Patent Application
20100132599
1. Field of the Invention
The present invention relates to round-bottom pyrometallergical furnaces with cylindrical containment shells for the smelting, converting, or melting of concentrates, mattes, or metals, and more particularly to methods and devices for applying and maintaining proper compression of the brick hearth in a furnace refractory so as to extend its service life.
2. Description of the Prior Art
One type of smelting furnace for winning iron from ore is built with vertical, cylindrical, steel containment shells with layers of refractory bricks inside the walls and a downwardly dished bottom. A hearth brick sub-layer on the bottom is covered with a brick hearth working layer. The refractory brick layers inside the steel containment shells can withstand the very high operating temperatures usual to the smelting of iron ore, and the outer shell provides the necessary containment and support.
Hearth bricks swell up in size over their operational lives as the bricks slowly absorb molecules of metal. Many expensive and complex ways have been devised over the years to keep the refractory bricks tightly pressed together as they swell so that liquid metal, matte, or slag cannot leak through the gaps. For example, so-called “flexible shells” bind adjoining overlapping plates together. The loose plate construction can allow for quite a lot of expansion and contraction. However, the cost of these kinds of containment shells is prohibitive.
Rigid hearth containment shells are much less expensive since they are constructed as a single rigid piece that does not require plate binding mechanisms. But conventional ways of keeping the hearth bricks together under the right pressures for these rigid shells accommodates only very limited growth in the hearth brick before shutdown and replacement with new brick is required.
Conventional systems are normally designed to accommodate the thermal expansion of the bricks, but do not maintain the pressure when the bricks cool down and shrink. This allows gaps to form which can invite molten materials to penetrate the brick joints. When the furnace finally reheats, the hearth is incrementally increased in diameter by the new material frozen in the joints. It therefore follows that extending the service life of the hearth bricks translates directly into big savings in the maintenance costs because fewer, less frequent shutdowns and brick replacements are needed over the life of the furnace.
What is needed are methods and devices that can maintain the pressures on the hearth bricks as the system cools down and the bricks shrink, and that can accommodate larger amounts of growth in the brick hearth over the campaign life, and that use the less expensive rigid shells to contain them.
Briefly, a brick hearth system embodiment of the present invention comprises a rigid containment shell in which a concave dished bottom is lined with a sub-layer and a working layer of hearth bricks. The outer perimeter of the hearth brick is ringed with thrust blocks to compress the whole radially toward the center and to thereby deny leaks from forming between the separate bricks. Many individual thrust rods penetrate the outer bottom of the containment shell, and such are used to transmit compression forces generated outside the shell to be applied inside against the thrust blocks in unison. Each thrust rod receives an adjustable amount of inward force from a spring, acting either directly or through a beam or rocker arm. These are anchored to and use the hoop strength of the containment shell as leverage. As the hearth bricks grow during their service life, the ring of thrust blocks grows too in diameter inside the margins provided within the containment shell. The individual thrust rod springs are periodically adjusted to keep the working pressures in an optimal range.
An advantage of the present invention is that replacement of the furnace hearth and the surrounding expansion material is less frequent compared to conventional systems.
A still further advantage of the present invention is that a system is provided that minimizes the long-term growth of the furnace by maintaining the pressures on the hearth bricks as they cool down and shrink, thus preventing seepage of molten material into to the interstitial joints after every cycle.
Another advantage of the present invention is that a system is provided that reduces the rate of expansion of the furnace hearth by increasing the load on the furnace hearth.
A further advantage of the present invention is conventional containment shells can be modified so the thrust blocks and rods of the present invention can be readily installed and put to work.
A still further advantage of the present invention is that a system is provided such that a hearth binding system could also be installed on rectangular furnaces with rigid binding systems, which do not already incorporate tie-rods and springs. The new additions would allow for controlled expansion of the hearth, and thereby reduce the rate of expansion due to the penetration of the hearth that would otherwise occur by molten materials.
Another advantage is embodiments of the present invention allow much longer campaigns before a furnace must be shut down, its hearth brick replaced, and the hearth compression thrust rods reset. When retrofitted with embodiments of the present invention, conventional furnaces with campaign lives of two to four years should be able to double this time before they need to be shutdown for hearth maintenance.
A further advantage of the present invention is that overstressing of the furnace shell caused by hearth refractory expansion can be eliminated.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
The outer perimeter of the hearth brick working layer 110 supports a wall brick 112. Gravity keeps the wall brick 112 tightly packed, and a ring of thrust blocks 114 inwardly compress and compact the hearth brick working layer 110. The radial compression towards the center is provided by many individual thrust rods 116 that each push inwardly against corresponding thrust blocks 114. The thrust rod forces are provided externally by adjustable spring assemblies 118 mounted on the outside of the shell wall 106. Simple holes, or pushrod guides may be provided for the thrust rods in the containment shell 102. The adjustable spring assemblies 118 take advantage of the hoop strength and solid construction of the shell wall 106 to generate the necessary leverage.
In operation, the adjustable spring assemblies 118 are periodically set to a predetermined pressure value. The hearth brick working layer 110 will inevitably grow in diameter as molecules of molten metal are absorbed into the refractory brick material and the minute spaces between them. Such growth necessitates routine readjustment of the adjustable spring assemblies 118, and so the conditions should be monitored.
Initially, a relatively wide margin of space is provided between the outer edges of the thrust blocks 114 and the inside of shell wall 106. Such can be filled with expansion boards or other crushable material that can be replaced or removed as the space diminishes. Fill material such as this is not required if a spring system embodiment of the present invention is sized to carry the full hearth load.
In some embodiments of the present invention, the thrust blocks 114 are provided with circulating coolant to draw off heat, and may be made of copper or copper alloy. A wall cooler 120 may also be provided which has horizontally or vertically oriented layers. The thrust block 114 is intended to either replace the brick skew, or apply load directly to it. The thrust blocks, as shown in
Alternatively, it could be kept shorter to the top of the brick skews, e.g., to keep only the brick within the hearth working layer tight. The possibility of leaks developing between wall brick and the skews would thereby be eliminated. The shapes of the thrust blocks shown in
The typical commercial furnace hearth size ranges from two to fifteen meters in diameter. The radial spacing of the hearth compression thrust rods 116 depends on the forces required, the stiffness of the inside thrust blocks 114, and should be arranged to avoid interference with tap holes and other openings. As an example, the spacing could be expected to range from one to two meters for larger diameter furnaces, or they could be arranged at 10-30 degree increments around the furnace shell 102.
Thrust rods 116 would generally be fabricated from steel, but may need to be made from metals that can resist corrosion and/or assist in cooling. The size and cross sectional shapes needed for the thrust rods 116 depend on the engineered forces required. Other contributing forces increase with the size of the hearth refractory, as well as the fluid pressure coming from the top of the hearth. The maximum fluid pressures will be observed at the hearth invert, or the lowest point of the hearth.
The devices could be used to impart initial compression of the hearth, which could result in an initial net shrinkage. The devices could be designed to typically accommodate 50-150 mm of hearth expansion. On a percentage basis, up to practical maximum of two percent of the hearth diameter. For larger hearth movements, differential expansion between the wall and the hearth becomes unmanageable, and the required size of the compression rods becomes unreasonable.
The minimum compression forces on the hearth refractory brick must be sufficient to keep interfacial pressures between the bricks greater than the fluid pressures trying to come between or float the bricks. One object is to limit penetration of molten metal, matte or slag that gets into the joints. Too rapid a penetration can induce a quicker than normal rate of expansion of the hearth over the long term. Too much molten metal penetration that gets under the bricks can cause individual bricks and sections of brick hearth to try to separate and float to the top of the matte. So, the hearth compression forces applied must be sufficient to maintain hearth stability, and overcome large buoyancy pressures if molten metals nevertheless get beneath the hearth brick working layer 110.
When embodiments of the present invention supply sufficient hearth compression to maintain hearth stability, and apply the minimum compression needed to limit melt penetration between the joints, their long-term hearth refractory rate-of-growth will not exceed that observed in conventional current hearth designs. And the service life will be greatly increased at very modest cost.
The rate of hearth expansion has been shown to be reduced with good hearth compression. Hence, increasing hearth compression could be used to reduce long-term hearth expansion. With existing hearth designs that use expansion boards between the skews and vertical shell plates, hearth expansion can be 2-50 millimeters (mm) after a furnace has been shut down cold and restarted. In usual practice, the expansion material must be replaced prior to the shell becoming overstressed.
Corrosion can be an issue in those environments where corrosive gases are produced as part of the smelting process. Gases like SO2 and SO, can readily form acids. Acid environments necessitate the use of stainless steel or nickel alloys to resist corrosion.
Parts that may be exposed to high heat loads or molten materials may require cooling. If a component is to be cooled, it may be fabricated from a conductive alloy of copper or other metal, to minimize stresses and to reduce the potential for cracking. For example, the internal member used for distributing the compressive forces to the hearth may be cooled with air, water or other heat transfer fluid or gas; it may have internal cooling passages for conveying the heat transfer fluid or gas.
In both
In
The inward forces generated by the springs 214 and 216 are directed at an angle theta (θ) 226, e.g., 90-degrees to the central vertical axis of the furnace, or at some appropriate obtuse or acute angle relative to the shell wall 210 to put them inline with the outer edges of a dished bottom hearth brick working layer.
Adjusting nuts 218 and 220 are set at the beginning, and then reset during the service life of hearth brick working layer 202 as it grows. The amount of pressure applied by particular springs can be interpreted from compression charts according to their present compressed spring length. Other mechanisms could also be used to estimate thrust rod pressures to keep the furnace in proper tune.
Seals can be included around each thrust rod where it penetrates the furnace shell to control leakage. But, if the spring system is doing its job keeping the bricks tight, such seals would not be necessary.
The refractory bricks inside a furnace are quite brittle. Uneven pressures on the bricks can allow uneven thermal expansion and possible break outs of molten material from the furnace.
Embodiments of the present invention use a circular band portion of the vertical walls of the containment shell for evenly applied inward radial leverage like a barrel hoop. In this way, the shell maintains hoop tension and naturally resists distortions. Dozens of radial thrust rods are used to uniformly compress a ring of thrust blocks and the refractory brick they encircle inside the furnace.
Heat transmission through the brick is improved by uniform compression of the refractory. The campaign life of the brick and the furnace is extended, because the rings of brick expand outward, increasing the pressures between the shell and the bricks.
In conventional rectangular furnaces, the bricks tend to pull away from the walls and coolers during each campaign because the bricks swell more on the hot faces than they do on the cooler backsides. The trapezoiding produces gaps between the lower edges of the brick, resulting in rapid erosion and chemical corrosion of the bricks. Such is well known to designers of circular and rectangular furnaces.
A hearth shell plate bolted to a vertical shell plate is considered “one piece”. The shell at the level of the compression devices should be one piece. However, if openings are needed, they must be reinforced to maintain overall integrity of the shell.
A thrust bar 514 in each compression assembly 510 is able to slide in and out on vertical pairs of threaded radial studs 508. Pairs of thrust springs 516 push the thrust bars 514 and thrust rods 512 radially inward against bottom refractory layer 502 using threaded radial studs 508 for leverage. Nuts (not shown) on the ends of the threaded radial studs 508 keep the springs 516 and allow some adjustment. Other spring arrangements are possible.
As the bottom refractory layer 502 grows in diameter during its campaign life, springs 516 will compress but still maintain more-or-less even pressure all around the circular perimeter of bottom refractory layer 502.
A tap hole 520 is water cooled and allows molten material to be tapped out, according to the elevation of its placement. Different elevations provide for the tapping of slag (molten oxides solution floating on top), matte (molten metal sulfide phases), or molten metal (the heaviest and at the bottom). The ends 522 of water cooled copper blocks can be seen all around the outside of furnace 500 in
As can be seen clearly in
Each thrust block 530 has a face 534 configured to be perpendicular to the plane of the bottom refractory layer 502 at its circular outer edges.
Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention.
This Application is a Divisional and Continuation-In-Part of U.S. patent application Ser. No. 11/674,123, filed Feb. 12, 2007, and titled, FURNACE HEARTH COMPRESSION, by the Present Inventor, Allan J. MacRae.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11674123 | Feb 2007 | US |
| Child | 12700721 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11674123 | Feb 2007 | US |
| Child | 11674123 | US |
