This application relates to the treatment of steelmaking slag to produce a product suitable for use as construction material.

BACKGROUND OF INVENTION Steelmaking slag, a by-product of steel production, can be used for a variety of high quality construction materials. For example, it as been used as a premium aggregate in road surfacing, specifically as a component of asphalt concrete. Its high strength and excellent skidding resistance make it a particularly suitable aggregate for this high-demand application. However, in-service hydration of excessive"free"or "unbound"lime (CaO) in the steelmaking slag aggregate can cause the aggregate to swell, which leads to premature deterioration of the road surface. This swelling phenomenon has also been observed in steelmaking slag when used for other construction applications, such as backfill and portland cement substitute.

It is therefore an object of the invention to provide a treated steelmaking slag product with which this problem is substantially reduced.

SUMMARY OF INVENTION According to the present invention, oxidic material is mixed with molten steelmaking slag containing free lime to modify slag chemistry and mineralogy to substantially eliminate the free lime and

make the final product a volumetrically stable construction material.

The nature of the oxide addition depends on the initial composition of the steelmaking slag and the desired application of the final product.

The oxidic material may be added before, during or after tapping the slag from a steelmaking vessel.

The preferred additive is fayalite slag with which no external heat source may be necessary. Fayalite slag consists mainly of iron silicate and is a waste product from the smelting of non-ferrous metals such as nickel and copper.

The steel and construction industries are at odds over lime requirements of the steelmaking slag. Steelmakers require a high lime content in the slag for effective impurity removal, while users of the by- product in the construction industry demand a low effective content of lime to control volumetric instability of their fabrications. The source of the"free"lime in the slag is twofold: (a) undissolved lime flux, and/or (b) lime precipitated from the melt during solidification and subsequent solid-state reactions.

The addition of fayalite slag, a by-product of the smelting of non-ferrous products, reduces the amount of"free"lime in the modified slag. As fayalite slag is composed primarily of silica and iron oxide, it is closest in composition to the"early"slag of oxygen steelmaking process which, based on fundamental scientific principles and previous research, is the best additive to eliminate the undissolved free lime, i. e. (a) above.

In addition, the silica of the fayalite slag is an acid slag component, which lowers the tendency for formation of precipitated free lime, i. e.

(b) above.

However, other additions instead of, and in addition to, fayalite slag can be made depending on the desired properties of the

final product. Additional heat, if any, that may be required to incorporate these additions can be supplied by external means, such as electrical heating.

The present invention can be used immediately in steel mills to create volumetrically stable construction material, such as aggregates.

Modified slag of desired chemical composition and volumetric stability in accordance with the invention will be the base for the development of new high-performance cement in the near future.

The present invention is more advantageous than present technologies for a number of reasons: -Unmodified steelmaking slag, in general, cannot be used as volumetrically stable construction material at the present time.

Chemical modification is a practical means of production and quality control to convert an industrial waste to a valuable product.

-As modifications can be performed after-tap, steelmakers do not have to compromise their current slag practices.

-Additives such as fayalite slag are themselves"waste"by- products of the metals industry with few re-use applications.

-The addition of fayalite slag (when used as the oxidic material) reduces the liquidus temperature of the melt, which makes extra superheat available for dissolution and incorporation of free lime.

-This method is more advantageous than traditional moisture aging and steam aging to control volumetric instability as these methods are likely to produce fines in excess for use as aggregate.

There is a known process, developed in Germany, in which silica sand is injected into slag for high temperature modification for reuse as a construction aggregate. The use of fayalite slag has many advantages over use of silica sand, including lowering the overall liquidus temperature of the melt, not requiring oxygen to be introduced for additional heat, and the fact that fayalite slag is itself another waste by-product.

DESCRIPTION OF THE DRAWINGS Specific examples of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Fig. 1 (a) shows the linear expansion results of control slag during a trial, with coarse gradation.

Fig. l (b) shows linear expansion results with fayalite-modified slag in the trial, with coarse gradation, Fig. 2 (a) shows linear expansion results of control slag in the trial, with coarse plus fine gradation, Fig. 2 (b) shows linear expansion results with fayalite- modified slag in the trial, with coarse plus fine gradation, and Fig. 3 shows linear expansion results of control and fayalite- modified slag in another trial, with coarse gradation.

DESCRIPTION OF EXAMPLES Example 1 High-Temperature Modification of Steelmaking Slag by Addition of Fayalite Slag to Control Volumetric Stability of Slag Aggregate A trial was carried out at Stelco Hilton Works in Ontario, Canada using fayalite slag (FS) from Falconbridge Ltd. (also in Ontario, Canada) as a high-temperature addition to steelmaking slag (SS) produced in Stelco's 136 tonne BOFs (Basic Oxygen Furnaces).

Initially, a reference SS sample, referred to as the"control slag", was tapped from a furnace into a slag pot during the first turndown of the steelmaking vessel. The vessel was tipped to allow personnel to determine the melt temperature and take both metal and slag samples.

Approximately 3 to 7 tonnes of slag were poured into a control slag pot at this time.

Approximately 1 tonne of FS was placed into a slag pot prior to tapping of the SS from a BOF. After tapping liquid steel from the tap-hole, the vessel was tipped onto its other side and SS was poured through the vessel's mouth into the FS-laden slag pot. Between 8 to 15 tonnes of SS were tapped onto the one tonne of FS. This amount of FS resulted in a range of addition of from about 6 to about 11 percent. The exact percentage was indeterminable due to errors in weight estimation and composition of the materials involved. Both slag pots, containing

the control slag and the FS modified slag, were taken to the slag yard. In normal operations, the still largely molten slag will be poured out of the slag pot into slag pits, where the slag would solidify and be made available for crushing and separation of metallics. For this trial, the two pots were moved to an isolated region of the slag yard, where the slag was allowed to solidify in the pots over a week-long period. The pots were then inverted so that the slag fell out, more or less as intact domes.

Slag samples were systemically sampled from different regions of the slag domes. The samples were then tested and compared to identify the extent of addition incorporation. In total, over 500 kg of slag was sampled for each trial. The slag was crushed and sieved to appropriate sizes for the two different aggregates gradations commonly used by pavement contractors, which will be referred to as (a) coarse gradation, and (b) coarse plus fine gradation. Particle size for the coarse gradation of aggregates ranges from about-16mm to about +4.75mm, while the coarse plus fine gradation ranges from about-16mm to about + 75, um.

The industry accepted accelerated test method of measuring volumetric stability of aggregates is the ASTM D4785-88 Standard Test Method for Potential Expansion of Aggregate From Hydraulic Reactions. In accordance with this test method, from 5 to 7 kg of steel

slag aggregate (SSA) was compacted into a proctor mould (which is essentially a cylinder closed at one end). A 2.2 kg piston was placed on the top, and the whole assembly was submerged in water at 71 ° C for a number of days. The only direction that an aggregate can swell (if any swelling occurs) is upward, and the linear measure of this upward expansion as a percentage is termed the linear expansion. Currently, the unofficial acceptance standard for coarse gradation is 1% linear expansion or less.

Linear expansion testing was performed twice on each gradation of slag from each region sampled. In addition, chemical analysis was performed on three samples from each region sampled.

Selected samples were also retained for microstructural investigation.

The linear expansion results of the trial are shown in Figs. 1 (a) and (b) for coarse gradation and Figs. 2 (a) and (b) for coarse plus fine gradation, where (a) and (b) are results for the control slag and for the FS-modified slag respectively. The ranges of these results are summarized in Table 1.

Table 1 Gradation Type of SSA Type Control FS-Modified Coarse 5 to 7 % 0.2 to 0. 6 % Coarse + Fine 8 to 10 % 0.4 to 2 %

The two highest curves in each of the FS-modified slag Figs. l (b) and 2 (b) have been excluded from the ranges in the table. The slag from these sources originate from the bottom centre location in the slag pot. Both observation and chemical analysis confirmed that slag from this region did not benefit from full incorporation of FS. An appreciable amount of undissolved FS was found in this region. Thus. it was concluded that this region did not benefit from FS addition and is not indicative of FS-modified SSA.

For at least the coarse gradation, linear expansion of the slag has been reduced by use of the invention from an unacceptable range (5 to 7%) to an acceptable one (0.2 to 0.6%). Although the results for the coarse plus fine gradation (0.4 to 2%) do not fall under the 1% limit, they show great improvement and promise for further trials. In any event, linear expansion results for coarse plus fine gradations are usually greater for most aggregates compared to coarse gradation alone.

Improved methods of FS addition and mixing may be used.

Such methods include injection technologies, vortex ring mixing, controlled charging into the molten tap stream, and alternate charging of solids and pouring of liquids.

Industrial trials in the steel making shop, based on the understanding that fayalite slag addition to molten steelmaking slag

promotes dissolution of free lime flux and suppresses the formation of free lime by solid state reactions, confirm the following: 1. FS is an effective additive to improve hydraulic volumetric stability of the modified slag aggregate.

2. FS modified aggregate falls within unofficial industry acceptance standards.

Example 2 High-Temperature Modification of Steelmaking Slag by Different Method of Fayalite Slag Addition.

In Example 1, inhomogeneous linear expansion resulted between modified slag from different sampling regions within the slag pot due to incomplete mixing of the fayalite slag addition. The industrial trial in this example was devised to more thoroughly mix the fayalite slag addition within the steelmaking slag. The fayalite slag was added to the steelmaking vessel after the steel was tapped, but before the slag was poured into the slag pot. The mixing took place during pouring from the steelmaking vessel to the slag pot. This modified slag was sampled and compared to a control slag taken during turndown as

described in Example 1. The 10-day linear expansion results for the coarse aggregate gradation are shown in Table 2 and Fig. 3. Six samples of modified siag taken from all regions of the slag pot were found to exhibit linear expansion results of less than 0.5% over ten days, which is below the unofficial acceptance standard of 1%, while the control slag expanded over 4.5% in the same period.

Table 2 SSATypeof ControlModified 4.50.26to0.48%4.7% Example 3 High Temperature Modification of Steelmaking Slag to create a High Performance"Super"Cement Steelmaking slag (SS) is very similar to portland cement (PC) in both chemical composition and mineralogy. Tables 3 and 4 show respective comparisons of the major chemical and mineralogical components respectively of SS and PC.

Table 3

Chemical Major chemical component (weight percent) Constituent Steelmaking slag Portland Cement CaO 4045 63-67 Si02 12-14 21-24 4-7Al2O32-5 2-4FenO20-30 1-2MgO8-10 P20511 MnO4<1 Table 4 Mineral Phase Present (Percent) Phase Cement nomenclature Steelmaking Slag Portland Cement Tricalcium silicate C3S 0-35 49 Dicalcium silicate C2S 20-50 25 Tetracalcium C4AF 15-30 8 aluminoferrite Tricalcium aluminate C3A 0 12 Magnesio-manganese-RO 25-40 0 calcio-wustite As shown in Table 4, SS may contain three of the four cementitious mineral phases in portland cement and does exhibit pozzolonic characteristics. Steelmaking slag, taken as-is from slag yards and ground to a fine mesh size, has been successfully utilized as a portland cement substitute in China. However, this application is limited to lower grade cements. The principal reason for this is the presence of free lime in the SS. High temperature modification of SS by

additions in accordance with the present invention can substantially eliminate such free lime.

The present invention is capable of producing a new series of cementitious products of higher performance than PC as well has matching its chemistry, mineralogy and characteristics. In PC production, iron oxide contents above 5 weight percent are detrimental due to the limitations with rotary kiln operation, where only a fraction of the components are liquid. Molten steelmaking slag is entirely a liquid silicate system with a higher iron oxide content. This higher oxide content results in a cement with high long-term strength and high abrasion resistance. This cement product is essentially alkali-free, very resistant to freeze-thaw cycles, has a low heat of hydration and is very impermeable to moisture.

Other oxide additions, such alumina-containing wastes or minerals, may also be added to the melt to engineer any desired mineralogy for many different high-demand applications such as airport runways, road surfaces and seawater exposed structures. If necessary, additional heat required to thoroughly incorporate the extra additions can be added in many ways, such as electrical.

This modified slag can be used either as a cement on its own or as a major component mixed with other cementitious materials (such

as PC, amorphous blast furnace slag, etc.) to form a blended slag cement. The components of the blended slag cement can be combined before, during or after the grinding state.

Other embodiments of the invention will now be readily apparent to a person skilled in the art from the foregoing description, the scope of the invention being defined in the appended claims.