Patent Application
20180245171
The present invention relates to liquid cooled oxidizing gas injection systems and other cooling applications for pyro-metallurgical furnaces, and more particularly those gas injection systems with water-based liquid cooling and heat transfer fluids that are intrinsically safe from boiling liquid expanding vapor explosion (BLEVE).
Water is an extremely explosive and dangerous material when it comes into contact with the super-heated baths inside pyro-metallurgical furnaces. Water will flash to steam very forcefully in such contacts, and even small amounts can instantly destroy the furnace containment and the building around it. These sorts of boiling liquid expanding vapor explosion (BLEVE) events have caused many deaths and injuries. So the introduction in pyro-metallurgical furnace applications of any water-based coolants into the cooling jackets of oxygen injecting lances and tuyere, and even bath containment cooler panels, must be done quite judiciously.
Water is such a compelling and useful coolant that it is worthwhile to find a safe way to use it free of the risks of BLEVE.
Several major heat transfer fluid cooled components used in and around furnaces run the risk of significant amounts of water leaking either onto the top of the bath submerged or injected into the bath. For example, water-cooled vertical lances, like subsonic Top Submerged Lance (TSL) in nonferrous furnaces, sonic lances used for steel making in Basic Oxygen furnaces. Also, furnace walls, roof cooling blocks, tap hole blocks, torches, launders, tuyeres, burner blocks, etc. for both ferrous and non-ferrous furnaces.
If there is a coolant leak onto the top of a bath, a crust of slag there can freeze. Then once the frozen slag cracks, free water can flow in to reach the matte or metal below and cause a BLEVE.
If a copper body in a cooler is worn away enough for an internal pipe to be exposed, such also could also leak cooling fluid into the furnace.
If any superheated metal contacts a block itself, it could thermally overload the cooling block's capacity to remove the heat, and that could lead to internal steam generation and the catastrophic failure of the cooling block. Steam inside the coolant passages significantly interferes with the ability of the block to remove heat because it introduces excess levels of thermal resistance to the liquid. Such then can lead to melting and rapid block wear, and end with a fluid leak into the furnace. This can also occur in contacts with superheated slag or matte, but the risk is often less than with the liquid metal itself.
If there is a large leak of molten slag, metal or matte out of the furnace and if it contacts a cooling fluid line, an explosion outside of the furnace could occur with water.
Water is very useful as a coolant because it has a relatively high specific heat, pumps very easily, is plentiful, inexpensive, and non-toxic. But in some applications as a coolant it needs to be enhanced. This is done conventionally with “antifreeze” glycols to lower water's freezing point and raise its boiling point. Common glycols put to these uses include monoethylene glycol (MEG), Diethylene glycol (DEG), and propylene glycol (PEG).
Ethylene glycol (ethane-1,2-diol), aka MEG, is an organic compound with the formula (CH2OH)2. It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odorless, colorless, sweet-tasting, viscous liquid. Unfortunately, ethylene glycol is moderately toxic. Because of its high boiling point and affinity for water, ethylene glycol is a useful desiccant. Ethylene glycol is widely used to inhibit the formation of natural gas clathrates (hydrates) in long multiphase pipelines that convey natural gas from remote gas fields to a gas processing facility. Ethylene glycol can be recovered from the natural gas and reused as an inhibitor after purification treatment that removes water and inorganic salts. Propylene glycol (propane-1,2-diol) is a synthetic organic compound with the chemical formula C3H8O2. Chemically it is classed as a diol and is miscible with a broad range of solvents, including water, acetone, and chloroform.
Glycols that have absorbed water and desiccated another liquid or gas are called “rich glycols”. Their ability to bind up water makes the water bound up that much harder to run free in a BLEVE.
TSL furnaces are used for non-ferrous production, and have submerged lances to sub-sonically inject oxygen to burn the sulfur fuel available in the ore. TSL furnaces are charged with concentrate for smelting, or matte for converting. The TSL is a chemical reactor with a relatively short residence, on the order of about fifteen minutes. Closely taken measurements and sampling of the feed and oxygen reagents are critical.
BOF furnaces are used for ferrous production, and have non-submerged lances above the bath that super-sonically inject oxygen to burn carbon-based fuels in the bath. BOF furnaces are fed with scrap iron, or pig iron, and the fuel.
The ore from mines must usually be concentrated before it can be smelted for its metals. In copper smelting, those ores are primarily chalcopyrite (CuFeS2), or other sulfides of copper and iron minerals. These are crushed and ground to release the target minerals from the “gangue” waste minerals. The powdered ore can then be more easily concentrated with mineral flotation techniques.
The concentrates are input as a feed material for smelting in top submerged lance (TSL) furnaces to produce a “matte”. Matte is a molten mixture of sulphides. And “slag” is a molten mixture of oxides and any unreduced sulphides. Copper matte can be readily converted and refined into anode copper. A copper matte is an intermediate product, e.g., in the extraction of copper from sulphide ores that naturally contain copper. TLS furnaces are used to treat many different feed materials, which are concentrates derived from ore, secondary products from smelting or converting, scrap, slimes or residues from metal refining, or waste. Metals treated are copper, nickel, zinc, aluminum, lead, tin, gold, platinum-group metals (PGMs).
The matte produced will vary in “grade” depending on how the furnace is being operated. For copper, the matte grade as we use it here means, and is defined as,
TSL furnaces are also used for the production of molten metal or fume. The majority of TSL furnaces have nearly dry solid feed material, but it is possible feed molten slag or matte to the furnace.
Typical TSL furnaces are characteristically upright-cylindrical shaped steel vessels that are lined inside with refractory bricks. The upper portion of the vessel may be sloped to permit the exit of process gases. Copper blocks with internal passages for the flow of a heat transfer fluid may also be present in the walls. A molten bath of slag, matte or metal sits in the well of the furnace.
The furnace types we are concerned with herein employ a steel lance that is lowered into the bath from above, and air, or oxygen-enriched air, is forcefully injected through the lance at supersonic speeds into the bath to agitate and oxygenate the bath. Feed to the furnace consists of one or more of mineral concentrates, matte, metal, flux, coal, coke, reverts, or recycled materials, which are dropped either through a roof opening into the bath, or fed down the lance.
Some TSL lances employ “swirlers” to spin the injection gas within the lance to promote mixing of the oxygen and fuel at the discharge end.
Movement of air and oxygen down the lance helps to cool the steel. Such cooling can help to freeze a layer of slag in a protective coating outside the lance. Layers of solid slag help protect the lance from wear and the high temperatures inside the furnace bath. TSL furnaces operate up to 1400° C.
The submerged tips of lances will eventually wear out. And lances that have not been cooled well enough and heated unevenly can also develop pronounced curves along their relatively long lengths. The lances can also develop holes or cracks on the side which can lead to the leakage of the enriched air or oxygen to the sides and walls, thereby increase refractory wear and splash.
Refractory bricks are used as an internal lining in the furnace to protect its steel shell from the heat inside. A lance that has curved too much will cause increased damage to the refractory from erosion or impingement when the injected gases, generated process gases, and agitation become too intense in the near wall region.
The metal, matte and slag smelting products are removed from furnaces through “tap holes” or outlets, either continuously or in batches. Fume to be collected and process gases exit via an opening in the roof to the off-gas system.
Top submerged lance (TSL) furnace vessels that run with the lance immersed are subject to high amounts of wear to their tips. Such wear will eventually require the lance to be replaced after as little as a day, or as much as a week or two of nearly continuous operation.
Lance immersion depths are normally controlled based on a measured amount of tip pressure. Operators also closely monitor the matte grade and bath temperatures. If the matte grade is too high, there is a risk of the bath foaming, and downstream processing in the converters will be more difficult. If the matte grade is too low, refractory wear will increase, especially downstream in the launders.
Refractory linings for nonferrous smelting and converting are most often constructed using high MgO bearing materials, which are subject to hydration via the formation of Brucite after contact with water. The elimination of water in any heat transfer fluids has this as a second benefit, in addition to the avoidance of BLEVE.
Lances for TSL furnaces conventionally have a carbonaceous fuel pumped down the center and discharged into the furnace that adds to the sulfur fuel in the ore. The most common fuels are oil and natural gas. Hence, using hydrocarbons in any heat transfer fluid would not present a new risk to the operation of such a furnace.
Furnaces which have concentrate fed through openings in the roof are subject to uneven heating and loss of concentrate. The concentrate can be carried out of the furnace in the off-gas stream. It is possible to feed solid concentrate down and directly into the bath with a cooled lance, if there are no internal swirlers included to rotate the gases.
Any lance, fuel or concentrate burner, probe, or cooling block which must be cooled with a heat transfer fluid to withstand the intense heat loads inside a pyro-metallurgical furnace would benefit if the fluid was not susceptible to BLEVE.
Briefly, a cooling system embodiment of the present invention for use in support of a pyro-metallurgical furnace includes a liquid heat transfer fluid blend of 10%-50% water with monoethylene glycol (MEG), diethylene glycol (DEG), or triethylene glycol (TEG), and corrosion inhibitors. When using such glycols, a minimum of 10% water prevents the heat transfer fluid from becoming too viscous for economical pumping, and a maximum of 50% water prevents BLEVE incidents inside the furnace. Such intrinsically safe cooling system circulates the liquid heat transfer fluid blend with an optimally sized pump, filtration, pressurization, and at flow velocities sufficient to avoid film boiling.
Water makes an excellent choice as a coolant because its low viscosity makes it easy to pump and its high specific heat means that coolant pumping volumes and speeds can be kept as low as is possible. A balanced combination of these considerations means the pumps in water-based cooling systems can be economized. But introducing water-based coolants into high heat ferrous and non-ferrous pyro-metallurgical furnaces runs a high risk of boiling liquid expanding vapor explosion (BLEVE).
Said another way, the coolant comprises a blended mixture of a single phase organic compound of glycol alcohol, and water, and a corrosion inhibitor, wherein the water is limited to a range of about 10% to 50% by weight. So all of the water is fully absorbed in the glycol with no excess water left to support a BLEVE.
“Glycol” is any of a class of organic compounds belonging to the alcohol family. The glycol molecule has two hydroxyl (—OH) groups attached to different carbon atoms. The term “glycol” is often applied to the simplest member of the class, ethylene glycol.
The minimum percentage of water that can be used is limited by the adverse impacts of increasing viscosity and reduced specific heat that bear on the acquisition and operating costs of liquid pump 104. As viscosity increases, it requires a greater pumping effort and a stronger liquid pump 104 to maintain a minimum coolant velocity 106. And as the specific heat of heat transfer fluid mixture 102 is decreased by diluting the water, the greater will be the pumping effort required of a larger capacity liquid pump 104 to maintain a higher, minimum level coolant velocity 106 that will compensate for the inefficiency.
In practice, the heat transfer fluid mixture must have a room-temperature viscosity of less than 20 mPa·s. And the heat transfer fluid mixture 102 must have a specific heat greater than 2.3 kJ/kg·K. Otherwise, the minimum requirements for a suitable pump 104 become unreasonable and/or unmanageable.
The maximum percentage of water that can be used safely is limited by the risks of BLEVE. Short of that threshold, the mixed coolant blend 102 will burn, and not BLEVE, if it escapes from a cooler 108 into a high heat ferrous or non-ferrous pyro-metallurgical furnace 110.
Intermolecular bond types determine whether two chemicals are miscible, that is, whether they can be mixed together to form a homogeneous solution. Here, the water and glycol in the heat transfer fluid mixture 102 form a homogeneous solution. When two chemicals like water and glycol mix, the bonds holding the molecules of each chemical together must break, and new bonds must form between the two different kinds of molecules. For this to happen, the two must have compatible intermolecular bond types. Water and MEG glycol do. The more nearly equal in strength the two intermolecular bond types are, the greater will be the miscibility of the two chemicals. Usually there is a limit to how much of one chemical can be mixed with another, but in some cases, such as with CH3OH (MEG) and H2O (water), there is no limit and any amount of one is miscible in any amount of the other.
As a consequence, the percentage of water in the heat transfer fluid mixture 102 will have a practical range between 10% and 50%. The optimum percentage of water plus corrosion inhibitors in the heat transfer fluid mixture 102 is generally about 25%. No excess water is left unabsorbed to support a BLEVE. And the water that is absorbed is impeded from BLEVE.
The heat transfer fluid mixture 102 is circulated in a closed system and pressurized by a pressurization system 112. Typical pressures run 2-7 bar. Raising the pressure inside the closed system raises the boiling point of the heat transfer fluid mixture 102. The minimum boiling point of the heat transfer fluid mixture 102 under pressure should be no less than 175° C.
A filter 114 is used to remove contaminants from the heat transfer fluid mixture 102 as it circulates. Otherwise, the contaminants can interfere with the coolant's efficiency. The coolants here are under very high heat loads.
A chiller or heat exchanger 120 is used to remove and dispose of the heat gained by the heat transfer fluid mixture 102 in circulation, e.g., a cooler 108 inside furnace 110. Such chillers and heat exchangers are conventional.
These glycols, like all low-molecular-weight alcohols, are soluble in all proportions in water. The heat transfer fluid 202 further includes corrosion inhibitors. MEG is preferred because it is so completely miscible with water. But its toxicity might not make its use a first choice.
Despite its being an organic compound, MEG is quite polar because of the differing electro negativities of the oxygen and carbon atoms it contains. Water is also as polar because of the different electro negativities of hydrogen and oxygen. Thus with both being very polar, they are excellent solvents for one another.
Dow Chemical Company DOWTHERM SR-1 and DOWTHERM 2000 inhibited ethylene glycol-based heat transfer fluids are useful herein, and such already include corrosion inhibitors. These inhibitors prevent corrosion of metals in two ways. First, they passivate the metal surfaces, and react with them to prevent acids from attacking. A passivation process results that does not foul the internal heat transfer surfaces. Conventional inhibitors, in contrast, usually coat heat transfer surfaces with a thick silicate gel that gets in the way of good heat transfer. Second, the inhibitors in DOWTHERM fluids buffer acids that form as a result of glycol oxidation. (All glycols produce organic acids as degradation products.) Such degradation will accelerate in the presence of oxygen and/or heat. If left in solution, such acids lower the pH and will contribute to corrosion. The formulated inhibitors used in DOWTHERM fluids neutralize such acids.
Ethylene glycol (MEG) disrupts hydrogen bonding when dissolved in water. Thus, the use of ethylene glycol not only depresses the freezing point, but also elevates the boiling point such that the operating range for the heat-transfer fluid mixture is broadened on both ends of the temperature scale. Ethylene glycol alone has a freezing point of 8.6° F. (−13° C.) and a boiling point of 388° F. (198° C.). But, ethylene glycol is toxic. DEG is not so toxic.
Heat transfer fluid mixture 202 has these elements in a blend in a combination that expresses simultaneous predetermined ranges of viscosity, and of specific heat. The heat transfer fluid mixture 202 is intrinsically incapable of a boiling liquid expanding vapor explosion (BLEVE) by including less than 50% by volume as water, or even less as empirically determined as appropriate in specific applications.
Water inside a pyro-metallurgical furnace can be catastrophic in two ways. First it can be the “fuel” that gets triggered to produce a BLEVE. And second, the refractory linings can be severely damaged by water absorption. It is therefore an object of the present invention to cool oxygen lances with liquids that cannot BLEVE, and with liquids that will not damage refractory.
Oxygen injection system 200 further comprises a cooling system 204 that extracts and disposes of heat from the heat transfer fluid mixture and that provides liquid pumping for minimum circulation velocities that preclude film boiling of the heat transfer fluid mixture. A cooling plant 206 can be a part of the general cooling of the pyro-metallurgical furnace that already exists. A heat exchanger 208 provides the isolation needed to maintain the heat transfer fluid mixture 202 in its required composition and purity. A pump 210 circulates the heat transfer fluid mixture 202. But the flow velocity achieved depends on where it's being measured because the cross sectional area and temperatures at various points vary widely along the path of the circuit.
However, at no point should the capacity of pump 210 be so insufficient as to maintain a velocity of the heat transfer fluid mixture that will prevent film boiling. At the hottest points in the circuit, that will typically be a minimum of 2.0 meters per second.
An oxygen lance 212 has a protective liquid cooled jacket 214 that extends its full length to a tip. The protective liquid cooled jacket 214 receives the heat transfer fluid mixture 202 from the cooling system 204. It returns a hot heat transfer fluid mixture. The cooling here of oxygen lance 212 will stop any tendency of thermal curving by precluding uneven heating of oxygen lance 212 during operation. To do that, the liquid cooled jacket 214 may include swirlers and restrictors, flows that maintain a minimum velocity flow at critical points to prevent film boiling. Oxygen lance 212 has as its basic purpose to provide for the injection of an oxygen flow 218 into a pyro-metallurgical furnace 220.
Gas injection system 200 also uses a heat transfer fluid mixture 202 that has a predetermined viscosity less than 20 mPa·s, and a predetermined specific heat greater than 2.3 kJ/kg·K. These two limits allow economical choices to be made for pump 210.
Mechanisms for swirling heat transfer fluids and for making tip replacements possible for lances are conventional and plentiful, and are therefore not necessary to describe in particular detail here. Both would of course enhance and improve most embodiments of the present invention.
A heat transfer fluid inflow manifold 314 forms an annular plumbing connection with a heat transfer fluid supply jacket 316. A heat transfer fluid mixture is directed under pumping pressure down to the copper lance tip 306 where it turns and flows back up outside in a liquid cooling jacket 318 to a heat transfer fluid outflow manifold 320. The velocity of the heat transfer fluid mixture turning back up inside the metal lance tip 306 is critical. The intense heat from submerging the metal TSL tip 306 in the furnace bath will provoke gas bubble formation and film boiling. Both can be combatted with high velocities for the heat transfer fluid. The specific heat and viscosity of the heat transfer fluid will determine the required velocity to prevent film boiling at a specific heat flux. The specific heat of the heat transfer fluid mixture will thus be prevented from degrading due to boil gases mixing in.
The down flowing and exiting oxygen and supplemental fuel assist in overall cooling of the copper lance tip 306. Pre-chilling them both is therefore helpful.
Top submerged lance (TSL) types of pyro-metallurgical furnaces smelt non-ferrous metals from ore sulphides that will burn and self generate heat with injected oxygen. Herein, we describe embodiments of the present invention that are applied as improvements to specific commercial products like the Glencore ISASMELT, Outotec AUSMELT, and other commercially marketed TSL furnaces as exemplars. A typical example is that of
Top submerged lances present a particular challenge, addressed here, in that uneven cooling and the resulting heat excursions can cause them to both curve and to wear too fast. Typically, a portion of any material fed in above the bath will be lost into the off-gas stream.
The MEG glycol we prefer here is highly hygroscopic, and most of the water vapor in the gas is absorbed by the glycol. The rich glycol clutches the absorbed water.
Lean, water-free glycol confiscates free water from the coolant by physical absorption. Absorption is a process that may be chemical (reactive) or physical (non-reactive). Absorption occurs when a substance is chemically integrated into another.
Therefore, the amount of water in the coolant mixture should not usually exceed the glycol in the same coolant mixture. In conventional applications of glycol as a desiccant, the “rich glycol” with absorbed water is dehydrated in a rejuvenation system. But in embodiments of the present invention, the water is left in the rich glycol state for the water's beneficial contributions to lowered viscosity and high specific heat. At the same time, there can be no unabsorbed water left in the coolant to BLEVE. Since the glycol used functions as a desiccant, any random water that does get into the coolant system, or even condense there, will be physically absorbed almost immediately.
Most of the energy needed here to heat and melt feed materials like chalcopyrite (CuFeS2), and other sulfide of copper and iron minerals, is derived from a reaction of oxygen forced down inside lance 402, with the sulfur “fuel” in a feed ore concentrate 404. Supplemental energy fuel 406 like coal, coke, petroleum coke, oil, natural gas, and other fuels are also injected down inside lance 402 to make up any fuel deficiencies. Solid, supplemental fuel is also sometimes added through the top of furnace 400, e.g., in with the feed ore concentrate 404.
TSL vessels that run with an immersed lance 402 universally experience high wear to the tip. Some tips may even simply burn off. So conventional lances are often constructed with replaceable tips to keep maintenance costs down. Other types of oxygen lances, like in basic oxygen furnaces, are run with their tips three hundred millimeters above the surface and provide a supersonic injection of oxygen and fuel that punches through the surface into the matte and slag floating on liquid metal.
The optimum depth to operate lance 402 is normally maintained with controls based on the tip pressure. Operators must also monitor the matte grade and bath temperature. Too high, and there is a risk of the bath foaming, and downstream processing in the converters will be more difficult. Too low, and refractory wear will increase, in particular downstream in the launders. The TSL is a chemical reactor with a relatively short residence, in the order of about 15 minutes. Measurement and sampling of the reagents (feed, oxygen) is therefore critical. Operators must monitor bath temperature and matte grade. A drop in bath temperature can indicate that the bath may be over oxidized and could foam.
A TSL 504 includes a cooling jacket 506. These combine to shoot a jet of oxygen/fuel 508 into a bath 510 for reaction. A bath surface 512 is subject to violent sloshing that varies in intensity and frequency with the magnitude of the exothermic reactions ongoing in the bath 510. The measurable pressure of a mixture of process gases 514 will also increase with increasing reactions ongoing in the bath 510. The sloshing and agitation of the heavy materials in the bath 510 will also produce observable vibrations in the vessel 502. And, of course, the matte grade will vary according to the reactions ongoing in the bath 510.
In one embodiment of the present invention, the tip of TSL 504 is stood off from an average of bath surface 512 by up to three hundred millimeters (300 mm), in others it is submerged into bath 510.
Although particular embodiments of the present invention have been described and illustrated, such is not intended to limit the invention. Modifications and changes will no doubt become apparent to those skilled in the art, and it is intended that the invention only be limited by the scope of the appended claims.
