WO1982002724A1

WO1982002724A1 - Method and apparatus for handling flux fines

Info

Publication number: WO1982002724A1
Application number: PCT/US1981/000169
Authority: WO
Inventors: Phillip C Howell
Original assignee: Phillip C Howell
Priority date: 1981-02-05
Filing date: 1981-02-05
Publication date: 1982-08-19
Also published as: AU7321481A; EP0071602A1

Abstract

A process for recovering and using fines of flux, such as limestone, used in a metallurgical process, such as oxygen steelmaking. The process involves collecting fines from a receiving zone (2), e.g., with blower and baghouse equipment. The fines so collected are then conveyed, as by a densephase conveyor (5), to a first processing zone (6) containing a first screening device (8) for screening flux material which is to be fed to the process. Material not passing through the first screening device (8) is fed to the metallurgical-process vessel, but fines passed by the first screening device (8) are collected and formed into briquettes. The briquettes are conveyed to a second processing zone which is located above the conveyor (5) which leads from the receiving zone (2) to the first processing zone (6) mentioned above. The second processing zone contains a second screening device (28) and a device for storing and releasing to the conveyor (5) the briquettes or portions thereof which have not passed through the second screening device (28). In particular, the second screening device (28) serves to remove from the briquettes their flashings, which pass via the conveyor (5) and the first screening device (8) to the briquetter (22) and are thus recycled and recovered.

Description

METHOD AND APPARATUS FOR HANDLING FLUX FINES

The invention concerns recovering flux fines and equipment used therefor. Often, no attempt is made to recover the fines in the limestone fed as flux in oxygen steelmaking. Sometimes the limestone is non-dusty (under 10% fines), but such material requires longer blowing time and gives lower throughput. Oftener, the limestone has 20 to 60 weight percent of fines. In oxygen steelmaking, such fines do not add to the flux available in the vessel, because there is a strong updraft and only larger particles can pass through it. Fines, if not lost elsewhere in receiving limestone and conveying it to the vessel, are rejected by the updraft.

Seldom are the fines recovered in useful form; when they settle, they are mixed with impurities. Until now, they have been a waste material, involving costs for clean-up and hauling. Also, there are costs for protecting workers from dust and for dealing with interruptions in operation (dust in bearings, seized conveyor rolls).

Providing equipment hooding the conveyors is not satisfactory. The equipment is expensive to install, operate and maintain, and it does not deal with particles made airborne by the updraft.

Hooding the oxygen vessel has been proposed (U.S. Patents Nos. 2,829,960 and 2,855,292), but such a system has not beeen used commercially.

Recovering fines with blower and baghouse means is known. Operators of oxygen steelmaking vessels have not known however, that for their process such means have an economic benefit. Recovering airborne flux fines in the fines-receiving zone is not economically attractive, absent a practical process for using the recovered fines, because it solves the problem only partly-conveying flux to the vessel makes more fines, and some fines become airborne only during an attempt to feed them to the vessel. Screening to remove such fines is unattractive, increasing costs and further degrading the flux material.

Briquetting fine limestone is known, but many known processes have been disappointing. Some processes, with improper conditions and procedures, give briquettes that degrade intolerably if handled or stored. Whether briquetting costs can be justified has been questionable, considering the pressures required for good briquettes, the cost of wear-resistant roll material, and the service life of rolls. But it is known that binderless briquetting of limestone fines to make briquettes that can be stored or. handled is physically possible, especially disregarding the "flashing" problem.

Agglomerators know that every briquette has, where the rolls meet, a ridge of "flashing". It is weak, and it protrudes and tends to be removed by contact with conveyors or other briquettes. A process dealing with this problem is desirable.

Limestone-fines briquettes often store poorly. Initially, such briquettes can be handled, but later, they absorb atmospheric moisture, swell, and are softened, and cannot be handled without degradation. Consequently, agglomerators have eschewed briquetting and favor other ways.

Thosed skilled in the art have not known that briquetting can be combined with other equipment (densephase conveyor, screen, weighing means) known per se to obtain an ensemble which briquettes and handles "flashing" and feeds fresh flux briquettes to a process, at considerable cost savings. Equipment payoff is rapid, and the problems with using 20-60% fines limestone in oxygen steelmaking are overcome.

The Drawing The sole Figure is a schematic diagram of the inventive apparatus, used for the inventive method.

Description of Preferred Embodiments Receiving Zone In receiving zone 2, fines-containing flux is accepted from a truck, railway car, or other transportation means.

The receiving zone and its equipment may vary in form, depending upon size and configuration of present equipment when the invention is used with an existing process. Principally, the zone 2 must have a dry collection means 4, whose objective is to recover fines becoming airborne in zone 2. Removing there all particles becoming airborne when fed to the vessel is not attempted. Existing installations usually lack room for screening equipment. large enough for such separation and recovery. Commonly, installations have a smallish cellar with an end of a belt conveyor 5 near its bottom. Sometimes, installing equipment to recover all fines is possible, but access to it for repair or maintenanace would necessitate expensive enlargement of the cellar. To avoid that and get other advantages, the separation effort in zone 2 is preferably limited to particles airborne there. Any conveying generates fines; it is undesirable to try removing all fines initially. The means 4 recovers all, or as much as practicable, of the fines becoming airborne in zone 2; these may be about 1 to 10 percent of the total fines that should be removed to prevent rejection of fines to atmosphere near the process vessel. Various means are known, including electrostatic precipitators, other precipitators, cyclones, and preferably baghouse means.

A baghouse means of modest capacity may be used. It has a blower such as one with capacity about 280000 liters per minute (10000 cubic feet per minute) at a static head of 25.4 to 45.7 centimeters (10 to 18 inches) of water at a dust loading of 11.6 to 23.2 milligrams per liter (5 to 10 grains per cubic foot), for an operation unloading 893000 kilograms (1000 U.S. tons) of flux per hour. Typically, unloading is done about 10 hours out of 24 if an oxygen vessel is operated at capacity, and otherwise correspondingly less. Engineers of ordinary skill can design the needed equipment. Relevant factors require consideration. These include (1) maximum percent of potentially airborne fines in flux, (2) size of collection zone (extent of free fall,) and (3) cross-section of column of falling flux. Blower size depends also upon size, length, and configuration of ductwork and blower location with respect to collection and discharge point or points, but necessary calculations require no inventive skill.

First Processing Zone After airborne fines are collected by means 4, the next step is conveying nonairborne material and airborne fines to a first processing zone 6, containing screening means 8 and preferably secondary collector means 10. Nonairborne material travels via belt conveyor 5. Airborne fines are conveyed with a dense-phase pneumatic conveyor 14. Screening means 8 separates its input into oversize and undersize portions. As at 16, the oversize goes to a process vessel, e.g., oxygen-steelmaking vessel. Preferably, a weighing means 18 is associated with the means conveying the oversize to the vessel. Feeding proper-sized particles to the vessel, as at 16, is done either directly or, more usually, indirectly. The means 16 may include means such as shortterm storage bins and weighing means for measuring predetermined weights of charge to the vessel. Keeping short the total time between briquetting and charging to the vessel is important, so that briquettes are fed before they absorb moisture and degrade. Usually, the time should be under 2 days, and preferably, under 24 hours.

A choice may be made about the size of particles passed by means 8. In feeding limestone to an oxygen vessel, satisfactory results come from a screen retaining as oversize the particles about 6 millimeters (1/4 inch) and larger. A means 8 somewhat coarser can be used, but in oxygen steelmaking, the flux desirably reacts as quickly as possible, so it is not especially desirable to use particles any larger than needed to enter the vessel despite the updrafts mentioned above. As received, flux may contain some particles as large as 75 millimeters (3 inches); naturally, the means 8 retains these as oversize and they pass directly to the vessel as at 16.

As mentioned, dense-phase conveyor means 14 conveys airborne fines to zone 6. These particles usually are quite small but possibly contain flaky particles of 12 millimeters (1/2 inch) in maximum dimension. A densephase conveyor resembles a pneumatic coneyor, but where a pneumatic conveyor's conduit contains mostly air and a little of conveyed solids, a dense-phase conveyor's conduit has in it mostly solids and a little air, the solids being suspended like finely divided solids in a fluidized bed. Usually a dense-phase pneumatic conveyor terminates in a closed receptacle, but with this invention, the means 14 terminates either above the first screening means 8 or just below it and above the hopper 19, but in any event near to a secondary collection means 10. The screening in zone 6 necessarily makes airborne fines, and usually some of the fines conveyed by the means 14 inevitably become airborne at the delivery end of the means 14. The secondary collection means 10 may be a small bag-type dust collector of capacity about 70000 liters per minute (2500 cubic feet per minute) for operations on the scale indicated above. For such an operation, the dense-phase pneumatic conveyor may convey lime dust with a density of 0.8 kilograms per liter (50 pounds per cubic foot) at over 382 kilograms per hour (840 pounds per hour) a distance of about 182 meters (600 feet), using a Schedule 40 pipe of 37 millimeters (1-1/2 inches) diameter and 840 to 1120 standard liters per minute (30 to 40 standard cubic feet per minute) of air at a pressure of 5.42 to 5.7 atmospheres (65 to 70 pounds per square inch gauge). Various suppliers, e.g., Consolidated Engineering, Chicago, Illinois, sell densephase conveying equipment like that used with this invention.

Hopper, Briquetting, and Storage Undersize material through screen means 8 goes as at 17 to a hopper 19. Material passes as at 20 from hopper 19 to a briquetter 22. Skilled agglomerators know suitable equipment and practices for binderless briquetting of fines of flux, e.g., limestone. Material is forced, as by a screw conveyor (controllable-pressure, positive-displacement means) into the nip of briquetting rolls, which also have a controllable force balancing the roll-separating force. Typical conditions for briquetting 10900 to 11800 kilograms (12 to 13 tons) per hour of lime fines include using rolls of 521 millimeters (20-1/2 inches) diameter operating at 20 to 40 revolutions per minutes to make pillow-shaped briquettes of about 6 cubic centimeters each. The roll-separating force is about 5370 kilograms per centimeter (15 tons per inch) of roll length. The precise conditions used vary with characteristics of the flux processed, and those of ordinary skill in the art can make any needed adjustments in process parameters.

After being made, the briquettes pass as at 24 to storage means 26. Desirably, the storage means holds enough briquettes to run the process for 2 to 24 hours. Because binderlessly briquetted limestone fines tend to absorb atmospheric moisture, swell, and become degraded, planning to use them promptly is necessary (preferably within 24 hours, nearly always within 48 hours). Ordinarily, it is uneconomical to give briquettes storage in a moisturefree atmosphere or coat them for moisture resistance, especially since by prompt use these cost-adding measures are avoidable. If the flux is different, e.g., fluorspar, the storage practices may be modified, as those skilled in the art will recognize.

Flash-Removal Means Desirably, the invention apparatus also comprises an optional flashing-removal means 28 located above conveyor 5. Briquettes come thereto from storage as at 30. Means 28 may be any means causing rubbing of briquettes against one another to remove flashings. Means 28 preferably comprises a vibrating screen; conveyor 5 receives both undersize and oversize therefrom. Deflashing is obtained not only from such means, if used, but also because of abrasion occurring during removal from the briquetting means and in transport to, through, and from the storage means and to conveyor 5. The screening means 8 removes as undersize the fines made by such deflashing.

At times, conveyor 5 feeds material from zone 2 to zone 6. At other times (no flux being fed to zone 2), conveyor 5 feeds .briquettes to zone 6. By observing weights sensed by means 18 during operation in the two different modes, the fraction of the vessel's flux feed coming from the inventive process is determinable. Alternatively, weighing means 8 only weighs briquettes fed to the process vessel. The location of means 18 can be varied to suit requirements. Means 18 can be eliminated if its information is unnecessary.

The apparatus and process described above improve productivity and yield considerable economic benefits. The benefits are large enough that the solving of the problems of clean-up and conveyor-belt breakage is essentially free of charge. There is also avoided the damage from a dusty atmosphere to other mechanical, electrical or hydraulic equipment. The economic value of the fines in their briquetted form, as a replacement for a part of the as-received fines-containing flux which would otherwise necessarily have been purchased in order to operate the process, is great enough by itself to justify installing and operating a system according to the invention.

Though discussed above principally as to handling calcined-limestone fines, the invention applies to handling other flux or coolant such as raw lime, dolomitic lime, dolomite, fluorspar, and ore.

Claims

1. Apparatus for processing flux material at a site whereat said material is received and is fed to a metallurgical-process vessel, said material containing in its form as received at said site a portion of fine particles of such size as to be rejected to the atmosphere at said process vessel said apparatus comprising a reciving zone having therein a first dry collection means for removing from said as-received material a portion thereof which becomes airborne in said receiving zone, a screening means serving to separate material fed thereonto an oversize portion and an undersize portion. means for removing non-airborne particles of said flux material from said receiving zone and conveying said non-airborne particles onto said screening means, hopper means for receiving said undersize portion, dense-phase pneumatic conveyor means for conveying to said hopper means material obtained from said first dry collection means, briquetting means for receiving material from said hopper means and producing briquetted particles in pieces of such size as to be classified by said screening means into said oversize portion of said material fed thereon, means conveying said briquetted particles to said screening means, and means conveying said oversize portion from said screening means to said process vessel.

2. Apparatus for processing flux material at a site whereat said material is received and is fed to a metallurgical-process vessel, said material containing in its form as received at said site a portion of fine particles of such size as to be rejected to the atmosphere at said process vessel, said apparatus comprising a receiving zone having therein a first dry collection means for removing from said as-received material a portion thereof which becomes airborne in said receiving zone, a screening means serving to separate material fed thereonto into an oversize portion and an undersize portion, means for removing non-airborne particles of said flux material from said receiving zone and conveying said non-airborne particles onto said screening means, hopper means for receiving said undersize portion, dense-phase pneumatic conveyor means for conveying to said hopper means material obtained from said first dry collection means, a second dry collection means- located in the vicinity of said screening means and a discharge end of said dense-phase pneumatic conveyor means for recovering airborne particles in said vicinity and feeding them to said hopper means, briquetting means for receiving material from said hopper means and producing briquetted particles in pieces of said size as to be classified by said screening means into said oversize portion of said material fed thereon, means conveying said briquetted particles to said screening means, and means conveying said oversize portion from said screening means to said process vessel.

3. Apparatus as defined in claim 2, said apparatus further comprising weighing means operatively associated with said means conveying said oversize portion to said process vessel.

4. Apparatus as defined in claim 2, wherein said means conveying said briquetted particles to said screening means comprises means for causing said pieces to be abraded against one another to remove flashings on said pieces and to be deposited upon said means for conveying nonairborne particles onto said screening means.

5. Apparatus as defined in claim 4, said apparatus further comprising weighing means operatively associated with said means conveying said oversize portion to said process vessel.

6. A method for handling limestone containing fines and delivering it to a metallurgical-process vessel, comprising separating and collecting an airborne portion of said fines at a receiving station, conveying the remainder of said limestone including a nonairborne-fines portion to a screening means via a belt conveyor, screening said limestone to recover an oversize portion suitable for direct feeding to said vessel and an undersize portion, briquetting said undersize portion in the absence of binder to form briquettes of minimum dimensions corresponding to that of said oversize portion, and feeding said briquettes to said vessel before they have undergone substantial absorption of moisture and degradation.

7. A method as defined in claim 6, said method comprising the additional step of separating and collecting airborne fines at the vicinity of said screening means and adding them to said undersize portion.

8. A method as defined in claim 6, said method comprising the additional step of conveying from said receiving station to the vicinity of said screening means by dense-phase pneumatic conveyor means said airborne portion of said fines.

9. A method as defined in claim 8, said method comprising the additional step of separating and collecting airborne fines at the vicinity of said screening means and adding them to said undersize portion.