WO2009024826A1

WO2009024826A1 - Production of secondary aggregates

Info

Publication number: WO2009024826A1
Application number: PCT/GB2008/050733
Authority: WO
Inventors: Colin Hills; Paula Carey
Original assignee: University Of Greenwich
Priority date: 2007-08-22
Filing date: 2008-08-21
Publication date: 2009-02-26
Also published as: GB0716360D0

Abstract

Secondary aggregates are prepared from metallic wastes, such as wastes or residues containing metal compounds resulting from metal extraction or metal processing, using a combination of accelerated carbonation and tumbling, such that aggregate particles composed of successive layers of solid carbonate-based reaction products form a hard aggregate suitable for use in concrete. The process may carried out by loading the starting materials, and if necessary, water into a cylindrical drum that is rotatable about its horizontal axis. The interior is charged with a carbon dioxide atmosphere, or the drum is mounted within a larger vessel that holds a carbon dioxide atmosphere. Aggregates are formed as coatings on core particles as tumbling proceeds, and the coatings are hardened by in situformation of carbonates by reaction with the carbon dioxide.

Description

PRODUCTION OF SECONDARY AGGREGATES

Field of the Invention

This invention relates to the production of secondary aggregates by combined carbonation and tumbling of wastes and residues from metal processing and metal extraction, referred to herein in general terms as 'metallic wastes'.

Background of the Invention

Previous work by the inventors, which is the subject of US Patent 5,997,629, proposed treatment of waste by accelerated carbonation to form (by non-hydraulic hardening) non-leaching granulates in which harmful components of the waste are embedded. This treatment involves the use of equipment, such as planetary mixers, to mechanically activate the reactive components of the medium to be carbonated through thorough mixing, and the removal of reaction products to expose fresh reactive surfaces.

Pressure on primary aggregate resources has lead to research into the production of secondary aggregates from inert and non-hazardous wastes, such as from sintered mixtures of clay and ash, which can be used in the production of concrete-based materials.

For example, the commercially available product Aardelite (RTM) is manufactured using the pozzolanic properties of residues such as fly-ash. By adding a binder to the residue, for example lime, and processing at a temperature of 90° C, the silica and alumina in the residue are transformed into cementitious minerals.

Summary of the Invention

The present invention is based on the finding that secondary aggregates can be prepared from metallic wastes, using a combination of accelerated carbonation and tumbling, such that aggregate particles composed of successive layers of solid carbonate-based reaction products form a hard aggregate suitable for use in concrete after only minutes of exposure to carbon dioxide.

Accordingly, the present invention provides a process for the preparation of aggregates useful in the manufacture of concrete, comprising tumbling Cθ₂-reactive i.e. carbonatable, metallic wastes, or metallic wastes and a Cθ₂-reactive i.e. carbonatable, binder in the presence of moisture and carbon dioxide, such that the carbonatable materials and the carbon dioxide react as the tumbling proceeds to form successive layers of hardened carbonates around core particles. Optionally further waste and/or binder is added to the system until the aggregate reaches a size which is suitable for its intended use, for example, aggregate for use in preparation of concrete, or aggregate used in road-making.

A useful carbonatable binder is ordinary Portland cement (OPC).

The process may be operated by tumbling single loads, or by batchwise addition of materials during the tumbling process, or by continuous feeding of carbonatable materials with continuous removal of aggregated product.

Brief Description of the Drawings

Figure 1 is a photograph showing untreated bauxite (left); and bauxite which has been tumbled with OPC and carbonated (right);

Figure 2 is a photograph showing untreated lead-contaminated waste (left); and lead- contaminated waste which has been tumbled with OPC and carbonated (right); Figure 3 is a photograph showing untreated oil-drill cuttings (left); and oil-drill cuttings which have been tumbled with OPC and carbonated (right).

Detailed Description of the Invention

The present invention is a departure from the mixing required to implement the carbonation process of US Patent 5,997,629, in that it involves a tumbling action, typically by use of a rotating drum or tray. In this invention, when materials that are susceptible to accelerated carbonation are mixed with inert wastes/fines such as quarry or washing fines, the mixture is hardened in a CO₂-rich gaseous environment, but in a way such that a series of successive layers of hardened material are formed (by the tumbling action) to produce a hard aggregated product, suitable as a replacement for stone, or sintered aggregate in, for example, concrete articles.

Also, unlike in US Patent 5,997,629 where materials are added in bulk and then processed in a CO₂ rich atmosphere, in this invention the materials are typically added to the tumbling system more sparingly, and water content is closely controlled to enable successive layers of reaction products/carbonate cement to build up on previous layers, to form a monolithic aggregated product. The accumulation of successive layers of carbonate cement that form the aggregate occurs during the short time that materials are exposed to CO₂. Experiments have shown that the CO₂ reactive components of the materials being carbonated can achieve up to 97% of their theoretical values during the processing by simple means that includes a modicum of mechanical activation energy.

Metallic wastes usable in the invention include metal ore residues, such as bauxite, metal processing wastes, such as aluminium smelting residue (red mud), lead- contaminated waste, iron foundry/steel waste and zinc smelter waste, and oil-drill cuttings.

Carbonatable binders that may be used in this process include cement kiln dust and Portland cement, and CO₂ reactive materials such as quicklime, combustion ashes containing free lime and calcium silicate, or magnesium based minerals. Advantageously, Portland cement, that is deemed "out-of-date" for concrete manufacture may be used in this invention.

For carbonation to take place, the residues suitably contain calcium or magnesium compounds, although carbonation can also occur with other metal compounds such as iron compounds. Residues containing carbonatable materials may also be supplemented with carbonatable binders, such as OPC, which may be essential if a carbonatable component is absent or minimal in the residue.

The controlled addition of materials and moisture content at which carbonation occurs, are important factors in the production of aggregates by accelerated carbonation. It is important to stress that the rapid development of strength that occurs during exposure to carbon dioxide gas in only minutes is unlike that of hydraulic cement-based systems, which harden over much longer time periods, i.e. several hours.

Small amounts of water are required to enable the carbon dioxide to react with the carbonatable materials. The amount of water required is much less than is used for hydraulic setting of Portland cement (hence the use of the term "moisture") and it may well be that the moisture content of the fines is sufficient for the carbonation reaction to take place. If not, then water is added to achieve a weight ratio to solids of not more than 0.5:1 , possibly not more than 0.4:1 or 0.3:1. Suitably the water to solids ratio is at least 0.01 :1 typically at least 0.1 or 0.2:1. The water to solids ratio may be assessed as an overall value for the process. In a continuous process the indicated water to solids ratio is preferably observed for the feed materials throughout the process. Also, in a batchwise process the indicated water to solids ratio is preferably observed for each batch of material added to the tumbler while building up a layer structure for the aggregate.

Suitably the process of the invention is carried out in an atmosphere that has a carbon dioxide content greater than that of natural air, for example containing at least 20%, 30% or 40% by weight carbon dioxide. Preferably carbon dioxide is the predominant component of the tumbling atmosphere, that is at 50%, 60%, 70% or 80% by weight. Most preferably the atmosphere substantially consists of carbon dioxide, that is at 90%, 95% or 99% by weight. Waste combustion gases with a high content of carbon dioxide may be suitable as the tumbling atmosphere.

The carbonation step is preferably carried out at or around atmospheric pressure, or up to a pressure not exceeding 30 psi (2 bar). The use of a CO₂ atmosphere at higher pressure does not greatly advance the process and requires more complex apparatus. The tumbled materials are able to combine with CO₂ in the presence of moisture, at ambient temperature and pressure. It is not usually necessary or desirable to employ heating or cooling during the tumbling.

It is preferred that the aggregate product does not retain hydraulic properties. Furthermore the product is preferably essentially non-hydrated in that the aggregate layers are formed by non-hydraulic hardening due to reaction with carbon dioxide.

The carbon dioxide used may be supplied from conventional sources of liquid or pressurised carbon dioxide. Alternatively, CO₂ rich gases discharged or recycled from processes such as cement making, or as combustion waste gases, may be used with the simultaneous environmental benefits of reducing greenhouse gas emissions. Suitably the process is carried out by loading the starting materials into a cylindrical drum that is rotatable about an axis that is horizontal or slightly inclined to the horizontal. The cylinder may be sealed at both ends so that its interior may be charged with a carbon dioxide atmosphere, or mounted within a larger vessel that holds a carbon dioxide atmosphere. On rotation of the drum, the material resting at the lowest point of the cylinder begin to "climb" the cylinder wall and then tumble back to the lowest level under gravity. Aggregates are formed as coatings on core particles as the tumbling proceeds, and the coatings are hardened by in situ formation of carbonates by reaction with the carbon dioxide.

When the drum is rotating about a horizontal axis the materials to be treated, and optionally water, may be added to the drum as a single batch or in more than one portion. The load, or each portion, is drummed until suitably hardened and, if necessary with addition of another portion, until the desired size is reached for use as an aggregate in concrete manufacture or, for example, as road-stone or gravel substitute.

When the rotational axis is inclined to the horizontal, the load is able to pass down the incline during rotation. The angle of inclination and the length of the drum is selected so that the time of travel of a load added at the higher end of the drum is such that it is aggregated to a suitable size as it discharges from the lower end of the drum. In this procedure, the materials may be added as a single load, or more preferably as a continuous supply. This process may be carried out in a static CO₂ atmosphere or by circulating a CO₂ rich atmosphere through the drum.

Alternatively, the process may be carried out using rotating pelletising pans or trays having a circular base and an upstanding peripheral wall around the circumference of the base. The base of the tray or pan may be horizontal, or inclined to the horizontal so that a tumbling action is imparted by the movement of the wall, as well as of the base.

In both the drum or tray form of apparatus, upstanding fillets may be provided on the surfaces in contact with the materials to be carbonated, to promote tumbling and to avoid the possibility that materials slide without tumbling. As an illustration only, an intermediate scale plant may be based on a tumbler drum of 1.5m diameter x 4.0m length with its main axis at an inclination of 1 -5 degrees from the horizontal. Operating at 1-5 RPM and velocity of 0.08-0.4 m/s, it is anticipated that a continuous feed of 1000-5000 kg/hr is feasible with a process time of 10 to 30 minutes, depending on waste. Such an apparatus may be mounted on pairs of rollers or wheels, which support the drum while allowing it to rotate. The drum may be rotated by a motor which drives one or more of the support rollers or wheels. This pilot scale plant may be scaled up in conventional manner for larger throughputs on an industrial scale, for example around 100 tonnes/hour.

The procedures of the invention are illustrated by the following experimental work.

Experimental

The CU₂-reactive component used may usefully be out-of-date Portland cement. However, cement-kiln dusts, flue dusts and slag can also be used in the process of the invention, in which aggregates are produced using a rotary drum mixer with a tumbling/rolling action charged with carbon dioxide with the internal pressure at around atmospheric.

The following materials, which are shown here as examples and should not be taken as an exclusive list of the materials that can be treated in this way, but merely as examples of what has already been achieved, have been successfully carbonated by the present invention The product size range can vary between that of a granular soil-like material (millimetres in diameter) and pellets (millimetres to centimetres in diameter).

1 ) Bauxite (aluminium oxide) was tumbled and carbonated together with Ordinary Portland Cement (as an example of a carbonatable binder). The resultant product is shown in accompanying Figure 1.

2) Industrial waste containing high quantities of lead was tumbled and carbonated together with Ordinary Portland Cement (as an example of a carbonatable binder). The resultant product is shown in accompanying Figure 2. 3) Oil-drill cuttings ware tumbled and carbonated together with Ordinary Portland Cement (as an example of a carbonatable binder). The resultant product is shown in accompanying Figure 3.

Claims

1. A process for the preparation of aggregates comprising tumbling metallic waste and optionally a carbonatable binder in the presence of moisture and carbon dioxide.

2. A process according to claim 1 comprising tumbling carbonatable metallic waste, or metallic waste and a carbonatable binder, in the presence of moisture and carbon dioxide such that the carbonatable materials and the carbon dioxide react as the tumbling proceeds to form successive layers of hardened carbonates around core particles.

3. A process according to claim 1 or 2 in which the metallic waste is bauxite waste, aluminium smelting residue (red mud), lead-contaminated waste, iron foundry/steel waste, zinc smelter waste, or oil-drill cuttings.

4. A process according to claim 1 , 2 or 3 in which the carbonatable binder is cement kiln dust or Portland cement

5. A process according to any preceding claim in which water is present at a weight ratio to solids of not more than 0.5:1.

6. A process according to any preceding claim in which the materials are tumbled in an atmosphere consisting predominantly of carbon dioxide.

7. A process according to any preceding claim in which the materials are tumbled in a cylindrical drum.

8. A process according to claim 7 in which the cylindrical drum rotates about a horizontal axis and materials are processed batchwise.

9. A process according to claim 8 comprising adding further fines and/or binder and/or water and continuing tumbling until the aggregate reaches a size suitable for its intended use.

10. A process according to claim 9 in which the cylindrical drum rotates about an axis inclined to the horizontal and materials are processed batchwise or continuously.