CA1303361C - Method and apparatus for melting metals - Google Patents

Abstract

METHOD AND APPARATUS FOR MELTING METALS
ABSTRACT
In the melting of metals in a furnace with the formation of dross on top of molten metal, the bridging and clogging of equipment with dross is overcome by using an impellor surrounded by a partly immersed, generally cylindrical, slotted shroud. The impellor is rotated and metal feedstock is added in the form of particulates to the resulting vortex. The particulates move downward into the vortex and melt readily. Dross separates from the melt and collects in a crusty layer on the melt surface outside the shroud. At least one vertically-positioned elongated opening in the wall of the shroud is adapted to allow melt to circulate through the opening while preventing dross collected outside the shroud from passing through. The shroud may have a downward taper with an angle of from greater than about 0°
to about 30°. The elongated openings may have a variety of shapes, but are preferably rectangular. The shroud and impellor are suspended from above the furnace and are preferably made of a ceramic material.

Description

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METHOD AND APPARATUS FOR MELTING METALS

This invention relates to the melting of metals and, more particularly to a method and apparatus for the melting of metals in particulate form.

BACKGROUND OF THE INVENTION

In the processing of metals and scrap metals, the metals are melted usually in the presence of impurities and oxidizing gases tha~ cause the formation of often appreciable amounts of dross. In many cases the dross collects in a crusty layer on top of the molten metal where it may impair the melting of freshly introduced metal feed stock. Often, the dross layer eventually bridges across the area to which solid metal is added to the melt, causing plugging of the equipment.

Particularly difficult is the melting of zinc particulate~
that result from processing dross obtained from the melting of zinc cathodes from the zinc electrowinning process.
Dross formation when melting zinc is assisted by the usual addition of ammonium chloride flux during the mel~ing process. The dross is often processed ~hrough a Williams ~ill for the recovery of its zinc content in the form of particulates which are returned to the melting furnace~ The particulates are extremely difficult to process~ Various methods such as feeding particulates in a stream of molten zinc, adding particulates into a vortex in a zinc melt, and adding particulates in~o a crucible placed in ~he mel~ing furnace and circulating molten zinc in a vortex throuyh the . ., ~r -~?336~
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crucible, all failed because of bridging of dross that caused vortices to collapse and the apparatus to plug. It would, therefore, be desirable to develop a system for the melting of metals and scrap wherein dross that is formed during melting is prevented from bridging and plugging the equipment.

DESCRIPTION OF PRIOR ART

Many systems exist for melting metals, secondary smelting, scrap melting and recycling metals. Most systems hav~ been designed such that the metal or scrap ls submerged by means of a mechanical mixing device or a vortexr Mixing impellors, rotators, paddles and vortex generators have been applied with some success, but problems still exist that have not been overcome. One of the most pervasive problems is ~he aforesaid formation of dross on top of the molten metal which clogs and bridges the system. Example~ of melting systems for particulate metal developed in the aluminum industry, where dross usually sinks to the bottom of the melt, can be found in U.S. Patents 3 873 305, 3 984 234, 3 997 336, 4 128 41~, 4 286 985, ~ 322 245, 4 386 764, 4 437 S50, ~ 486 228, 4 491 474, 4 491 475 and 4 518 424. These systems include the use of blades, vortex generators and specially designed rotators to submerge the metal below the melt surface. These systems also include specially designed, rather complex furnaces including a plurality of separate, often interconnec~ed9 bays for heating, pumping, charging and collecting of melt, scrap and dros~.

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Metaullics Systems has recently developed a shrouded auger melting system which operates in conjunction with a circulation or gas injection pump. The shrouded auger is submerged in a melt of metal and provides a strong axial flow with a very small radial component. This system is said to submerge scrap quickly without attendant oxidation.
The system is, however, said to be sensitive to its positioning in the melt bath and the use of a separating baffle, that divides the bath in two compartments, is desirable. In some cases incomplete submersion requires the use of two such systems in series. Older examples of the use of shrouded agitators or augers can be found in U.S.
Patents 1 386 503, 2 038 221, 2 067 394, 2 426 389, 2 515 478, and 2 786 755. Neither one of these systems prov~des means to maintain the area of the agitator free of dross that collects on top o~ a melt. Consequently, none of these systems overcomes the problem of bridging when large amounts of dross are formed or are present on top of molten metal.

SUMMARY OF THE INVENTION

We have now discovered that the problems encountered in the prior art, especially those of bridging and clogging with dross on molten metal, can be overcome by positioning a mixer, agitator or impellor device, referred to as impellor hereinafter, in a bath of a molten metal in a melting furnace, surrounding the impellor with a slotted shroud and feeding particulate metal into a vortex created ~y rotating , .~

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the impellor inside the shroud. The shroud is suspended in the melt and is only partly immersed therein.

The slotted shroud has at least one vertically positioned elongated opening of predetermined dimensions. The dimensions, i.e. width and height, of each opening are such that dross is retained outside the shroud and molten metal is allowed to circulate through each opening. The opening can have a variety of shapes, a substantially rectangular shape being preferred~ The shroud has a generally cylindrical shape with open top and open bottom. In a preferred embodiment, the generally cylindrical shape is tapered down toward its lower end such that the shroud has a truncated conical shape.

The rotating impellor creates a vortex in the melt inside the shroud. Particulate metal is fed into the vortex and the particulates are drawn rapidly into the vortex and are submerged in the melt. The préferred conical shape of the shroud causes an increasing velocity for the submersion o~
the particulates into the melt. The particulates melt and dross forms. The dross rises to the surface, mostly outside the shroud, and collects with the dross on the melt surface to form an in~erface therewith. The interface is maintained opposite the at least one elongated shroud opening such that melt and partly melted particulates can circulate through each opening in the shroud and are promptly drawn downward into the vortex without rising to the melt surface inside the shroud. The dross layer on the surface of the molten metal surrounding the shroud bridges across each opening.

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The collected dross is thereby prevented from passing through the opening and entering the melt inside the shroud, whereas the molten ~etal under the dross is free to flow through the opening under the overlaying layer of drossO
The impellor and the partly immersed shroud are supported by support means situated above the melt, i.e. above the dross surface.

Accordingly, it is an object of the present invention to provide a method for the melting of particulate metal.

It is another object to provide a method for melting particulate metal without dross bridging in a manner to impair the melting process.

It is still another object to provide a method for the efficient melting of particulate metal in a vortex using a partly immersed, slotted shroud surrounding a rotating impellor.

It is yet another object to provide a method for melting metal that is tolerant of fluctuations in the level of the melt surfaceO

It is a further object of the invention to provide an apparatus for the melting of particulate metal without collected dross bridging the apparatus in a manner to impair ~he melting process.

It is yet a further object to provide an apparatus for melting particula~e metal wherein dross is retained outside ~ 3~3~:~33;~
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the apparatus and molten metal can circulate through the apparatus.

These and other objects of the invention can be achieved according to one embodiment of the invention by providing a method for the meltin~ of particulate metal into a melt of molten metal with formation of dross and collection of dross on the melt surface which method comprises the steps of establishing a bath of molten metal; partly immersing a shroud in said bath, said shroud having a wall with an upper end and a lower end, and having a generally cylindrical shape being open at said upper end and open at said lower end, and said wall having at leas~ one substantially vertically positioned elongated opening adapted to prevent dross collected on the surface of said bath outside said shroud from passing through said opening and adapted to allow molten metal to pass freely through said opening;
positioning an impellor centrally inside said shroud such that the impellor i5 submerged in said bath and is at least partly surrounded by said shroud; rotating said impellor to create a vortex in said bath; ~eeding metal in par~iculate form into said vortex; melting ~he particulate metal in said bath with formation of dross; collecting dross in a layer on top of said bath outside said shroud, the layer of dross having an interface with said bath; and maintaining said 2S interface at a level opposite said opening.

According to another embodiment, ~here is provided an apparatus for melting particulate metal in a melt of molten metal with formation of dross and collec~ion of dross on the i3(1~336~

melt surface in a furnace which apparatus comprises an lmpellor; means for rotating said impellor; a shroud at least partly surrounding said impellor; said shroud having a wall with an upper end and a lower end, and having a generally cylindrical shape being open at said upper end and open at said lower end; and said wall having at least one substantially vertically-positioned, elongated opening therein adapted to prevent dross collected on the surface of said melt outside said shroud from pas ing through said opening and adapted to allow molten metal to pass freely through said opening.

Preferably, the shroud is tapered downward toward said lower end and has a truncated conical shape.

Preferably, each opening has a top and a bottom and the height of each opening is such that the melt-dross interface is at a position between the top and the bottom of each opening while allowing melt to pass through at least a portion of each opening, and wherein said at least one opening has a width sufficient to allow the melt to pass and circulate freely through the opening while restric~ing ~he flow of dross such tha~ it will bridge and be prevented from flowing through said opening.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to the accompanying drawings wherein:

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Figure 1 is a sectional view of a part of a melting furnac2 with a preferred embodiment of the apparatus of the present invention suspended therein, Figure 2 is a cross-section of one embodiment of the shroud of the invention, and Figure 3 is a cross~section of a preferred embodiment of the shroud of the invention.

In the drawinqs, like numbers refer to like parts.

DETAILED DESCRIPTIQN

The method and apparatus are useful for the melting of metals and scrap metals wherein dross forms that rises to and collects on the top of molten metal, and wherein said metals are in particulate forms such as pellets, shOtr shreds, chips, granules and the like. The method and apparatus are especially useful for the melting of particulate zinc such as obtained from treating zinc dross in a Williams Mill which yields zin~ particulates containing an amount of 10 included dross.

With reference to Figure 1, a melting furnace generally indicated in part at 10 contains a bath of molten metal 12.
On ~he surface of the bath 12 is a layer o~ dross 14 forming an interface 16 between ~he molten metal and the dross. An impellor 18 is submerged in bath 12. The shape of impellor 18 may be one o~ a number of known shapes suitable for drawing a vortex 20 into ~ath 12. Impellor 18 is preferably made of a ceramic material. Impellor 18 is mounted on a ~3V336~

shaft 22 for rotation by means of a motor 24. The lower portion o shaft 22 may be enveloped in a protective sleeve 26 which prevents corrosion of shaft 22 by molten metal.
Protective sleeve 26 is preferably made of a ceramic material. Motor 24 with suspending shaft 22 with impellor 18 is mounted on support means generally indicated at 28 situated outside and above furnace 10. Feed means, generally indicated at 30, is provided for feeding particulate metal into vortex 20.

With reference to Figures 1, 2 and 3, at least partly surrounding impellor 18 and shaft 22 with sleeve 26 is a shroud generally indicated at 36. Shroud 36 can be made of one of a number of suitable materials but, because of the corrosive nature of some molten metals, is preferably made of a ceramic material. Shroud 36 has a wall 38 which has a generally cylindrical shape, as shown in Figure 2, with an upper end 40 and a lower end 42, both ends 40 and 42 being open. In a preferred embodiment, the shroud may be tapered downward towards its lower end 42. ~n the preferred embodiment, the shroud has a truncated conical shape as shown in Figures 1 and 3. The generally cylindrical shroud may, therefore, have a downward taper with an angle in the range of from greater than about 0 to about 30C. The preferred truncated conical shape has a taper with an angle in the range of about 5 to 20~.

Wall 38 of shroud 36 i~ provlded with at least one substdntially verticall, positioned opening 44. Preferably, . ~ -' .: -:

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a plurality of openings, such as, for example, two openingsas shown in Figure 2, or four openings as shown in Figure 3, is used. It is understood of course that other numbers of openings may be used. Each opening 44 has a generally elongated shape and is positioned in wall 38 between upper end 40 and lower end 42 of shroud 36. Opening 44 may have one or more of a number of shapes. The preferred elongated shapes of opening 44 are rectangular, triangular, trapezoid, oval or the like. The most preferred ~hape, as shown in Figure 3, is a substantially rectangular shape. The-shape and dimensions of each opening 44 are adapted such that dross 14 that has accumulated on melt 1~ outside the shroud can not pass through to the insidè of shroud 36 but melt 12 under the dross layer 14 can freely pass and circulate through each opening 44 under the overlaying layer of dross.
Each opening has a top 46 and a bottom 48. The height of each opening is adapted such that the interface 16 between melt 12 and dross 14 is at all times maintained at a level opposite the opening and situated between top 46 and bottom 48 of each opening and at a distance above bottom 48 sufficient Por melt 12 to pass and circulate through each opening 44 at all times. The height of each opening 44 is also adapted to allow variations in~he level of the melt 1~
in furnace 10. The width of each opening 44 is sufficient to allow the melt to pass and circulate fully through the openings while restricting the flow of dross such that it will bridge and be preven~ed from flowing througb the openingsO For example, for a substantially rectangularly-shaped opening, the width o an opening may be in the range ~ ~IL3(~

of about 1 to 5 cm, while the height may be in the range of about 10 to 30 cm.

The impellor 18 and shroud 36 are positioned in fixed positions in the furnace. Impellor 18 is centrally located in and at least partly surrounded by shroud 36. When impellor 18 is partly surrounded by shroud 36, i.e. extends below lower end 42~ vortex 20 is most effective in drawing particulates into melt 12. Impellor 18 is submerged in melt 12 to a depth sufficient to draw a vortex 20 in melt 2 upon rotation and adequate to draw particulates added to vortex 20 from feed means 30 into the vortex and submerge the particulates in melt 12. When the generally cylindrical shroud has a truncated conical shape, the drawing down of particulates into the vortex is accelerated. The truncated conical shape of the shroud has the added advantage of centering the impellor so that a bearing on the shaft at the impellor end is not necessary. Shroud 36 is immersed in melt 12 to a depth sufficient to maintain interface 16 along opening 44, as described~

Shroud 36 is suitably suspended from support means 28, for example, by means of at least two rods 32 that are attached to a support ring 34 surrounding ~he top portion of shroud 36. The ring may be made of mild steel~ In the preferred embodimen~ of the shroud, the suspension of shroud 36 in mel~ 12 is facilitated by its conical shape.

The height of shroud 36 should be sufficient to accomodate melt level variations in the furnace and to support th*
shroud ~rom above the furnace conten~s. The height of the :..

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shroud may conveniently be in the order of 30 to 60 cm. The diameter of shroud 36 should be sufficient to accomodate impellor 18 and allow impellor 18 to draw an adequate vortex inside the shroud. .Similarly, the open lower end 42 of the shroud should have a diameter adequate to accomodate impellor 18 and vortex 20. Di~meters of the shroud are conveniently in the range of 15 to 50 cm.

In the operation of the method according to the invention~
furnace 10 is operated with a bath of molten metal 12, the level of which may vary depending on the rates of additions to and withdrawals from the furnace. A dross layer 14 forms on top of melt 12 with an interface 16. With variations in levels of melt and dross in the furnace, the level of interface 16 may vary between a high and a low level indicated by arrows A and B, respectively.

The submerged impellor 18 is rotated by shaft 22 and motor 2~ at a rotational speed adequate to cause the formation and the maintenance of vortex 20 in melt 12. For example, adequate speeds are in tha range of about 300 to 800 rpm that cause a vortex to be formed and maintained with a depth of between about 10 and 20 cm.

Particulate metal is added into vortex 20 from feed means 30. The particulates are drawn into vortex 20 and below the surface of the melt wherein ~hey dissslve with formation Qf dross. When zinc particulates from a Williams Mill are melted, a relatively large amount of dro~s is formed because :

~L3~?;3361 the particulates already contain oxides and non-metallic inclusions.

The particulates readily melt and released dross rises to the surface of melt 12 and accumulates in the dross layer 5 14. The dross layer 14 in furnace 10 is usually of a crusty nature caused by surface cooling. The design of the shroud 36 takes advantage of this crusty n-ature of the dross by preventing dross from following the stream of melt through opening 44 into the vortex. The dimensions of each opening are such that the dross of layer 14 effectively bridges across each opening and is held back while melt can still flow through the opening. Any particulate not fully melted remains in the melt circulating from the vortex through the melt and through each openiny, is drawn back into vortex 20 through each opening and is not caught up in the dross.
Similarly, any dross not accumulated in dross layer 14 remains in ~he circulating melt and is drawn back into vortex ~0 until accumulated in dross layer 140 Thus, the system is increasingly efficient in removing dross, including oxides and non-metallic inclusions, into the dross layer 14 while retaining metal in the melt 12. In the preferred embodiment, the conical shape of shroud 36 enhances the velocity of the downward circulation of the melt and causes particulates to go deeper into the melt.

The vortex 20 remains substantially fre~ o any drQss and the process can consequently be operated without any interference with or impairment o~ the melting that could be caused by any accumulation of dross inside the shroud.

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The interface 16, which can vary between levels A and B, is located opposite and along the length of each opening 44 between its top 46 and bottom 48 but some distance above bottom 48 such that melt 12 is always permitted to flow S through a portion of opening 44.

The method and apparatus of the present invention have a number of important advantages. Metals and scrap can be efficiently melted without the process being impaired by - dross. A large amount o~ dross can be present in the melting furnace. No circulation pump is required. The position of the apparatus in the melt is not sensitive.
Fluctuations in melt levels in the furnace can be readily accomodated. ~he method and apparatus can be operated inside a furnace with a minimum of operator attention. No complexity is required in the furnace or the melting, pumping, charging, collecting, or separating of melt~ scrap, or dross. Finally, the apparatus is simple, inexpensive and easy to install and operate.

The invention will now be illustrated by the following non-limitative example.

Example Zinc particulates obtained from a Williams Mill containing99.9 % ~inc were melted in a melting furnace containing a bath of molten zinc~ An appara~us according to the invention, as described above and shown in Figures 1 and 3, was suspended in the melt. The impellor, which had a height of 10 cm, was positioned in the sh~oud and extended 2.5 cm ~L3~?3;~

below the bottom o~ the shroud~ The impellor was rotated at 500 rpm drawing a vortex in the melt to a depth of 15 cm.
The shroud surrounding the impellor was made of a ceramic material. The shroud was of truncated conical shape and partly immersed in the melt to a depth of 10 cm from the top of the shroud. The wall thickness of the shroud was 2.5 cm and the shroud was 45 cm high. The inside diameter of the shroud at its upper end was 40 cm and at its lower end 20 cm, giving a taper of 12.5. The shroud had four vertically positioned openings equally spaced around the shroud's circumference, the bottom of each opening being at 10 cm above the lower end of the shroud. Each opening was 20 cm long and 4.4 cm wide.

Zn particulates were fed continuously into the vortex over a 3 hour period at a ra~e of 3000 kg/h. Molten zinc was periodically withdrawn from the furnace, causing the level of the melt and dross to vary over a distance of 15 cm~ It was observed that the particulates readily melted and that the surface of the vortex remained clear of any surfaGe accumulation during that period.

It is understood that changes and modifications can be made in the embodimen~s of the me~hod and apparatus according to the present invention without departing from the spirit and scope of the appended claim~.

Claims (17)

1. A method for the melting of particulate metal into a melt of molten metal with formation of dross and collection of dross on the melt surface which method comprises the steps of establishing a bath of molten metal; partly immersing a shroud in said bath, said shroud having a wall with an upper end and a lower end, and having a generally cylindrical shape being open at said upper end and open at said lower end, and said wall having at least one substantially vertically positioned elongated opening adapted to prevent dross collected on the surface of said bath outside said shroud from passing through said opening and adapted to allow molten metal to pass freely through said opening;
positioning an impellor centrally inside said shroud such that the impellor is submerged in said bath and is at least partly surrounded by said shroud; rotating said impellor to create a vortex in said bath; feeding metal in particulate form into said vortex; melting the particulate metal in said bath with formation of dross;
collecting dross in a layer on top of said bath outside said shroud, the layer of dross having an interface with said bath; and maintaining said interface at a level opposite said openings.

2. A method as claimed in claim 1, wherein said at least one opening has a top and a bottom and the height of each opening is such that said interface is at a position between the top and the bottom of each opening allowing molten metal to pass through at least a portion of each opening, and wherein said at least one opening has a width sufficient to allow the melt to pass and circulate freely through the opening while restricting the flow of dross such that it will bridge and be prevented from flowing through said opening.

3. A method as claimed in claim 1, wherein said generally cylindrical shape has a taper downward toward said lower end with an angle in the range of from greater than about 0° to about 30°.

4. A method as claimed in claim 1 or 2, wherein said generally cylindrical shape has a taper downward toward said lower end with an angle in the range of about 5°
to 20° to form a truncated conical shape.

5. A method as claimed in claim 1, 2, or 3, wherein said opening has a substantially rectangular shape.

6. A method claimed in claim 1, 2, or 3, wherein said bath is a bath of molten zinc and metal fed in o said bath is zinc in particulate form.

7. A method as claimed in claim 1, 2, or 3 wherein said bath is a bath of molten zinc and metal fed into said bath is in the form of zinc particulates obtained from a Williams Mill.

8. A method as claimed in claim 11 2, or 3, wherein said vortex is maintained by rotating said impellor in the range of about 300 to 800 rpm.

9. An apparatus for melting particulate metal in a melt of molten metal with formation of dross and collection of dross on the melt surface in a furnace which apparatus comprises: an impellor; means for rotating said impellor; a shroud at least partly surrounding said impellor; said shroud having a wall with an upper end and a lower end and having a generally cylindrical shape being open at said upper end and open at said lower end; and said wall having at least one substantially vertically-positioned, elongated opening therein adapted to prevent dross collected on the surface of said melt outside said shroud from passing through said opening and adapted to allow molten metal to pass freely through said opening.

10. An apparatus as claimed in claim 9, wherein said generally cylindrical shape has a taper downward toward said lower end with an angle in the range of from greater than about 0° to about 30°.

11. As apparatus as claimed in claim 9, wherein said generally cylindrical shape has a taper downward toward said lower end with an angle in the range of about 5°
to 20°.

12. An apparatus as claimed in claim 9, wherein said impellor is centrally located in said shroud and extends below said lower end of said shroud.

13. An apparatus as claimed in claim 9, 10, or 12, wherein said opening has a substantially rectangular shape.

14. An apparatus as claimed in claim 9, 10, or 12, wherein said opening has a height in the range of about 10 to 30 cm.

15. An apparatus as claimed in claim 9, 10, or 12, wherein said opening has a width in the range of about 1 to 5 cm.

16. An apparatus as claimed in claim 9, 11, or 12, wherein said opening has a substantially rectangular shape with a height in the range of about 10 to 30 cm and a width in the range of about 1 to 5 cm.

17. An apparatus as claimed in claim 9, 10, or 12, wherein said impellor and said shroud are made of a ceramic material.