Metallurgical apparatus – Means for treating ores or for extracting metals – By means applying heat to work – e.g. – furnace
Reexamination Certificate
2000-06-12
2001-08-28
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Means for treating ores or for extracting metals
By means applying heat to work, e.g., furnace
C266S241000
Reexamination Certificate
active
06280681
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to furnace crucibles, and more particularly to the copper cooling blocks used behind refractory layers in the walls of the crucibles.
2. Description of Related Art
The high temperatures used in metal furnaces is enough to erode even brick-lined crucibles. Refractory materials are conventionally used to line the insides of crucibles, and the prior art has adopted the use of cooling blocks behind such linings. The operational result is a thin layer of the molten slag, matte and/or metal freezes on the walls and helps stabilize them against break-out. Such cooling blocks are also used for burner blocks, launders, tuyeres, staves, casting molds, electrode clamps, tap-hole blocks, and hearth anodes.
Most modern pyro-metallurgical furnaces use cooling systems to stabilize the unavoidable erosion of wall, roof and hearth refractories. Cooling blocks are typically arranged in a number of different ways. Walls, roofs and hearths that include them are used in cylindrical furnaces, oval furnaces, blast furnaces, Mitsubishi-style flash smelting and converting furnaces, IsaSmelt furnaces, electric arc furnaces, both AC and DC, basic oxygen furnaces, electric slag cleaning furnaces, rectangular furnaces, Outokumpu flash smelting and converting furnaces, Inco flash smelting furnaces, electric arc furnaces, slag cleaning furnaces, and reverbatory furnaces.
Cooling blocks can also be arranged in layers, with alternating courses of refractory. A refractory brick and/or castable refractory sometimes is used for the hot face of the block and may be smooth or have pockets and/or grooves machined or cast-in.
A problem develops when the cooling pipes and the metal castings are not the exact same materials. Different materials will have different coefficients of thermal expansion, and the strength of the bonding between the pipes and the castings will also vary. Constant thermal cycling can work the pipe loose of the casting, and when this happens the thermal efficiency drops significantly.
However, pipes made of materials with melting points that are higher than the molten casting metal are desirable because such resists softening or break-through during the casting pour. One prior art way to work around this problem is to tightly fill the pipes with sand so they are reinforced against collapse. Such sand is washed out after the casting has cooled.
Some combinations of cooling pipe and metal casting materials are known in the prior art as being able to provide at least an acceptable service life. For example, Falcon Foundry (Lowellville, Ohio) has produced Monel-400 pipes cast in copper cooling blocks since the 1960's. (Monel-400 is a trademark brand for an alloy of about 63% nickel and 31% copper.) Other companies, ElectroMelt (now defunct) and American Bridge (a former division of U.S. Steel), have designed cooling blocks utilizing Schedule-40 or Schedule-80 Monel-400 pipe coil assemblies which allow cooling chambers to be well defined. No cooling of the pipes is required during the casting pour of copper, as is normally the case with pure-copper pipes.
Unfortunately, failure analyses have shown that the copper cooling blocks are not in complete contact with the Monel-400 pipe. Many defects can be seen to exist when the blocks are destructively tested and the Monel-to-copper bond is evaluated. Such bonding defects reduce the thermal transfer efficiency and introduce unknowns into the overall furnace-cooling patterns.
Prior art cast copper and low-alloy-copper cooling blocks and design engineering have also been commercially supplied and/or designed by Hatch (Mississauga, Canada), Outokumpu OY (Finland), Kvaerner (Stockton, England), Demag (Germany), Hundt & Weber (Siegen, Germany), Tucson Foundry (Tucson, Ariz.), Thomas Begbie (South Africa), Alabama Copper (Alabama), Niagara Bronze (Niagara Falls, Canada), Hoogovens (Netherlands), and others.
Outokumpu, and others, design and manufacture copper cooling blocks from copper billet with longitudinal holes drilled for water passages. Extruded holes have also been used for the water passages, but some of these have been the subject of failures. Transverse drill holes with internal plugs have also been included to form internal cooling-water circuits.
The drilled and extruded designs all require plugs to be installed in all the open drill ends around the edges of the billet blocks. Solder, welded, and pipe-thread type plugs have all been tried. But many such blocks leak nonetheless, and such leaks are very dangerous in a metallurgical furnace.
The size and shape of such kinds of blocks is limited by the ability to cast or forge the copper billets. The internal water passage layout is often very constrained by having to fashion the passages from combinations of interconnected drill bores.
In contrast, cast blocks can be made in a wide variety of block shapes and sizes, and almost any layout is possible with the internal piping. Cast blocks can be used with much larger heat loads, compared to drilled and plugged blocks.
The fabrication of drilled blocks and cast blocks each present their own challenges. In casting, the water pipes can be both flow and pressure tested before and after. The danger of a leak through a copper cooling block with fabrication voids is very low because the pipe walls will contain the water.
Conventional cast cooling blocks are typically manufactured by forming a water pipe into a desired layout and pressure-testing it, before and after, to 150% of the design operating water pressure for at least fifteen minutes. Before the casting pour, the outside of the pipe is cleaned to minimize gas bubble formation that can result in porous casting sections at the pipe-coil and cast-copper interfaces. Sand is sometimes used to fill the inside of the pipes to stiffen them against softening, but only when using a pipe coil material that does not have a melting point significantly higher than the casting temperature of copper. For example, Monel-400 pipe does not ordinarily need to be packed with sand before casting.
The casting molds are made with extra allowances for machining off of porous sections, gates, risers, and shrinkage. Such molds are typically made from sand mixed with a bonding agent. The original shapes which are pressed in the sand are made from wood and other easily formed materials.
The pipe coils are securely located in the correct position inside the sand mold. Copper from a melting furnace is poured into a ladle. A de-oxidant may be necessary if the copper is melted in a non-inert environment. Any oxide slag is skimmed off. A sufficient superheat of the copper over its melting point is used to prevent the copper from prematurely solidifying during handling or pouring. The liquefied copper from the ladle must be sufficiently fluid to fill the mold, completely cover the pipe coils, and flow to the top of the risers. Any gas bubbles will rise high up to the surface of the risers.
Once the deoxidized copper is poured into the mold from the ladle, the casting is allowed to cool until it has totally solidified. The risers and gating systems are mechanically removed. Any excess material is machined or cut away, and hot-face grooves and/or pockets are formed or finished. On the outside surface, the holes are drilled and tapped for either locating, mounting or block lifting. The mating surfaces, between blocks, are normally machined. The amount of machining needed is dependent on the end use for the block.
Any surface imperfections may or may not be repaired, depending on the requirements of the end user. Such imperfections are ground out, weld filled, and machined smooth. The completed blocks are inspected using one or more inspection x-ray, visual inspection, infrared-thermal inspection, and hydrostatic or pneumatic pressure testing for leaks. Thermal and/or electrical testing is used to check that the block meets minimum thermal and electrical conductivity. Dimensional tolerances are also checked. Samples can be used in a destructive testing
Kastler Scott
Main Richard B.
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