LU88548A1 - Process for recycling iron-rich sludge - Google Patents

Description

Process for recycling iron-rich sludge.

The present invention relates to a process for recycling an iron-rich sludge.

Many steel operations give rise to the production of sludge containing sometimes high proportions of iron. Examples of such operations include, among other things, the hot rolling of steel in various forms, or the production of steel in an electric furnace. The rolling mill sludge is essentially composed of iron particles, often in the form of oxides, torn from the surface layers of the products during rolling; this sludge usually contains a small amount of used rolling oil. Electric furnace sludge essentially contains iron dust, generally oxidized, as well as a certain humidity; they can also contain lime.

The recovery of this iron-rich sludge is of undeniable ecological interest, since this sludge must no longer be landfilled even under controlled conditions. The economic benefit of such recovery is also obvious, since it allows on the one hand to reduce the consumption of raw materials and on the other hand to escape the increasingly heavy costs of the storage of this waste.

However, this sludge cannot generally be recycled in the raw state, that is to say as it comes from the operations which produce it. They must undergo an appropriate preparation treatment, so that they can be easily handled and used, in particular in the steel industry.

For this purpose, the most common way consists in manufacturing by compacting briquettes of various shapes and sizes, intended mainly to be loaded in a steelworks furnace or in a blast furnace.

Experience has shown, however, that conventional briquettes do not always have satisfactory resistance to impact and crushing, and that they do not withstand bad weather, in particular rain. As a result, they must be stored in relatively small heaps, and preferably under shelter.

The object of the present invention is to propose a process for recycling an iron-rich sludge, by means of which it is possible to manufacture briquettes resistant on the one hand to impact and crushing, and on the other hand to humidity, especially rainwater.

According to the present invention, a process for recycling an iron-rich slurry, in which said sludge is mixed with a binder and briquettes are formed from this mixture, is characterized in that a binder is added to said sludge organic in a proportion of between 2% and 25% by weight, relative to the weight of the mixture, in that briquettes of said mixture are formed and in that the curing of said briquettes is carried out thermally for a period of time between 1 a.m. and 3 a.m.

By way of organic binders, it is advantageous to use by-products, such as 1ignosulfonates, molasses or candy bagasse.

The proportion of organic binder added to the mud can be expressed differently depending on the physical state of both this binder and the mud.

In particular, the 1ignosulfonates can be presented either in the powder state or in the diluted state in water.

In the powder state, the 1ignosulfonates are added in a proportion of between 2% and 15%, and preferably between 3% and 10% of the weight of the mixture.

In certain cases, in particular with steelworks sludge, it may be advantageous to hydrate the 1ignosulfonates beforehand, by adding a certain amount of water, in order to contribute to forming a smooth paste.

This amount of water depends on the initial degree of humidity of the mud. In particular, if a dry mud is considered, a weight of water at least equal to the weight of powdered lignosulfonates is added.

If they are already diluted in water, the 1 ignosulfonates are added in a proportion which depends on their degree of dilution; for a dilution of 50%, the 1ignosulfonate solution is added in a proportion of between 15% and 25%, and preferably about 20% of the weight of the mixture. A slight addition of water or sawdust can optionally be added to the mixture to obtain a smooth paste.

In the case of rolling mill sludges containing oily scale, this sludge becomes fluid during its mixing with the 1ignosulfonates in an industrial mixer, due to their thixotropic nature. There is therefore no need to add water. On the contrary, it is preferable to add a plasticizing agent to them to allow the extrusion of the briquettes and give them a sufficient consistency before cooking. This consistency must be such that the continuous product leaving the extruder does not crush under its own weight and can be handled without difficulty between the extruder and the baking oven.

The proportion of plasticizing agent to be added depends in particular on the water content of the sludge and on the nature of the plasticizing agent used, in particular its particle size. By way of example, said plasticizing agent can consist of powdered clay or blast furnace dust with a low zinc content.

As a general rule, it is considered that the content of plasticizing agent must be between 15% and 35% by weight of the mud.

According to a particular implementation, briquettes are formed having a thickness of at least 20 mm.

According to a first embodiment, applicable to sludges containing small quantities of used oil, said briquettes are baked at a temperature between 100 ° C. and 250 ° C. This cooking ensures the polymerization of the 1ignosulfonates.

Preferably, the briquettes are baked at a temperature between 150 ° C and 2000C for a time between 1 h and 2 h.

The cooking of these briquettes is preferably carried out in an oven.

In the case where the briquettes are more specifically intended for blast furnaces, it is advantageous to then bring the briquettes to a temperature between 350 ° C. and 450 ° C. and to maintain them at this temperature for a period of between 15 min and 60 min. This operation allows the oil contained in the briquettes to be distilled and used directly as fuel for cooking the briquettes. Another advantage of this technique is that this avoids any distillation of oil in the blast furnace.

According to another mode of implementation, applicable in particular to sludges containing lime, the organic binder is added, and an at least equal weight of water, and the briquetting of the mixture is carried out and the briquettes are taken with the room temperature, for a period of between 1 h and 2 h. The lime content is generally between 5% and 15% by weight.

In the case of the addition of powdered lignosulfonates, a mixture is preferably formed containing 60 - 80% of mud, 10-20% of lignosulfonates and 10-20% of water.

The heat given off by the hydration of the lime causes the polymerization of the lignosulfonates and consequently the hardening of the briquettes.

According to an additional characteristic, the cured briquettes, respectively hardened, can be coated with a hydrophobic material, to give them good resistance to weathering, and in particular to rainwater. For this purpose, it is possible in particular to use a secondary reagent such as a used foundry wax, forming an impermeable coating.

In the case of foundry wax, this coating is however hard and brittle, and it can therefore crack in the event of impact.

A certain quantity of a lubricating agent, for example a used rolling lubricant, can then be added to the wax, in order to give the coating a certain flexibility which improves its impact resistance. The proportion of lubricant to be added may vary depending on the nature of the coating material. In the case of a coating of used foundry wax, 30% to 60% by weight are advantageously added, and preferably approximately 50% by weight of lubricant relative to the weight of the wax / lubricant mixture.

The invention will now be illustrated by examples of implementation relating respectively to the recycling of hot rolling mill sludge and of an electric steelworks furnace.

Example 1

It concerns a laboratory test on a hot rolling mill sludge, which typically has the following composition, by weight: iron: 60-70% humidity: 10 - 15% oily materials: 5 - 15% volatile materials: 3 - 5 % carbon: 2 - 5%.

The rolling mill sludge used in the present example contains: iron: 63.6% humidity: 12% oily materials: 6.5% volatile materials: 4% carbon: 3.4%.

4.5 kg of this slurry were mixed with 0.5 kg of powdered lignosulfonates, in the presence of 0.6 kg of water. A smooth paste was thus obtained, from which briquettes of 50 × 50 × 20 mm were produced. These briquettes were baked in an oven at 150 ° C for 2 h.

The cooking of briquettes has several phases.

We first observe a phase of elimination of water, during which the dough tends to spread on the plate which carries it; then, the briquette goes through a rubbery phase, which seems to correspond to the softening temperature of the lignin present in the ligno-sulfonates. Towards the end of the cooking, the soft material turns into a very hard material, probably because Von has reached the glass transition temperature, which was defined in the article by D.A.I. GORING, Thermal Softening of Lignin, Hemicellulose and Cellulose. Pulp and Paper Magazine of Canada, December 1963, pp. 517-527.

After cooking, the briquettes were coated with a mixture of equal amounts of used rolling oil and used foundry wax.

An impact test, consisting of a free fall from a height of 15 m on a concrete floor, causes only a few surface chips, rarely breaking the briquette into two pieces, while briquettes made with conventional binders such as cement, polymers, starch, are completely pulverized by this test.

A crushing test by a 65 mm diameter piston revealed a breaking load of the briquettes of between 650 kg and 750 kg.

Example 2

This example, carried out in the laboratory, relates to briquettes made from sludge from an electrical steel plant, containing lime. A mixture composed of 78% steelworks slurry, 11% of 1 powdered ignosulfonates and 11% of water was prepared. Briquettes in the shape of a square section pyramid trunk were made with the following dimensions: large base: 55 x 55 mm, small base: 37 x 37 mm, height: 55 mm.

The water caused the hydration of the lime, and the resulting heating ensured the hardening of the briquettes by polymerization of the lignosulfonates, without external heating. The hardened briquettes were then coated with a mixture of oil and wax, as in Example 1.

The impact test, that is to say a free fall from a height of 15 m on a concrete floor, also caused only small surface chips, but no break in the briquette.

The crushing test using the 65 mm diameter piston indicated for these briquettes a breaking load of between 500 kg and 600 kg.

In addition, the coating of the briquettes was tested by complete immersion in water for more than 2 hours. No solubilization of the lignosulfonates was demonstrated at the end of this immersion test.

Example 3

A mixture composed of 85% rolling mill mud and 15% bagasse was prepared, also in the laboratory, and hardened briquettes were made by baking at 150 ° C for 2 hours. The briquettes were coated as in the previous examples.

The impact test caused the briquettes to break into 3 to 4 cm pieces.

The crush test indicated a load of approximately 105 kg. It is however difficult to determine this load precisely, because the material is not fragile and it undergoes a certain deformation before breaking.

Example 4

Was still formed, still in the laboratory, a mixture containing, by weight, 90% of mill slurry and 10% of a binding agent, to which the necessary mixing water was added. The binding agent contained 10% xanthan gum, 10% sodium alginate and 80% air-crosslinkable polymer.

Briquettes were made, baked and coated as in the previous examples.

During the impact test, these briquettes broke into pieces of 2 - 3 cm.

Their crush resistance was on average 95 kg.

It appears that the briquettes produced according to Examples 3 and 4 are preferably suitable for handling by transport and loading buckets.

Example 5

To make briquettes for loading into a blast furnace, a mixture was formed in a pilot plant containing 57 kg of oily rolling mill scale, 3 kg of lignosulfonate and 17.5 kg of low-grade blast furnace dust zinc as a plasticizer. The 60 x 40 x 30 mm briquettes, obtained by extrusion, were baked at 165 ° C for 1 h 10 min and then kept at 400 ° C for 30 min.

The crush test indicated a resistance of 1740 kg.

The impact test, consisting of a fall from a height of 15 m on a concrete surface, caused the rupture into fragments of 2 to 3 cm.

The process of the invention makes it possible to recycle, without compaction, various types of waste, such as iron-rich sludges containing either oily materials or lime, organic waste such as 1 ignosulfonates, molasses, or candy bagasse and, to a lesser extent, used foundry waxes and used rolling oils. The briquettes obtained by this process have a high resistance to impact and crushing. They can be stored in the open air, without deteriorating, thanks to their protective coating. They are used, for example, as a temperature control charge in oxygen converters or in electric ovens.

The invention is not limited to the modes of implementation described in the examples. Different modifications can be made by a person skilled in the art, in particular with regard to the shape and dimensions of the briquettes, or even the proportions of the different materials mixed with the sludge.

Claims (9)

1. A method of recycling an iron-rich mud, in which said mud is mixed with a binder and briquettes of this mixture are formed, characterized in that an organic binder is added to said mud in an amount between 2% and 25% by weight, based on the weight of the mixture, in that briquettes are formed from said mixture and that the curing of said briquettes is carried out thermally for a period of between 1 h and 3 h . 2. Method according to claim 1, characterized in that said organic binder is chosen from a group containing lignosulfonates, as well as molasses and candy bagasse. 3. Method according to claim 2, characterized in that lignosulfonates are added in a proportion of between 3% and 10% by weight. 4. Method according to either of claims 1 to 3, characterized in that a plasticizing agent is added to said sludge in a proportion between 15% and 35% of the weight of the sludge. 5. Method according to either of Claims 2 and 3, characterized in that a weight of water at least equal to the weight of lignosulfonates is added to the mixture. 6. Method according to either of claims 1 to 4, characterized in that said sludge contains 5% to 15% by weight of oily materials and in that the briquettes are baked at a temperature between 100 ° C. and 250 * C. 7. Method according to claim 6, characterized in that the briquettes are baked at a temperature between 150 ° C and 200 ° C for a time between 1 h and 2 h. 8. Method according to either of Claims 6 and 7, characterized in that the briquettes are then brought to a temperature between 350 ° C and 450 ° C and in that they are kept at this temperature for a period of between 15 min and 60 min. 9. Process according to either of Claims 2 to 4, characterized in that the said sludge contains 5% to 15% by weight of lime, in that lignosulfonates are added in a proportion of between 10% and 20% by weight, in that a weight of water at least equal to the weight of lignosulfonates is added to the mixture, in that briquettes are formed and that the briquettes are hardened to room temperature for a period of between 1 h and 2 h. LO. Process according to either of Claims 1 to 9, characterized in that the said briquettes are coated with a layer of hydrophobic material.