WO1997006280A1 - Transportable apparatus for mill scale reduction - Google Patents

Abstract

A new use is proposed in relation to previously disclosed particulate iron reduction methods, where a refractory chamber and associated gas handling apparatus are constructed according to the methods of these previous disclosures, where the chamber and apparatus are sized for transport on a rail car or other transport means. These transportable reduction means may be moved to diverse sites and renders economic the processing of mill scale and other sources of iron oxide comingled with oil, commonly produced in small quantities. These waste products are commonly stockpiled or disposed of without recycling. The present invention enables the disposal and recycling of large quantities of hitherto wasted iron oxides with reduced pollutant emissions.

Description

Transportable Apparatus for Mill Scale Reduction

Cross References to Related Applications

This invention makes use of methods disclosed in my related patent applications "Method for Reducing Particulate Iron Ore to Molten Iron with Hydrogen as Reductant" serial number 08/258,572, filed 1994 June 10, and "Method for Reducing Particulate Iron Ore to Molten Iron Using Natural Gas as Reductant" serial number 08/489,127, filed 1995 June 8.

Background of the Invention

1) Field of the Invention

The present method relates to the smelting reduction of iron oxide and specifically the reduction of iron oxide in the form of mill scale to molten iron within transportable reduction means.

2) Description of the Prior Art

Mill Scale Residues

Mill scale is iron oxide waste from steel production; for example, the waste generated during the rolling of steel into sheet and strip steel. Mill scale may additionally include iron oxide fines carried as dust in blast furnace top gas. During the rolling process, hot steel ingots are compressed between rollers. A layer of the steel oxidizes due to exposure to air or water at the elevated temperatures. During the rolling process, the steel sheds flakes and splinters of iron oxide. Oxide also clings to the outer surfaces of the rolled steel, and must be removed from these surfaces, generally by abrasive means. Still other sources of mill scale are oxide layers which cling to vessel walls, and which must be removed. It is estimated that up to 3% of iron passing through a steel mill is lost in the form of mill scale waste.

Large quantities of oil are used in the production of sheet and strip steel products, and a percentage of this oil typically becomes mixed with the mill scale to form a sludge. This sludge is typically stockpiled as a hazardous waste, the storage or disposal of which may cause environmental concerns.

Although the iron oxides are typically of a high grade, they are physically unsuitable for use in conventional reduction facilities. The fines need to be pelletized, but facilities do not exist for pelletizing iron oxide at steel mills. While the flakes and splinters of mill scale are of varying sizes, mill scale is typically of too fine a consistency to be used as a sinter bed.

Although many thousands of tons of iron oxides are produced in the form of mill scale at a typical steel mill, the quantities are not sufficient to make it worth separating the oils from the oxides and transporting the mill scale to a conventional pelletizing plant. Mill scale is thus stockpiled or landfilled.

The stockpiles of mill scale have reached, worldwide, something over a billion tons, the rough equivalent of one year's global supply of iron oxide. This material is either left where it is, to the possible detriment of the environment, or disposed of by companies specializing in the disposal of industrial waste, for a fee, representing a further expense to the steel manufacturer.

Research in the field has concentrated on the recovery of oil from the various sludges produced by steel plants. US Patent 3,008,894, 1961 November 14, W. J. Culbertson is exemplary of such recovery.

Heat Carrier Solids have been used to remove hydrocarbons from solid wastes, and in the 1970's, Tosco used aluminum oxide balls to recover oil from oil shale.

However, there is virtually no prior art addressing the recovery and use of the iron oxides in mill scale. Owing to the environmental concerns and costs, extensive research is being conducted to determine how best to dispose of mill scale, however no large scale viable method of use has been disclosed.

Research in the field is primarily directed toward the possibility of agglomerating mill scale so that it can be used again at the plant which produced it.

Reduction Technologies

Conventional blast furnace operation involves a counter flow of ore and/or other iron units (hereafter referred to simply as ore) and reducing gas in a furnace. The ore is fed as particles which may be lump ore, sinter feed, pellets, briquettes, or other agglomerates. In such a blast furnace, the ore descends as a burden in which individual particles of ore rest atop other particles forming a porous mass. The reducing gas rises through channels in the burden, and exits at the top of the furnace. The use of ore burdens introduces several difficulties related to the mass of the burden. First, the walls of the furnace must be able to support the mass of the burden. Second, the ore must be able to support itself in order to provide for gas flow (as will be described below) . Finally, the thermal mass of the burden makes initiating or halting furnace operation a lengthy process involving extensive fuel use. In a furnace which melts the burden as it descends, improper shutdown can render the furnace unusable and filled with a solid fused mass of ore, slag, and metal.

The need for gas channels is a major factor in process parameters. The ore must meet strict size and strength requirements which restrict mass fraction present as fines and insure that ore particles do not undergo excessive degradation into fines. Additionally, if the ore melts, a material such as coke is required to maintain gas channels throughout the molten material. The size requirements are problematic because many beneficiation processes are better suited to producing smaller ore particles. If the beneficiated ore particles are too small for the blast furnace, then an agglomeration step is needed to produce suitable larger particles. In the conventional blast furnace, the coke serves both to maintain the gas channels and to supply the reductant gas. Coke is a major cost in blast furnace operations.

Direct reduction processes eliminate the need for coke in the reduction step. The ore is never melted, and thus the needed gas channels are maintained by the ore itself. Reductant can be totally supplied by external means. Internal reductants, if used, do not need to meet the strength requirements of coke, thus ordinary coal and fuel oils can be used. Natural gas used as a reductant has proven viable in regions where natural gas is plentiful. The product of the direct reduction furnace is sponge iron, a high surface area material consisting of metallic iron intermingled with the gangue from the ore. This material needs to be melted to separate the iron from the gangue, generally in a separate step involving an electric arc furnace. The material must be passivated prior to transport, as it is subject to catastrophic oxidation. The product is not suitable as the primary feed for steel conversion processes other than the electric arc furnace. Specifically, direct reduction does not produce the molten hot metal that is needed by the basic oxygen furnace or by other self powered steel conversion processes. Direct reduction processes make use of a burden similar to that of blast furnace processes. Ore must meet similar strength, size, and stability requirements to that of the ore used in blast furnace operations. Substandard ore and/or incorrect charging can cause problems with gas flow, burden stability, or agglomeration.

Of particular relation to the present invention are the various processes which make use of freely falling ore particles. These processes avoid the problem of having to maintain an acceptable burden (vis. strength, gas flow, etc.) by eliminating the burden. The falling particle processes allow for furnace operation above the melting point of the ore. Higher temperatures enable more rapid reduction reactions, as well as molten product. Eliminating the burden also reduces the requirements for ore strength, and makes the use of fine ore particles, such as mill scale, not only permissible, but desirable. Finally, because the melting and reduction phases are coterminous, furnace complexity and cost are greatly reduced.

U.S. Patent 2,066,665 to Baily (1937) is an early example of the freely falling reduction furnace. Baily teaches that particles of ore are caused to descend in an upward flow of reducing gas, which is produced through the partial combustion of fuel in air. The particles of ore are swiftly reduced, and allowed to fall into a pool of molten metal and slag. The reducing gas considered in the aforementioned patent is composed of carbon monoxide, generally mixed with hydrogen. No attempt is made to recycle the reducing gas, which by virtue of reaction equilibrium can only be partially used. While an attempt is made to recover the heat energy entrained in the used reducing gas, much of the available energy is wasted.

U.S. Patent 2,951,756 to P.E. Cavanaugh (1960) discloses a Jet Smelting Process, in which ore particles are entrapped in a turbulent reducing flame consisting of natural gas and oxygen. As compared to the work by Baily, higher ore temperatures are achieved and more rapid reaction facilitated. However, little or no effort is made to recycle the 'top gas' (atypically drawn off of the bottom of the reaction) , and the gas flow is not counterflow to the ore glow. While this reduction method is extremely simple, it requires excessive natural gas for operation.

Maeda et al.. Patent Number 4,886,246, dated 1989 December 12, addresses the issue of recycling the waste gases. Two furnaces are conjoined in tandem, a prereduction furnace, and a smelting furnace. Hot gases from the smelting furnace are conveyed to the upper, prereduction furnace, to provide the heating for that furnace.

This method allows particles of different sizes to be dealt with, the larger particles falling through from the prereduction furnace to the second furnace, while the smaller particles circulate in the waste gas, are recovered, and pneumatically injected into the smelting furnace. Maeda's disclosure does not demonstrate the suitability of the device for reducing mill scale, which, although it consists of oxidized iron particles of varying sizes as envisaged by Maeda, is also frequently commingled with oil, to form a sludge.

Maeda's tandem arrangement of two furnaces would not be so easy to adapt for a transportable apparatus as a device requiring only one furnace. Maeda also uses substantial quantities of Carbon Monoxide in his device, an extreme toxin, which also further handicaps its suitability for transportable applications, as its usefulness would be limited to those areas where the risk of Carbon Monoxide emissions were deemed acceptable.

More recently, the process of Keogh et al., Patent Number 5,489,325, dated 1996 February 6, offers an ingenious solution to the problem of waste. Keogh teaches that a rotating furnace vessel has the effect of pushing waste gases and liquids to the side of the furnace vessel, so that a greater proportion are contained within the vessel and do not escape through the vessel mouth. Oxidizing gases are introduced to the vessel through tuyeres, the force of their introduction imparting the rotational spin.

Keogh also suggests that fine particles tend to combine under the turbulent conditions of the vessel chamber, forming larger lumps of ore which are, in turn, more likely to fall into the pool of molten metal at the bottom of the furnace, rather than escape the chamber. Keogh does not examine the possibility of reducing mill scale by this process, although it is possible that the commingled oil of mill scale might be separated from the iron oxide particles as part of the rotational process, the oil then burning in the chamber and contributing to the gases present. But this is by no means clear, especially at the preferred rotational speed of between 2m/s and 15m/s, nor does it appear to have been attempted.

However, Keogh's process, like Maeda's, also envisages a reductant gas containing a substantial quantity of Carbon Monoxide.

The suitability of Keogh's process for transportability is also questionable. While the disadvantage of Maeda's double furnace is met, the need to provide a stable axle of rotation, or spindle, without recourse to earthbound posts and fixings, would present an additional challenge.

Further, Keogh's process is most advantageously utilized with a furnace vessel of rotational symmetry about a vertical axis. To render such an apparatus transportable on a rail car, for example, the chamber's diameter would therefore be restricted to the width of the rail car upon which it was mounted. Keogh's process also prefers a height for the vessel as a whole of 4 to 7 times the depth of the molten metal within the vessel, a further limitation upon the size of vessel which could be practically be transported.

All of the above methods are characterized by the use of relatively large, static processing plants to which iron ore must be transported for processing. It follows also that such plant requires a substantial quantity of ore to be processed at any time, in order to achieve cost effective operations.

In my application "Method for Reducing Particulate Iron Ore to Molten Iron with Hydrogen as Reductant" serial number 08/258,572, filed 1994 June 10, I disclosed a method for reducing particulate iron oxide to molten iron utilizing hydrogen as the reductant in a reducing furnace, in which the ore freely falls during the melting and reduction process. Reacted top gas is purified and recirculated through the reduction furnace, establishing a counterflow relative to the particulate iron oxide, thereby heating and reducing the oxide. The heat for reduction is generated by combusting a fraction of the hydrogen reductant with oxygen. Heat may also be supplied via the use of electric arc heating. Oxygen may be partially or wholly replaced with water in order to maintain reaction oxidizer ratios while reducing heat supplied to the reaction.

In my further disclosure of "Method for Reducing Particulate Iron Ore to Molten Iron Using Natural Gas as Reductant" serial number 08/489,127, filed 1995 June 8, I further disclosed a method for reducing particulate iron oxide to molten iron utilizing natural gas as the source for process heat and reduction potential, in which the ore freely falls during the melting and reduction process.

Both of these processes make use of fine particles of freely falling ore, high temperatures which facilitate rapid reduction, and short furnace residence times which facilitate high production rates. Both of these processes are suitable for the smelting reduction of iron oxide fines such as mill scale.

Portable Furnaces

Portable furnaces are rare, though not unknown to the art. The Portable Melting-Furnace of Herman W. Falk, Patent No. 587,718, dated 1897 August 10, discloses a melting furnace mounted upon a rail car. The furnace may be moved along tracks to the point at which the metal is needed, and the liquid metal poured from the cupola. Falk intended his device primarily for use in casting joints used in track- laying.

Falk's furnace is not a reduction furnace, but a melting furnace. It is charged with ingots of iron or some other metal, which are melted down and poured into casts at the location where the casting is required. Falk does not disclose a reduction process which is sufficiently compact as to be capable of portable construction, nor does he disclose a furnace which could be used to agglomerate or reduce or otherwise usefully process iron oxide fines in the form of mill scale.

Theodore Masel's Furnace-Controlling Mechanism, Patent No. 1,608,801, dated 1926 November 30, continues the theme introduced by Falk.

Masel's mechanism is an ingenious arrangement of cables and drums which enables a cupola to be heated electrically, melting a charge of metal placed therein, and then moved along rails and tilted to pour the molten metal into molds for the casting of ingots and other forms.

Like Falk's melting-furnace, the Masel device cannot reduce iron oxide to iron, nor can it reduce, agglomerate or otherwise usefully process iron oxide fines in the form of mill scale. Nor is it envisaged by Masel that his Furnace- Controlling Mechanism be transportable, in the sense that it may be moved from foundry to foundry. It is designed to be supplied with electrical power by extemal cables, and to be moved only short distances between molds, laid out upon the foundry floor. Objects and Advantages

Accordingly, besides the objects and advantages of the methods of the smelting of iron ore described in my above patent, several objects and advantages of the present invention are the following:

It is an object of the present invention to provide a portable reduction method for the consumption of mill scale stockpiles.

An advantage of the present invention is the consumption of waste iron oxide as a source of useable iron.

An advantage of the present invention is the freeing of valuable storage space.

An advantage of the present invention is the conversion of a waste liability into a profitable product.

An advantage of the present invention is the removal of mill scale waste without the capital investment of a permanent installation.

An advantage of the present invention is that reduction plants may be built in small sizes to suit the specialized economic requirements of disposing of mill scale waste products.

It is an object of the present invention to provide a reduction method amenable to rapid startup and shutdown.

An advantage of the present invention is that a reduction plant may be brought on site, operation swiftly initiated, and then swiftly removed when all mill scale has been consumed.

An advantage of the present invention is that a reduction plant in permanent installation may be operated as needed with little expense related to 'banking'.

An advantage of the present invention is that reduction furnace operations can be adjusted to accommodate variation in economics and fuel supply, at little marginal cost.

Further objects and advantages of the present invention will become apparent from a consideration of the ensuing descriptions.

Description of the Preferred Embodiment

Briefly, the present invention consists of mounting an ore reduction facility on a rail car. Said rail car is transported to a steel mill, supplied with necessary reductant fuel, and fed with mill scale from the steel mill stockpile. Said mill scale is reduced to metallic iron, in the form of molten hot metal, and returned to the steel plant for further processing into final product. Said rail car mounted reduction facility may further be transported to a landfill location and used to process landfilled mill scale into transportable reduced iron, for example, pig iron. Other suitable transportation may be used in place of a rail car. For example, a smaller scale reduction plant may be operated from a trailer suitable for road transport, or a larger scale plane operated from a barge.

The freely falling particle reduction furnaces disclosed in my previous two patents, mentioned above, are ideally suited for this service. The reasons for selecting this particular reduction technology are as follows:

1) Avoiding a burden greatly reduces the strength requirements for the reduction chamber.

2) Gas flow may be used to prevent contact of hot metal with the reduction chamber wall, thus reducing refractory requirements of the reduction chamber.

3) High specific production rates mean that a small reduction chamber may be used.

As envisioned for use on a conventional rail car, a reduction chamber of active diameter between 40 and 120 cm would be used, with a total diameter of between 60 and 150 cm. Reduction chamber lengths would be on the order of 1200 cm to 2000 cm. For transportation, the refractory lined chamber should be fitted for removal and horizontal placement, using suitable flange and bolting means. At the scale envisioned, the expected production rate for hot metal is 1.8 million metric tons per year per square meter of active cross section. Thus a 40 cm diameter reduction furnace would produce about 225 thousand metric tons hot metal per year. Such a system will fit, along with the requisite gas handling and recycle equipment, on a standard rail car.

As mentioned in the background section, mill scale is often contaminated with oils from rolling operations. Such oils will become part of the reductant gas, and will thus serve a useful purpose in the reduction process. Sludges containing such oils will be best utilized by dilution in dry mill scale prior to reduction. The implementation of the present invention will be obvious to an individual skilled in the art upon review of the above mentioned patent disclosures.

Ramifications and Scope

It may be seen from the above descriptions, that the nature of my previously disclosed methods lends itself to the manufacture of reduction furnaces which may be small and light enough to be transported from site to site, and thus rendering a cost effective apparatus for processing relatively small quantities of iron ores in the form of mill scale, wastage from grinding, and other sources.

The possibility of using detachable feeds and pipes, which may be held in situ, rendering it even more easy to transport the chamber to where it is needed, while not mentioned in my previous disclosures, provides for the practical construction of a portable steel making plant.

Problems of environmental pollution are solved in that mill scale and similar wastes may be processed instead of stockpiled, the processing itself being considerably less polluting than the discarding of wastes to leach their oils into the surrounding environment.

Lightweight materials, such as heat resistant composite ceramics, may be used in the construction of the refractory chamber itself, thus reducing the weight of the chamber and rendering it less subject to damage during transport.

Accordingly, the scope of the invention should be determined not by the embodiment illustrated, but by the appended claims and their legal equivalents.

Claims

Claims

I claim

6) a method of reducing oxidized iron particles to molten metal comprising the steps of

(a) introducing said oxidized iron into a refractory lined furnace means along with fluxing means for the production of slag;

(b) passing said oxidized iron through a flame generated by combusting oxygen with excess reductant gas, so as to fully melt said oxidized iron particles;

(c) regulating the rate of said introduction of said oxidized iron particles so that said oxidized iron particles are able to freely fall through said furnace means;

(d) introducing reductant gas into said furnace means through inlet means and causing said reductant gas to flow upwardly through said furnace means, in counterflow to said oxidized iron particles;

(e) introducing oxygen into said furnace means through inlet means for combustion with said reductant gas

(f) removing hot gaseous by-products from said furnace means; and

(g) collecting said molten particles in then reduced state,

wherein said reductant gas is compounded from the group of gases comprising hydrogen and natural gas, alone or in combination with reformation products of said gas. wherein said steps form a concurrent cycle of steps

the improvement wherein being the use of said method wherein mill scale or oxidized iron commingled with oil, or mill scale commingled with oil is the source of said oxidized iron particles.

7) the method of Claim 6 wherein said method includes the mounting of said furnace and associated materials on transport means and use of said reduction apparatus for the reduction to iron of said mill scale or said oxidized iron commingled with oil at the location of such said mill scale or said oxidized iron commingled with oil.

8) the method of Claim 7 wherein said transport means comprises a rail car or rail cars.

9) the method of Claim 7 wherein said transport means comprises waterborne transport means

10) the method of Claim 7 wherein said transport means comprises a road trailer or trailers

11) the method of Claim 6 wherein said commingled oil is combusted with said reductant gas in said furnace, whereby said combustion is augmented.