The iron is first removed from the steel slag finely ground in this way, using a magnetic field.

The resulting steel slag from which the iron has been removed is then introduced into a stronger magnetic field and by this means, on the one hand, a fraction which is richer in ferrites and, on the other hand, a fraction which is lower in ferrites are obtained.

The ferrite-richer fraction is recycled to the blast furnace, in order to replace iron ore.

The lower-ferrite fraction is granulated using an aqueous binder to form granules on a granulating tray or by a sintering process and the granules are then hardened to give gravel-replacement material.

EXAMPLE V

Sandlime brick is formed by incorporating 20 % of the porous granulated steel slag according to Example I, which has been finely ground to a particle size of 63 µ, in the composition to be used for such a sandlime brick.

The characteristics of such sandlime brick are the same as those of normal sandlime brick.

EXAMPLE VI

Cellular concrete is formed by incorporating finely ground porous granulated steel slag according to Example I in the concrete mixture.

The building product obtained, in the form of a tile, has the same characteristics as concrete products obtained using ground normal blast furnace slags.

EXAMPLE VII

Porous granulated blast furnace slag according to Example I is used as an embankment material for raising a ground surface.

Because of the low weight per unit volume the granulated steel slag according to the invention does not sink away into a soft substrate or even a body of water. Consequently a ground surface can be brought to the desired height very successfully.

EXAMPLE VIII

The lower-ferrite fraction obtained according to Example IV is used as cement fraction, to replace Portland cement clinker or blast furnace granules, and a self-setting cement is obtained which has the same characteristics as Portland cement or blast furnace cement, respectively.

Replacement of the lower-ferrite fraction by an amount of porous granulated steel slag gave comparable results.

On the other hand, although replacement of the amount of porous granulated steel slags by air-cooled and finely ground steel slags yielded a cement having a somewhat slower onset of setting, the product obtained after hardening for 28 days amply met the values specified for use as cement in respect of bending strength under tension and compression strength.

It is pointed out that the use of porous granulated steel slags as a cement constituent is economically advantageous because the grinding energy required for grinding to cement fineness can be appreciably restricted.