FIELD OF THE INVENTION

The present invention relates to a cooled refractory wall inside a slag cleaning furnace for FeCr and Steel Melting Shop (SMS) slag reduction and settling. The wall enables partial separation of metal droplets present in molten slag input feeds and metal droplets generated during the reduction reaction. This allows one furnace to produce two metal products with different chemical content, simultaneously. The main driver is to provide a possibility to generate high and low nickel ferrochrome from the above mentioned slag streams.

BACKGROUND

Currently, according to the state of the art technology, slags from ferrochrome production are allowed to cool and metallic particles are separated from the cold slags by mechanical separation methods, usually by jigging, dense media and magnetic separation. The total yield of chromium is normally below 50% in these kinds of recovery processes since the slag phase contains a large amount of chromium oxides not separated by the mentioned methods. Depending on the smelter process and grade of chromite ore used, the slag can contain 5-10% of FeCr metal and 5-20% of chromium oxide.

Slags from stainless steel production in current state of the art technologies are currently treated by allowing them to cool, reducing the particle size of the slags and separating metallic particles into individual streams using screens or magnets. The total yields of nickel in these processes are usually over 95%, since nickel is normally present in slag only in metallic particles.

One of the known method for combined liquid slag treatment has been described in Finnish patent application No 20195153, “Combined Smelting of molten slags and residuals from stainless steel and ferrochromium works” by Parviainen & Vallo. The application describes a furnace or converter method for treating the above-mentioned slag streams in one furnace to recover valuable iron, nickel and chromium units also from oxides of those metals by carbothermic reduction. However, nickel units present in stainless steel slags as droplets are diluted into the total metal stream of oxides and droplets, hence decreasing product nickel composition. Typically, ferrochrome with high nickel content has a larger value for instance for austenitic stainless steel manufacturers, and low nickel ferrochrome is suitable for the manufacturing of ferritic stainless steel grades.

In the present invention, a method for the production of high and low nickel FeCr product using a single furnace is disclosed.

SUMMARY OF THE INVENTION

The invention is defined by what is disclosed in the independent claims. Preferable embodiments are set out in the dependent claims.

According to the invention, metal droplets present in slag streams fed to a furnace are separated into two distinct metal pools by a wall, made of a suitable refractory material. Metal particles generated in carbothermic reduction processes of metal oxides are separated partially to pools on the respective sides of this wall. A part of these droplets will settle into a first pool. A further part of these droplets will settle into a second pool. The separated metals have different compositions: A first pool will have a high nickel content and a second pool will have close to zero nickel content. Chromium content in pool two is slightly higher than in pool one since no diluting effect of nickel is present. The actual metal composition in both pools is related to feed material characteristics and metal versus metal oxide contents.

Concentrating nickel to a lower metal volume increases its value. Typically, ferronickel alloys are rated in value based on their nickel content. Higher nickel content provides greater income from material streams. This innovation allows a high separation rate of nickel, increasing its composition and the total value of materials produced.

DETAILED DESCRIPTION

The present invention relates to a method for smelting metal- and metal oxide containing wastes and side streams, such as slags, dust, scales and sludges generated in stainless steel and ferrochrome manufacturing processes. The invented method is a physical separation process of liquid metals produced in a combined ferrochrome and stainless steel slags cleaning furnace. Metal streams are separated mainly for usability and value reason.

Embodiments of the invention relate to a method for cleaning slags. For the purposes of the present description, the term slag includes all metal and metal containing wastes and side streams such as slags, dust, scales and sludges generated, for example in stainless steel and ferrochrome manufacturing processes. In the process of the invention, slags are fed into a furnace 100 from one side. On the opposite side of the furnace 100, the purified slags are tapped out 20 of the furnace 100. In the lower part of the furnacelOO is provided a refractory wall 200 creating two metal pools 30, 50 beneath a slag layer 70 flowing from the inlet towards the outlet 20. The wall 200 is high enough to separate the metal pools 30, 50 but low enough to allow the slag layer 70 on top of the metal to move freely over the wall 200 from the inlet side 10 towards the outlet side 20. In the furnace 100 bottom on each side of the refractory wall 200 are tapping arrangements for liquid metal 40, 60.

Thus, in an embodiment a method for cleaning one or more slags in a furnace 100 comprises the steps of feeding one or more slags into the furnace 100 via one or more inlets 10, to form a slag layer 70 which flows through the furnace 100, gravitationally separating metal droplets present in the slag from the slag into a first metal collection pool 30, gravitationally separating further metal droplets formed in metal oxide reduction processes from the slag into a second metal collection pool 50 separated from the first metal collection pool 30 by a refractory wall 200, recovering metal from the first metal collection pool 30 through an outlet 40, recovering metal from the second metal collection pool 50 through an outlet 60, and recovering cleaned slag through an outlet 20.

In suitable embodiments one or more of the inlets 10 are slag feed launders. In a particular embodiment one or more of the inlets 10 are adapted to feed molten stainless steel slag into the furnace 100. In a further embodiment, one or more of the inlets 10 are adapted to feed molten ferrochrome slag

In some embodiments the slag fed into the furnace 100 is nickel rich. For the purposes of the present description, the term nickel-rich slag should be understood to mean that the slag comprises nickel compounds. In a particular embodiment, a nickel-rich slag has a nickel compound to non nickel metal compound ratio of at least 1 :1 , for example a nickel compound to chromium compound ratio of at least 1 :1 , preferably 3:2, suitably 2: 1 .

In the metal pool closer to the slag feed, the first metal collection pool 30, nickel- rich metal accumulates, since metal droplets occurring in the slag stream settle into that pool. In the metal pool closer to the outlet for tapping of purified slag, the second metal collection pool 50, material originating mainly from metal oxide reduction reactions and being very low in nickel, will settle.

Thus, in an embodiment, the metal recovered from the second metal collection pool is substantially different to the metal recovered from the first metal collection pool.

Solid slags of stainless steel manufacturing can be used as a raw material to increase metal production or to adjust slag chemistry. If solid slag or metal oxide streams from stainless steel production are used as raw material input for the furnace, the feed should be directed to an area where nickel recovery to the first metal collection pool 30 is highest, i.e. above the first metal collection pool 30, preferably close to the slag feed side of the first metal collection pool 20 to allow complete melting of the materials. There slag melts and nickel oxides are reduced to nickel metal in the slag layer and nickel containing droplets settle to the first metal collection pool 30. Thus, in an embodiment the one or more slags comprises nickel oxides

In a further embodiment the one or more slags comprises chromium oxides. Solid ferrochrome production slags can be used to increase metal production as a raw material or to adjust slag chemistry. For maximizing the second metal collecting pool 50 Chromium content, the feed area of solid slag or metal oxide streams from ferrochrome production should be close to or above the refractory wall side of the second metal collecting pool 50 e.g. downstream from the refractory wall. There slag melts and its iron and chromium oxides are reduced to ferrochrome metal in slag layer and metal dropelts settles to second metal collection pool 50. Thus, in one embodiment the one or more slags are fed into the furnace via slag inlets 10 to a position downstream of the refractory wall 200 to bypass the first metal collection pool 0. In a further embodiment the one or more slags are fed into the furnace via inlets 10 to a position upstream of the refractory wall 200 for the separation of nickel from the first metal collection pool 30.

The slag fed into the furnace 100 may in embodiments be solid and in further embodiments may be a liquid. In still further emboidments, the slag fed into the furnace 100 may comprise a combination of both liquid and solid slags.

Further embodiments relate to a furnace 100 for cleaning one or more molten slags. In an embodiment the furnace 100 comprises one or more inlets 10 for feeding slag into the furnace to form a flowing slag layer 70, a first metal collecting pool 30 configured to collect metal droplets gravitationally separated from the flowing slag layer 70, a second metal collection pool 50 downstream to the first metal collection pool, the first 30 and second 50 metal collection pools being physically separated by a refractory wall 200, and one or more outlets 20 for removing cleaned slag from the furnace. In one embodiment the slag layer 70 flows directly over the metal collecting pools. In an embodiment the one or more outlets 20 may be slag tapping lauders.

In a further embodiment the first metal collecting pool 30 comprises one or more outlets 40 for recovering metal. Such an outlet 40 might, in an embodiment, be a tap adapted for tapping liquid metal. In a further embodiment the one or more outlets 40 might comprise one or more tapping holes and lauders.

Similarly, in an embodiment the second metal collecting pool 50 comprises one or more outlets 60 for recovering metal. Again, such an outlet might, in an embodiment, be a tap adapted for tapping liquid metal. In a further embodiment the one or more outlets 60 might comprise one or more tapping holes and lauders.

In a further embodiment the furnace 100 further comprises one or more electrodes 80 positioned between the one or more inlets 10 and the one or more outlets 20. Electrodes 80 are needed to supply electric energy to heat up material streams up to reaction temperature, to 1600-1650°C, and provide heat for actual reduction reactions.

In an embodiment the one or more inlets 10 comprise one or more slag feed lauders. In a further embodiment the one or more outlets 20 comprise slag tapping lauders.

Inlets 10 are positioned with respect to the first and second metal collection pools 30, 50 depending on the composition of the slag to be fed into the furnace 100. In one embodiment one or more inlets 10 are positioned upstream of the first and second metal collection pools 30, 50. In a further embodiment, one or more inlets 10 are positioned downstream of the first metal collection pool 30.

The actual position of the refractory wall 200 is dependent on the furnace operating temperature, the metal and slag density difference, and the slag viscosity at the operating temperature. Thus, in an embodiment the refractory wall is positioned to optimize nickel separation capability. If the wall is too close to the inlet end or too far away from the inlet end, nickel separation might suffer.

The refractory wall 200 can be cooled internally by suitable cooling media to increase its lifetime. Suitable cooling media are for instance nonflammable oils or air. Use of water can cause safety issues.

The refractory wall 200 structure can be independent from the furnace refractories to avoid design issues related to the thermal expansion of refractory materials.

In the alternative, the refractory wall 200 can be designed to be a functional part of the furnace refractories to avoid design issues related to the buoyance effect relating to the different densities of refractories, metal and slag.

The purpose of the refractory wall 200 is to divide a metal pool in the furnace into two sections 30, 50, one with high nickel and one with low nickel.

Further embodiments describe the use of a furnace. In one embodiment the furnace 100 described herein above is used in a method for cleaning one or more slags. In a further embodiment, the furnace 100 described hereinabove is used in a method as described hereinabove

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated in more detail referring to the attached drawings where

Fig. 1 shows schematically a rectangular furnace with electrodes and a refractory wall metal divider according to the invention. In the rectangular furnace, the metal pool inside the furnace is divided into two sections, one with droplets settling and one for metals from oxide reduction reactions. EMBODIMENTS ILLUSTRATING THE INVENTION

Fig. 1 illustrates an embodiment where molten FeCr and Stainless Steel (SS) slags are fed to a rectangular six-Soderberg-electrode in-line furnace 100 through feeding inlets 10. The electrode arrangement in the figure is exemplary.

The combined slag layer 70 flows through the furnace 100 where purified slag is tapped to (for example) slag pots 20.

Inside the furnace 100, metal droplets present in the feed slag are settled by gravitation to the first metal pool 30. From the first metal pool 30, metals are tapped from the furnace 100 through tapping holes and lauders 40.

Inside the furnace 100, metal droplets produced by metal oxide reduction processes can settle by gravitation to both metal pools 30, 50. If feeding of chromium-rich solid feed materials is arranged to the downstream side of the refractory wall 200, settled metals will not dilute the nickel content in the first metal collecting pool 30. Chromium oxides present in molten slag feed will enter both pools. Nickel oxides present in solid feed materials should be fed to the upstream side of the furnace 100 to enhance nickel recovery to the first metal collecting pool 30. From the second metal collecting pool 50, metals are tapped from furnace through tapping holes and lauders 60 like the first metal collecting pool 30.