ENERGY EFFICIENT SALT-FREE RECOVERY OF METAL FROM DROSS

Abstract

A process and an apparatus are disclosed for improved recovery of metal from hot and cold dross, wherein a dross-treating furnace is provided with a filling material with capacity to store heat. This filling material is preheated to a desired temperature by injection of an oxidizing gas to burn non-recoverable metal remaining in the filling material after tapping of the recoverable metal contained in the dross and discharging of the treatment residue. When dross is treated in such furnace, the heat emanating by conduction from the filling material is sufficient to melt and separate the recoverable metal contained in the dross, without addition of an external heat source, such as fuel or gas burners, plasma torches or electric arcs and without use of any salt fluxes. Furthermore, the recovered metal being in the molten state can be fed to the molten metal holding furnace without cooling the melt.

Claims

1.-29. (canceled)

30. A process for treating dross containing a recoverable metal, in order to recover said metal, comprising the steps: (a) charging a batch of dross into a furnace containing a filling material preheated to a high enough temperature to insure that said dross is thereby heated above the melting point of the metal to be recovered; (b) providing an inert atmosphere in the furnace to prevent oxidation of the dross during the process; (c) rotating or oscillating the dross within the furnace to ensure proper transfer of heat between the hot filling material and the dross and heating of the dross to a temperature above the melting point of the recoverable metal, its separation from the dross residue and from the filling material and its agglomeration at the bottom of the furnace; (d) removing from the furnace the recoverable free metal in a molten state; (e) transferring the recovered molten metal to the holding furnace for pouring in the melt; (f) removing the dross residue while leaving inside the furnace the filling material and a fraction of non-recoverable metal; (g) injecting a controlled amount of an oxidizing gas into the furnace while rotating or oscillating the furnace, so as to oxidize sufficient non-recoverable metal within the filling material to evenly heat and store in the filling material sufficient energy for treating a new batch of dross; (h) stopping the oxidation reaction by providing an inert atmosphere in the furnace by filling the furnace with inert gas; and (i) charging into the furnace the new batch of dross and repeating the process.

31. (canceled)

32. A process for treating dross containing a recoverable metal, in order to recover said metal, comprising: (a) charging a batch of dross into a furnace containing a filling material preheated to a sufficient temperature to insure that said dross is thereby heated above the melting point of the metal to be recovered by transfer of energy stored in the filling material; (b) providing an inert atmosphere in the furnace by filling the furnace with inert gas, to prevent oxidation of the dross during the process; (c) rotating or oscillating the dross within the furnace to ensure proper transfer of heat between the filling material and the dross and heating of the dross to a temperature above the melting point of the recoverable metal, a separation thereof from the dross residue and from the filling material and agglomeration thereof at the bottom of the furnace; (d) removing from the furnace the recoverable free metal while leaving inside the furnace the filling material and a fraction of non-recoverable metal.

33. The process according to claim 32, wherein the filling material is a dross residue produced in the treatment of previous batches of dross.

34. The process according to claim 32, wherein the inert gas is argon.

35. The process according to claim 32, wherein the inert gas is replaced with nitrogen.

36. The process according to claim 32, wherein the oxidizing gas is injected at a controlled rate.

37. The process according to claim 32, wherein the oxidizing gas is oxygen.

38. The process according to claim 32, wherein the oxidation reaction is stopped upon achieving satisfactory heating of the filling material by injecting an inert gas into the furnace.

39. The process according to claim 38, wherein the inert gas injected to stop the oxidation reaction is one of argon and is replaced by nitrogen.

40. (canceled)

41. The process according to claim 32, wherein there is provided a slight overpressure of inert gas to prevent any air inflow into the furnace.

42. The process according to claim 32, wherein a controlled amount of oxidizing gas is also injected into the furnace just prior to removing free metal so as to provide a controlled oxidation of some free metal and thereby increase the temperature of the metal in the furnace when required.

43. The process according to claim 32, wherein a batch of hot dross, charged into a cold empty furnace filled with an inert gas, is further heated by injection of an oxidizing gas into the furnace, while rotating and oscillating the furnace, so as to oxidize sufficient recoverable metal within the dross to evenly heat the dross above the temperature required for metal tapping.

44. The process according to claim 32, wherein a small amount of the filling material is ignited with an external heat source, such as fuel or gas or oxy burners, plasma torches or electric arcs, only for starting the oxidation of the filling material leading to its complete heating at the temperature required to treat the next batch of dross, with all subsequent heating of the filling material being done through the oxidation reaction as described in step (e).

45. The process according to claim 32, wherein the recoverable metal is one of aluminum, zinc, lead and nickel.

46. The process according to claim 32, wherein the filling material acts as a thick blanket which protects the furnace refractory wall from both mechanical and thermal shocks as the cold dross chunks are charged and tumble before crumbling.

47. The process according to claim 32 to treat aluminum dross, wherein the mostly aluminum oxide residue can be recycled as a cover for the aluminum electrolytic cell as it is not contaminated by salt.

48. The process according to claim 32 to treat zinc dross, wherein the mostly zinc oxide residue can be recycled as a cover for the zinc leaching step.

49. The process according to claim 32 to treat zinc dross, wherein the process residue is a valuable by-product consisting of mostly zinc oxide, the contaminants having been eliminated during the high temperature processing of the dross in the furnace.

50. The process according to claim 32, wherein the recovered metal following tapping and kept molten in a suitable container such as a refractory lined ladle is returned to the molten metal holding furnace and is poured into the melt of that holding furnace, thus avoiding loss of heat, metal oxidation and cooling of the holding furnace melt as would have occurred if the recovered metal was left to cool down before being reintroduced in the plant production line.

51.-60. (canceled)

61. A process for treating dross containing a recoverable metal, order to recover said metal, comprising the steps: (a) charging a batch of dross into a furnace containing a filling material preheated to a high enough temperature to insure that said dross is thereby heated above the melting point of the metal to be recovered; (b) providing an inert atmosphere in the furnace to prevent oxidation of the dross during the process; (c) rotating or oscillating the dross within the furnace to ensure proper transfer of heat between the hot filling material and the dross and heating of the dross to a temperature above the melting point of the recoverable metal, a separation thereof from the dross residue and from the filling material and agglomeration thereof at the bottom of the furnace; (d) removing from the furnace the recoverable free metal in a molten state; (e) transferring the recovered molten metal to the holding furnace for pouring in the melt; (f) removing the dross residue while leaving inside the furnace the filling material and a fraction of non-recoverable metal; (g) injecting a controlled amount of an oxidizing gas into the furnace while rotating or oscillating the furnace, so as to oxidize sufficient non-recoverable metal within the filling material to evenly heat and store in the filling material sufficient energy for treating a new batch of dross; (h) stopping the oxidation reaction by providing an inert atmosphere in the furnace by filling the furnace with inert gas; and (i) charging into the furnace the new batch of dross and repeating the process.

62. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Reference will now be made to the accompanying drawings, showing by way of illustration an illustrative embodiment of the present invention, and in which:

[0085] FIG. 1 is a side elevation view of a rotary furnace in accordance with the present invention, and shown in a run/tapping mode thereof;

[0086] FIG. 2 is a front elevation view of the furnace in the run/tapping mode;

[0087] FIG. 3 is a side elevation view of the furnace in an emptying mode thereof;

[0088] FIG. 4 is a front elevation view of the furnace in the emptying mode; and

[0089] FIGS. 5a to 5e are six (6) schematic representations of successive steps of the present process.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

[0090] Scale-up to industrial operation will have to be possible, safety issue requiring discharging of both the metal and the residue at low temperatures will be addressed as well as issues of protection of the environment, non-discharge of greenhouse gas and great attention to energy savings.

[0091] Furthermore, recovery of the metal will be achieved without any use of salt fluxes and with a significantly reduced off-gas generation requiring smaller gas cleaning equipment.

[0092] Reintroduction of the recovered metal into the holding furnace will be made in such a way as to avoid (i) oxidation of metal, (ii) perturbation of the holding furnace operation and (iii) the loss of heat.

[0093] In essence, the present process for treating dross containing a recoverable metal, such as aluminum, in order to recover this metal, comprises the following steps, which are also represented in the illustration below: [0094] (a) charging a new batch of dross into a refractory-lined furnace containing a sufficient amount of a filling material, such as dross residue produced in the treatment of previous batches of dross, previously heated in the furnace, under an inert atmosphere, to a high enough temperature to insure that this new batch of dross is thereby heated above the melting point of the metal to be recovered by transfer of energy stored in the filling material; [0095] (b) rotating or oscillating the dross within the furnace to insure (i) proper heating of the dross to a temperature above the melting point of the metal to be recovered, (ii) separation of that metal from both the dross residue and from the filling material and finally, (iii) accumulation of the recoverable free metal at the bottom of the furnace; [0096] (c) removing from the furnace, at low temperature, both the recoverable free molten metal and [0097] (d) the dross residue while leaving inside the furnace the filling material and a fraction of non-recoverable metal which stays within this filling material as it cannot be recovered; [0098] (e) thereafter, injecting a controlled amount of an oxidizing gas, such as oxygen, into the furnace while rotating or oscillating the furnace, so as to oxidize sufficient non-recoverable metal within the filling material and through resulting exothermic oxidation reaction to evenly transfer to the filling material sufficient energy to heat the filling material to a temperature suitable for treating a new batch of dross and therefore repeating the process. The oxidizing gas can be injected through a super alloy or ceramic tube protruding through the refractory shell or furnace door.

[0099] It should be mentioned that, before the very first charge, a small amount of the filling material, placed in the furnace, is ignited with an external heat source; once that small amount is burning, oxidizing gas is injected and the combustion propagates rapidly to the rest of the filling material leading to its complete heating at the temperature required to treat the first batch of dross, with all subsequent overheating of the filling material being done, without any external source, but simply through the oxidation reaction with oxidizing gas injection. The controlled amount of oxidizing gas injected to carry out the exothermic oxidation reaction is normally introduced into the reactor at a controlled rate to heat the filling material at a predetermined rate and to a predetermined temperature. The thermitting rate is controlled by monitoring the temperature and adjusting the oxidizing gas flow rate. Any runaway reaction is prevented by completely stopping the oxidizing gas injection and initiating inert gas injection.

[0100] The novel process may be carried out in a closable rotary refractory lined furnace, the rotation frequency of the furnace being adjusted to promote tumbling of the charge in the furnace barrel in order to maximize mixing of the cold dross charge with the hot filling material. The rotation may be carried out in a continuous or intermittent manner. It should be noted that U.S. Pat. No. 4,952,237 also considers the injection of oxygen into a dross treatment furnace after discharge of the metal. However, the objective of such operation is not to provide processing energy as is the present case and it is, therefore, totally different. Furthermore, the energy produced by the process disclosed in U.S. Pat. No. 4,952,237 is not sufficient to treat the cold dross being treated by that process.

[0101] In the present process, the complete processing of the dross is carried out under inert atmosphere in order to prevent oxidation of the recoverable metal; the injection of oxidizing gas to induce exothermic reaction in the filling material is only allowed once the tapping of the recoverable metal has been achieved and part of the dross residue has been discharged. However, in some exceptional circumstances it is also possible to inject a controlled amount of oxidizing gas into the furnace just prior to removing the recoverable free metal in order to provide a controlled oxidation of some free metal and thereby increase the temperature in the furnace if and when required.

[0102] It is also preferable to maintain a slight overpressure of inert gas, such as argon, during the steps (a), (b), (c) and (d) described hereinabove, to prevent any air inflow into the furnace chamber which otherwise would oxidize some of the metal during the steps of charging, processing or discharging from the furnace.

[0103] FIGS. 5a to 5e provide schematic illustrations of successive steps of the present process, wherein FIG. 5a shows dross charging; FIG. 5b shows dross heating; FIG. 5c(i) shows metal discharging through the door; FIG. 5c(ii) shows metal discharging through the tap hole; FIG. 5d shows residue discharging; and FIG. 5e shows filling material heating.

[0104] Now referring to the appended figures, the present process and apparatus will be further described, wherein the same reference numbers are used to describe the same parts.

[0105] A furnace 10 suitable for the purposes of the present application is shown in the run/tapping mode in FIGS. 1 and 2 and similarly in the emptying mode in FIGS. 3 and 4. To show the positioning of the furnace 10 more clearly, a framework 15 in FIGS. 1 and 3 is drawn. The furnace 10 comprises a hollow steel cylinder 11 having its interior lined with a high temperature resistant refractory wall 12. As wall 12, one may use a high alumina castable refractory, for example.

[0106] One end of the cylinder 11 is closed by an end wall 11a while the other end has an opening 13 (see FIG. 3) which is closable by a door mechanism shown generally as 14. The above structure forms an enclosed furnace chamber 27 for treatment of dross when the door mechanism 14 closes the opening 13.

[0107] The cylinder 11 is rotatable and tiltable, supported by the framework 15. The framework 15 allows the cylinder 11 to rotate on its longitudinal axis on rollers and trunnions 16 or a gear ring rigidly connected to the cylinder 11 and a chain which passes around the gear ring. The rotation is driven by a motor capable of rotating the cylinder 11 either intermittently or continuously in either direction at speeds of up to 20 R.P.M. The arrangement of the rotating system is conventional and is not shown in the drawings. The framework 15 also permits the cylinder 11 to tilt about pivot 17. Tilting may be effected by a hydraulic piston which moves a cradle 18 within the framework 15.

[0108] The door mechanism 14 is supported by a framework 19 which can be tilted about pivots 20 with respect to the main framework 15. The door mechanism comprises a door mount 21 used to support a circular refractory lined door 22 so that the door can sit properly in the opening 13 of the cylinder 11 when the furnace 10 is in the run mode. The door 22 has a hole 23 which acts as a gas vent to permit escape of furnace gases to the exterior. The vent is covered by an exhaust conduit 24 enclosed within the door mount 21. Controlled amount of inert gas, such as argon, or oxidizing gas, such as oxygen, may be injected in the furnace using piping (not shown) mounted in the wall of the exhaust conduit 24 (see FIGS. 2 and 4) and a nozzle (not shown) located in the hole 23 of the door 22.

[0109] When the furnace 10 is in the run mode, the refractory-lined door 22 can be lowered and allowed to sit on the cylinder 11. In the run mode, the refractory-lined door 22 rotates with the cylinder 11. Escape of gases between the periphery of the opening 13 and the door 22 is prevented by a gasket 25 made of compressible material capable of withstanding high temperatures, like ceramic fiber rope. In the run mode, the door 22 is normally held closed simply by the pressure due to its own weight; however, a latch (not shown) may also be provided to further compress the gasket 25.

[0110] The apparatus described above is operated in the following manner:

[0111] The filling material content of the furnace 10 in the run position as illustrated in FIG. 1 has been preheated as a result of the exothermic oxidation of the non-recoverable metal remaining in the filling material of the previous batch. This is done by injection of an oxidizing gas, such as oxygen, at a controlled rate into the inert gas filled furnace 10 until a desired temperature is reached. The door 22 is seated on the cylinder 11 to prevent the energy stored in the filling material to escape to the exterior. As already mentioned previously, when initially starting the furnace 10, the filling material may be preheated using, for example, a gas burner, a plasma torch or an electric arc. A hot dross charge is prepared in a charging device (not shown) adapted to allow charging of the furnace chamber 27 when the cylinder 11 is tilted upwardly as shown in FIGS. 1 and 2. Then, the door 22 is opened and the charge of hot dross is dropped into the inert gas filled furnace chamber 27; in order to avoid damaging the refractory wall or lining 12 it may be desirable to tilt the furnace 10 horizontally as shown in FIGS. 3 and 4, in order to allow the charge to be pushed inside the furnace chamber 27 using a tool similar to an ember rake instead of being dropped in. The total dross charge, including the filling material, is such that it occupies about one quarter to one third of the total interior volume of the furnace chamber 27. The furnace cylinder 11, being in the run mode position (tilted upwardly), the door 22 is lowered to close tightly, compressing the gasket 25. The tilting angle of the cylinder 11 is such that maximum use is made of the volume of the furnace chamber 27 without affecting the tumbling effect of the charge which is normally needed for maximum recovery of metal contained in the dross by mixing and heat transfer with the overheated filling material followed by agglomeration of the metal droplets contained in the dross.

[0112] The cylinder 11 of the furnace 10 is then either rotated or preferably oscillated in the case when large blocks of dross were charged, low amplitude oscillation being preferred in that case to prevent damage to the refractory lining 12 which could result from the tumbling of the heavy dross blocks within the furnace 10. The tumbling noise produced by the large blocks of dross may be monitored using a sound monitor mounted in the gas exhaust conduit 24 and full rotation of the furnace would only be allowed to proceed once the tumbling noise signal is below a predetermined level. As the furnace is rotated, heat transfer occurs between the dross charge and the filling material. The temperature of the dross charge is monitored using a thermocouple mounted in the gas exhaust conduit 24 and several thermocouples mounted inside the refractory wall 12. For example, radio frequency (RF) transmission thermocouples can be used on the rotating furnace. Once the charge has reached a predetermined temperature as monitored by the thermocouples, the separated molten metal is tapped off into a suitable crucible. Tapping is carried out through a taphole 26 located at the lowest point in the cylinder 11 of the furnace 10 when in the upward tilt position (FIG. 1). The taphole plug is lined with refractory material that is replaced after each tap. While tapping the furnace 10, the door 22 remains sealed and the atmosphere in the furnace 10 is an inert gas such as argon. If preferred, tapping could also be made through the door opening 13.

[0113] The tapped metal can then be kept molten in a suitable container such as a refractory lined ladle, returned to the molten metal holding furnace and is poured into the melt of that holding furnace, thus avoiding loss of heat, metal oxidation and cooling of the holding furnace melt as would have occurred if the recovered metal was left to cool down before being reintroduced in the plant production line.

[0114] After the metal has been tapped, it is desirable to rotate the furnace 10 again for a certain period of time because repeated tests have shown that the solid residue floating on the molten metal bath remain wetted with appreciable amount of metal; in one example, following a first tapping of aluminum, the furnace 10 was rotated for a further five minutes, allowing a second tapping of an amount of metal corresponding to more than 20% of the first tapping.

[0115] After the recoverable metal has been tapped, the taphole 26 is closed, in the case where tapping was made using a tap hole. The furnace door 22 is then lifted, the furnace cylinder 11 is tilted forward as shown in FIG. 3 and the residue is discharged while rotating the furnace 10, leaving a fraction of the residue inside the furnace 10 which will act as the filling material for the next batch. Once the right amount of residue has been discharged, the rotation is stopped, the furnace cylinder 11 is placed in the run position illustrated by FIG. 1, and the furnace door 22 is closed to prevent heat loss by radiation.

[0116] In the case of aluminum dross, the mostly aluminum oxide residue can be recycled as a cover for the aluminum electrolytic cell, as it is not contaminated by salt.

[0117] In the case of zinc dross, the mostly zinc oxide residue can be recycled as a cover for the zinc leaching step.

[0118] In the case of zinc dross, the high temperature treatment step acts as a means of volatilizing contaminants, such as chlorides, sulphur, ammonia, and volatile metals, such as thallium. The contaminants having been eliminated during the high temperature processing of the dross in the furnace, the residue is a fine powdery product consisting of mostly zinc oxide, which can be marketed, for example, as an activator for rubber vulcanization or as an additive or filler to plastics, ceramics, glass and cement.

[0119] Once the furnace door has been closed, a controlled amount of oxidizing gas is injected into chamber 27 of the inert gas filled furnace 10 through the nozzle located inside the hole 23 of the door 22. Controlled oxidation of the non-recoverable metal contained in the filling material is thereby produced; the temperature of the filling material is monitored using the thermocouples previously mentioned. The furnace is rotated while the metal contained in the filling material is reacting with the injected oxidizing gas in order to evenly transfer the energy produced in the reaction to the filling material. Once the predetermined amount of oxidizing gas has been injected, or if the temperature monitored by the thermocouples indicates a temperature value at or above a predetermined level, the injection of oxidizing gas is stopped and the furnace 10 remains filled with inert gas.

[0120] Preheating of the cold furnace 10 is carried out using a fuel or gas burner or plasma torch or electric arc mounted on a support installed in front of the furnace with the door 22 opened. When the required temperature is reached, the preheating is completed, the external heat source is removed and the operating cycle described above can be initiated.

[0121] Preheating of the cold furnace 10 can also be achieved by first charging a batch of hot dross into the chamber 27, followed by the injection of an oxidizing gas into the chamber 27. Controlled oxidation of the metal contained in the dross will occur, resulting in an increase in the temperature in furnace 10, which will be monitored using the thermocouples previously mentioned. The furnace is rotated while the exothermic reaction is occurring in order to evenly distribute the heat to the dross charge in the furnace 10. Once the predetermined amount of oxidizing gas has been injected, or once the temperature monitored by the thermocouples indicates a temperature value at or above a predetermined level, the injection of oxidizing gas is stopped and the furnace 10 remains filled with inert gas.

[0122] The present process is further illustrated by the following example:

Example

[0123] The hot aluminum dross formed at the surface of the molten aluminum bath of the molten aluminum holding furnace is skimmed into containers before being transferred to the dross house for treatment in a DROSRITE furnace. During the transfer, the dross, in contact with air, continues to oxidize and therefore its temperature does not decrease. In fact, measurements have shown that the temperature of the dross remains high for several hours because of the heat generated by this oxidation. To prevent oxidation during cooling, for example, Alcan is marketing a dross cooler where argon is injected in the dross container to prevent contact of the dross with ambient air (c.f. “The Alcan Process for Inert Gas Dross Cooling” in the Journal of Metals, February 1991, pp. 52-53). Cooling the dross under inert atmosphere, such as argon, is of interest as it prevents a loss of metal which could otherwise be recovered by a subsequent treatment; however the energy contained in the hot dross is lost during cooling in the Alcan cooling box.

[0124] On the contrary, with the process and apparatus proposed herein, the energy content of the hot dross is not lost as it is charged right away in a preheated furnace which also contained the amount of preheated filling material required to treat that dross charge. In our example, corresponding to an industrial situation, we consider a batch of 6 metric tons of such dross, with 50% free metal content, charged into an inert gas filled furnace such as above furnace 10, which already contains 10 metric tons of filling material heated to a temperature of 1000° C. The mean temperature of the dross charge is assumed to be 400° C., although measurements in industry have shown the temperature to be much higher, of the order of 600° C.

[0125] The objective is to transfer energy from the overheated filling material into the hot dross charge to bring the total furnace content to 700° C. Once that objective is reached, both the metal and a portion of the dross residue will be discharged at the “low” temperature of 700° C., leaving inside the furnace the 10 metric tons of residues/filling material required for the treatment of the next batch of hot dross. Then, a controlled amount of oxygen is injected into the furnace to bring the filling material back to the original temperature of 1000° C. by burning sufficient non-recoverable metal within the filling material to evenly heat and store in the filling material sufficient energy for treating the next batch of hot dross.

[0126] The following calculations show that, in the 10 tons filling material, the energy available for transfer from 1000° C. to 700° C. to the hot dross corresponds to the energy required by the 6 tons hot dross to be heated from 400° C. to 700° C.:

Energy available: 1,06 MJ/t ° C. (1000-700)×10t=3 180 MJ

Energy required: 1,06 MJ/t ° C. (700−400)×6t+50%×398MJ/t×6t=3 102MJ

[0127] To the energy required for the process, 3 102 MJ, which is lost as it remains with the discharged metal and residue, must be added the energy lost to the outside by the furnace and estimated at about 1 200 MJ for a furnace of that capacity over the 3 hours of the treatment cycle, for a total of about 4 302 MJ.

[0128] That 4 302 MJ of energy, produced by a controlled oxidation of non-recoverable metal in the filling material requires the burning of the following amount of metal:

4 302 MJ/31.32 kg/MJ=137 kg of aluminum.

[0129] This amount of aluminum corresponds to: 137 kg/10 t=1.4% of residual metal in the filling material. This residual metal is part of the non-recoverable metal which remains in any of the various processes which are in operation for the recovery of metal from dross. Measurements have shown that the amount of such residual metal in the residue after treatment is higher than 5%.

[0130] Of the 4 302 MJ of energy produced by oxidation of part of the residual metal, 3 180 MJ is therefore used to bring the filling material from 700° C. to 1 000° C. The remainder is used to maintain the refractory inside surface at the same temperature as the filling material in spite of heat conduction through that same refractory; that energy is also lost as it is transferred to the outside through the furnace wall.

[0131] It should be mentioned that the above-described preferred embodiments are in no way limitative and various modifications obvious to those skilled in the art can be made without departing from the spirit and scope of the present invention.

[0132] Finally, although the present invention has been described hereinabove by way of embodiments thereof, it may be modified, without departing from the nature and teachings of the subject invention as described herein.