Field of the Invention

This invention relates to the treatment of steel mill waste to produce

secondary material suitable for recycling to steel-making furnaςes.

Background of the Invention

The steel industry produces more waste materials than any other

manufacturing business. For example, approximately 120 million tons of steel

per year are produced in North America and this, in turn, produces about 12

million tons of waste in the form of slag, dust, mill scale, grindings, shot dust,

metallic slag fines and sludges, etc. Over half of this is slag which is usually

used in aggregates and road materials. The remaining waste mainly comprises

iron oxide. By recycling such waste for metal recovery, it may be reclassified

as secondary material.

Over the last thirty years, the industry has been required to comply

with ever more stringent environmental standards. The dusts and sludges are especially polluted with heavy metals such as lead, zinc, cadmium, chromium

and nickel, etc. as a result of processing contaminated scrap metal.

Consequently, such dusts and sludges have been classified as hazardous.

Numerous processes have been developed to treat and recycle steel mill

secondary materials and waste, but these have failed to provide comprehensive solutions because they only deal with some of the waste and also are very costly. Some of these processes use, for example, plasma arc furnaces,

briquetting machines or pelletizing systems.

It is an objeα of this invention to provide a process for treating steel

mill waste inexpensively to produce a product which can be stored safely as

a non-polluting secondary material suitable for recycling in various furnaces,

for example electric arc furnaces, basic oxygen furnaces or blast furnaces.

Summary of the Invention

According to the invention, a method of treating steel mill waste

containing iron oxides and silica comprises reacting the steel mill waste with

an alkali solution in an amount sufficient to raise the pH to at least about 7

to solubilize sihca to form soluble sihcate compounds and silica gels and

produce a chemically reactive mixture. The mixture is reacted with an

alkaline silicic compound to produce a solid, stable, non-polluting material.

The alkali may comprise sodium hydroxide, and may be produced by

reacting sodium carbonate with calcium oxide in situ.

The alkaline silicic compound may comprise ground blast furnace slag

containing calcium silicate, a mixture of calcium sihcate and tri-calcium

aluminum sihcate, or a mixture of calcium sihcate cement in the form of

ground blast furnace slag and tri-calcium aluminum sihcate in the form of

Portland cement, which may be Type I or Type HI.

The steel mill waste may comprise from about 5 to about 12% by

weight carbon dust and from about 1 to about 10% by weight sihca. The

alkali may comprise from about 0.1 to about 5% by weight sodium

carbonate. The silicic compound may comprise from about 1 to about 20%

by weight calcium sihcate. The weights are percentages of the weight of the

mixture and the alkali. The pH may be raised to a value in the range of from

about 7 to about 14.

The present invention also provides a method of treating steel mill

waste containing from about 10 to about 80% be weight iron oxides, from

about 1 to about 65% by weight carbon and from about 1 to about 15% by

weight sihca, the method comprising reacting the steel mill waste with from

about 0.1 to about 5% by weight soluble alkali to produce a strong alkali

solution and from about 1 to about 10% by weight lime to raise the pH to

at least about 7 to solubilize sihca to form soluble sihcate compounds and

sihca gels and thereby produce a chemically reactive mixture, and reacting the

mixture with from about 1 to about 20% by weight ground blast furnace slag

containing calcium silicate to produce a solid, stable, non-polluting material.

Portland cement may be used instead of the slag, or a mixture of slag and

Portland cement may be used. The weights are percentages of the weight of

the chemically reacted mixture and the slag and/or Portland cement.

The amount of alkali required depends on the nature of the steel mill

waste, particularly the unstable polluting dusts, and may be determined by

trial and error in any particular instance. 'Some or all of the alkali may already be present in the waste.

The alkali and steel mtll waste may be mixed together by a simple

heavy-duty type of mixing apparatus which can be selected according to the

nature and stiffness of the mix, and the best apparatus to be used in any

particular case will be readily apparent to a person skilled in the art. For

example, a readi-mix concrete truck may be especially useful because the

mixing can take place on route to delivery.

The amount of silicic compound required depends on the required

setting time and, in particular, the required hardness desired for the end use.

The amount of silicic compound required is determined by trial and

experiment and is mixed into the chemically reactive mixture previously

described.

UsuaUy, large batches of materials can be processed within 30 minutes

to two hours, and the processed material is laid out to set and harden. This

is usually well advanced within 24 hours, and the solidified material can be

used within 10 to 40 days for its end use.

Specific examples of the invention will now be described.

Example 1

Steel mill waste including dusts and sludges were obtained from an

integrated steel company which operated blast furnaces (BF) and basic oxygen

furnaces (BOF). These materials were: BOF dust, BF dust, mill scale, ore

pellet fines, BF kish (metallic slag fines), carbon dust and lime dust.

The following initial mixture was made up, the percentages by weight;

10% silica

10% silica

15%

25%

15%

10% carbon dust (top dust from BF), and coke fines used to reduce oxides to iron

5% waste lime dus (This eliminates the need to add lime).

1% sodium carbonate to create strong alkali (sodium hydroxide) from the lime dust.

9% (initially separate) fine ground blast furnace slag

(calcium sihcate) comprising the silicic compound.

The initial blend (less the blast fumace slag) contained sufficient water

to make a combination that was mixable to dissolve and disperse the alkali.

After thorough mixing, the fine ground blast furnace slag (calcium

silicate) providing the silicic compound was added and mixed, thereby raising

the pH to about 14. The mixture started to noticeably stiffen, and it was

then placed on the ground to await completion of the reaction and hardening.

The material was hard within 24 hours, and a penetrometer reading

indicated a hardness of over 1,000 psi. After ten days, the material was

broken up to load into a furnace and heated to a temperature above the

melting point of steel. Examination of a cooled sample showed that the

material had reduced itself to steel and slag.

An unmelted sample was leached with distilled water after the sample

had cured for about 10 days, and an analysis of the water showed that the pH

had dropped to about 9, indicating that the alkaH had reaαed with the

silicates. Chromium, lead and cadmium values were all below about 0.1 ppm,

well below the recommended Regulatory Limits of 1 to 5 ppm. Example 2

Steel mill waste including bag house dusts were obtained from a ferrous

scrap steel minimill which operated electric arc furnaces (EAF). These

materials were EAF bag house dust, mill scale and metal grinding dust.

The following initial mixture was made up, the percentages being by

weight;

10% carbon

10% EAF bag house dust

(containing metal oxides and sihca) 15% metal grinding dust

50% mill scales (Fe ₃ 0 _< )

5% reagents (1% sodium carbonate and 4% lime) 10% (initially separate) fine ground slag (calcium sihcate) providing the silicic compound

The materials (less the fine ground slag) were mixed together with

sufficient water to a wet, mixable, mortar-like consistency, raising the pH to

about 14.

The fine ground slag (calcium sihcate) providing the silicic compound

was then mixed in. Before the resultant mixture became too stiff to mix

further, it was poured out onto the ground to set, harden and cure. After 20

days, it was broken up and fed into an eleαric furnace where it was

consumed without producing any difficulties in the production of steel and

slag.

Example 3

Samples of waste materials were obtained from a specialty steel mill

which produced stainless steel from scrap steels in an eleαric arc furnace

(EAF). These materials were bag house dust and mill scale.

The bag house dust and mill scale contained valuable amounts of

chromium, nickel and molybdenum along with the usual iron oxides, lead,

cadmium, zinc and sihca.

The following initial mixture was made up, the percentages being by

weight:

10% carbon

20% EAF dust (contains metal oxides including silica) 55% mill scale (Fe ₃ 0,)

6% reagents, namely 2% sodium carbonate (soda ash) and 4% calcium oxide (lime) 9% (initially separate) ground BF slag (calcium sihcate) providing the silicic compound

The materials were mixed together with the soda ash and lime

and water to make a flowable wet mix with a pH of about 9. The ground

BF slag, (calcium silicate) providing the silicic compound was then mixed in.

When thoroughly mixed, and before setting started, the batch was dumped

on the ground to harden. After several weeks, the material was broken up

(it had a hardness of over 1,500 psi) and was used as feedstock to a fumace to

recover the metal values.

Example 4

Examples 2 and 3 were repeated using Type I or Type HT Portland

cement (tri-calcium aluminum silicate): instead of slag. Sufficient alkali was

present in the waste to produce a pH of about 9. The results were an

improvement over the results in the earlier examples in that the products

hardened sufficiently in two or three days. Mixtures of slag and Portland

cement may also be used.

Numerous other experiments have been conduαed using varying

amounts of dusts and waste materials and the results have all been essentially

the same, although wide variances in setting times and hardness have been

experienced.

The method according to this invention advantageously makes use of

a variety of steel mill waste products, and consolidates such waste into an

environmentally safe and stable product which has significant stmctural

integrity suitable for disposal or for recycling to supplement raw materials fed

to steel-making ^" furnaces.

Other embodiments of the invention will be readily apparent to a

person skilled in the art, the scope of the invention being defined in the

appended claims.