CA2270850C - Metal recovery from skimmed slag - Google Patents

Abstract

The invention relates to a process for recovery of metals such as Cu, Ni, and Co from slag formed during the smelting process. Upon smelting, a slag layer is skimmed from a matte layer, containing valuable metals. Metal from the matte layer becomes entrained in slag skimmed from matte. To recover metal from matte-enriched slag, slag skulls are formed which comprise layers of solidified matte-enriched slag. Slag skulls are cooled and broken into pieces selected for high metal concentration, which. may then be either fed directly to a smelter, be further concentrated by grinding and flotation or may be upgraded using conventional gravity separation techniques such as a shaking table. Valuable metals so recovered may be added back into a smelting process.

Description

1V~ETAL RECOVERY FROM SKIMMED SLAG
The present invention relates to a process for recovery of metal from smelting waste, and more particularly for recovery of metals from skimmed slag.
BACKGROUND CIF~THE INVENTION
The smelting; of metal sulphides for the recovery of metals has been practised for many years and is the major process for producing valuable metals, such as copper and nickel. During smelting and converting a matte is formed, which contains the majority of copper and nickel. A slag is formed containing iron and other elements, which is immiscible with the matte and is of a lower specific gravity.
The slag floats on the surface of the matte, and is separated from the matte in furnaces and conven:ers by a process referred to as "skimming". Skimming relies on the differences in specific gravity between the matte which contains the valuable metals and the slag which contains mainly the waste products.
Once skimmed from the matte, slag is conventionally transferred into ladles or pots for transportation to a waste disposal site. Unfortunately, during the skimming process, some of the matte is entrained with the slag, resulting in the loss of valuable metals when the skimmed slag is discarded.
Conventional solutions have attempted to recover the valuable metals from skimmed slag. Such solutions have included the use of fuming processes utilizing oxygen, fuel and red.uctant in order to obtain metal enriched fiunes from slag (e.g. US
4,820,340; US 5,588, 436; and US 4,252,563). In US 4,009,024 and US 3,891,428 there is disclosed the; recovery of metals from slag that comprises the injection of fuel and air into the molten slag to reduce the slag, followed by metal recovery.
Typical metal recovery involves the use of settling units which allow the entrained matte to separate followed by a second slag skimming, or grinding and flotation of the slag to recover the sulphide metals into a flotation concentrate. GB 1,435,371 discloses a method for separating metal-containing slag using a wind tunnel, with separation arising as a result of differences in specific gravity. Alternatively, attempts have been made to reduce the amount of valuable metal entrained in the slag prior to skimming, for example by addition of reductants to the molten metal or slag which reduce dissolved metal oxides, thereby increasing their specific gravity. This allow the reduced metals to selparate into the matte layer. Inconveniently, these conventional approaches may require large capital expenditures, specialized equipment, and increased operating <;osts. The cost of milling and flotation of all of the slag generated from the smelting process may exceed the value of the metal recovered using conventional recovery methodology.
There is a need within this art for a cost effective and simple method for the recovery of metals from the slag produced during the steel making process. It is an object of the invention to provide a simple and efficient process for the recovery of copper, nickel, cobalt or other metals of interest from skimmed smelting slag.
SUMMARY OF THE INVENTION
The present invention relates to a process for recovery of metal from smelting waste, and more particularly for recovery of metals from skimmed slag.
According to the invention, there is provided a process for recovery of metal from skimmed slag, the process comprising the steps of: storing skimmed slag in a container for a period of time sufficient for at least a portion of the metal to precipitate or separate from the slag and freeze to the walls of the vessel, removing molten slag from the; container while leaving a slag skull within the container, cooling the slag skull, and recovering metal from the slag skull. This invention also comprises this method wherein the period of time sufficient for at least a portion of the metal to precipitate from the slag is from about 15 min. to about 12 hours.

Furthermore, the above method is suitable for the recovery of metals selected from the group consisting of copper, nickel, cobalt, and combinations thereof.
Metal recovered from a slag skull would otherwise have been discarded as a waste product of the: smelting process. Advantageously, the process allows for greater recovery of :metal and reduces the waste as a result of smelting.
Furthermore, the step of storing the skimmed slag, as defined in the above method, may take place during transport of the slag to a slag disposal site.
Additionally, the invention provides a process for recovery of metal from a slag skull comprising; breaking a slag skull into pieces, grinding the pieces of slag skull into fragments, and separating the fragments to obtain fragments concentrated in metal. The separated fragments may then be further processed by smelting.
Conventional. approaches to the recovery of metals from slag may require large capital expenditures, specialized equipment, and increased operating costs. The present invention is directed to providing a cost effective and efficient method for the recovery of metals from metal-enriched skulls, and the recovery of the metal from these skulls.
DESCRIPTION OIL PREFERRED EMBODIMENTS
The present invention relates to a process for recovery of metals from skimmed smelting slag. The metals recovered according to the invention include any metal typically found in the matte portion upon smelting. The metals recovered generally have a greater specific gravity than slag, and thus settle out of molten slag.
Examples of metals recovered from slag include, but are not limited to copper, nickel and cobalt.
A layer of srr~elting slag is skimmed off of the matte. During the skimming process, a small amount of the matte typically becomes entrained in the slag.
The matte content of skimmed slag is typically low between 0.25 and 0.35% by weight, but it can be much higher under upset or abnormal operation, up to, for example, about 20% by weigr~t.
Once skimmed, molten slag and any matte content entrained therein is deposited into a container, such as a ladle or a pot and may be transported to a slag disposal site. Due to the higher specific gravity of matte relative to slag, the matte entrained in the slag precipitates to the bottom of the container while the slag is in liquid form. As a result, a layer comprised of slag enriched with matte forms in the bottom of the container.
Any metals entrained within the slag are allowed to settle for a time sufficient to permit at least a portion of the metal to separate from the slag and freeze. For example, this period of time may comprises from about 15 minutes up to about hours or more, depending on the size of the pot, and the slag composition.
Longer periods of time may be employed for larger pots, where slag may remain in a molten state for a longer period. The slag cools in such a way that the molten liquid first solidifies near the base and wall of the pot while the inner content of the container remains in molten state due to heat retention. The metals may settle our of the slag en route to a slag disposal site, or while remaining stationary for a period of time. In this way, the bottom layer within the pot, which comprises slag enriched with matte and metals, solidifies ag~unst the bottom of the container.
Once the desired amount of solidification of the slag has been achieved, either by design or through the time required to transport the slag to a dump site, the remaining molten slag is removed from the container. This portion of slag may be disposed of as waste or may be poured into a secondary container, to allow further settling of metals contained therein. The solidified layer containing matte-enriched slag remains encrusted to the bottom of the container. The solidified layers of material that encrust the pot are called "skulls".

The emptied container, having the layer formed along the bottom thereof, may then be re-filled with freshly skimmed molten slag and the cycle repeated.
Upon repetition of the above-described process, layers of combined matte-enriched slag build up along the swface of the container, gradually decreasing the available volume of the container. T'he skull is removed from the container either when the available volume of the contaainer is inadequate for further waste disposal of molten slag, or when the size of the skull is optimum for metal recovery as determined by testwork.
Factors affecting this optimum include, container geometry, waiting time, travel time to disposal facilities, outside temperatures and operating temperature of the smelting unit.
The degree crf recovery of metals may depend on the degree of oxidation of the metals in the slag (see for example US 4,588,436 and US 4,009,024). For example, the higher the degree of oxidation, the less recovery of the metal of interest. Thus, metals in the molten slag may be reduced from the oxidized form prior to settling in the pot, thereby increasing recovery of the metals.
To remove the solidified layers, the container is allowed to cool to a partially frozen state, allowing the combined layers to solidify. The skull is removed from the container and cooled to a frozen state, preferably to ambient temperature. The skull contains layers of rr~atte-enriched slag, and is thus more concentrated, than is slag, in valuable metals.
The skull m.ay represent up to about 2 to about 5% of the total mass of the slag produced, depending on operating conditions, but the skull comprises about 3 to 6 times more valuable metal than slag. The skull contains a high concentration of the metal of interest, thus avoiding the need to process all of the slag generated.
After cooling, the skull is broken up into pieces, for example, using a pneumatic or hydraulic hammer into fragments that can be manually sorted, however, the use of machine-assisted sorting may also take place. Skull pieces containing high concentrations of the metal of interest are then selected by any suitable process. Such processes include, but are not limited to, visual inspection, the use of metal detectors, specific gravity, or weight. The pieces of skull identified and sorted according to the concentration of the metal of interest contained therein, may be re-processed by smelting and further refining.
According to an embodiment of the invention, once the skull has been broken up, those pieces containing a layer rich in the metal to be recovered may then be identified and selectively separated from the pieces of broken skull. The metal of interest may then be recovered by milling the pieces of skull into smaller fragments, followed with further grinding to achieve appropriate sized fragments containing the metal of interest for further processing, for example, by flotation separation or gravity separation using a conventional shaking table.
The ground fragments may be returned to smelter or may be further separated on the basis of specific gravity, using any acceptable method of separation such as flotation separation. Metals of interest are generally of a greater specific gravity than slag, and will thus settle out from the remaining slag content of the skull.
By using the process as described herein, a significant portion of the metal present in slag, and up to about 80% of the metal present within the skull may be obtained. The concentration of the recovered metals within the product have been up to 45% Cu, Ni, and Co are typically of about 15 to about 25%. The weight ratio of materials processed to product is variable but is typically from about 20:1 to about 30:1 Examples arc: provided below which describe particular embodiments of the invention, but are not to be construed as limiting.

Example 1 Slag is skimmed from a furnace at about 1100 degrees Celsius into steel ladles or pots which hold between 50 and 100 tons. These are transported to a slag dump and during the transfer (betwf:en l and 2 hours) heat is lost through the walls of the vessel and the slag begins to freeze. During this time any entrained matte which contains the valuable metals, settles to the bottom of the vessel and preferentially freezes. The slag is poured off at the dump site leaving behind in the vessel a skull, that is, a skin of slag enriched with matte. The quantity of solidified material varies with many factors as discussed earlier. In a typical operation the skull can represent between about 1 and 3 tons per cycle. After a number of cycles the skull is removed from the vessel, allowed to cool, broken into pieces and the high grade metal bearing layers recovered.
Recovery includes father feeding directly to a smelter or further concentration by grinding and flotation or upgraded using conventional gravity separation techniques such as a shaking table. Recovery of the valuable metals from the entrained matte that is skimmed with the' slag carp be up to 80% with this process.
Various modifications rnay be made without departing from the invention. It is understood that the invention has been disclosed herein in connection with certain examples and embodiments. However, such changes, modifications or equivalents as can be used by those skilled in the art are intended to be included.
Accordingly, the disclosure is to be construed as exemplary, rather than limiting, and such changes within'~he principles of the invention as are obvious to one skilled in the art are intended to be included within the scope of the claims.

Claims (12)

1. A process for recovery of metal from skimmed slag comprising:
(a) storing said skimmed slag in a container from 15 minutes to 12 hours to permit at least a portion of said metal to separate from said slag thereby forming a slag skull;
(b) removing remaining molten slag from said container, and leaving said slag skull within said container;
(c) cooling said slag skull; and (d) recovering metal from said slag skull.

2. The process according to claim 1, wherein steps (a) and (b) are repeated prior to step (c).

3. The process of claim 2, wherein said metal separates during transport of said skimmed slag to a slag disposal site.

4. The process according to claim 1, wherein step (d) comprises removing said slag skull from said container, breaking said slag skull into pieces, and selectively retaining pieces having high concentrations of said metal.

5. The process according to claim 1, wherein step (d) comprises removing said slag skull from said container, breaking said slag skull into pieces, grinding said pieces into smaller fragments, separating said fragments to obtain a product concentrated in said metal.

6. The process according to claim 5, wherein separation of said fragments takes place on the basis of specific gravity or by flotation separation.

7. The process according to any one of claims 1 to 6, wherein said metal recovered at step (d) is returned to a smelting process.

8. The process of claim 1, wherein said metal is selected from the group consisting of copper, nickel, cobalt, and combinations thereof.

9. A process for recovery of metal from a slag skull, the process comprising the steps of:
(a) breaking a slag skull into pieces;
(b) selecting said pieces having a high concentration of said metal to obtain selected pieces;
(c) grinding said selected pieces of slag skull into fragments; and (d) separating said fragments to obtain fragments concentrated in said metal.

10. The process of claim 10, wherein in step (d) fragments are separated on the basis of specific gravity.

11. The process of claim 10, wherein said fragments concentrated in said metal are further processed by smelting.

12. A process for recovery of metal from a slag skull, the process comprising the steps of:
(a) breaking a slag skull into pieces;
(b) selecting said pieces having a high concentration of said metal to obtain selected pieces; and (c) processing said selected pieces of slag skull by smelting.