This invention relates to a method for improving the reduction degree of metal components in a material to be treated when smelting ferroalloy, as ferrochrome suitable for manufacturing of stainless steel. According to the method nickel-bearing material is fed into ferroalloy.

It is known from the WO patent publication 2010/092234 a method wherein nickel ore and/or nickel concentrate or an intermediate product precipitated from solutions of nickel ore and/or nickel concentrate is agglomerated in the manufacturing process of ferrochrome so that it is first produced from nickel- containing material together with iron-containing chromite concentrate and binder agent pellets, and the drying and calcination of nickel-containing material is carried out advantageously within one-stage heat treatment of pellets, sintering. With the heat treatment of pellets the object are strengthened so that the heat treated objects are conveyable, when desired, essentially complete between separate process stages. If needed, the pellets can be preheated before sintering. Heat treated objects can be conveyed, when desired, essentially complete between separate process stages. Heat treated objects can be downsized, when desired, when conveying object between separate process stages or process units. Sintered and thus strengthened pellets are used as material in a smelting process carried out in reducing conditions, in which case it is received as a smelting product nickel-containing ferroalloy, ferrochromenickel.

The above mentioned WO patent publication 2010/092234 thus relates mainly to the production of nickel-containing pellets by sintering. Instead, smelting conditions of the sintered pellets are not exactly described. When describing the energy efficiency it is, however, mentioned that nickel containing in pellets catalyses chromium reduction in pellets and thus decreases the specific consumption, advantageous carbon, of the reducing agent in the ferroalloy production.

It is now surprisingly observed that nickel containing in pellets not only catalyses the reduction of chromium in chromite pellets, but nickel containing in the feed of a furnace used for smelting of chromite improves in the smelting process the reduction of all essential metal components, iron, chromium and nickel, containing in the feed of the smelting furnace. The object of the present invention is to utilize this surprizing finding and to achieve a more effective method than before for increasing the reduction degree in the smelting process of chromite material in which method the reduction of metal components in chromite during the smelting is improved by alloying into the material to be gone into smelting nickel-containing material and simultaneously to achieve a prealloy, ferrochromenickel, suitable to the production of stainless steel. The essential features are enlisted in the appended claims.

According to the invention, it is alloyed into the raw material, as chromite, to be smelted in the ferroalloy production before the smelting nickel-containing material, in which case nickel-containing improves the reduction of metal components containing in the feed material simultaneously when nickel- containing material itself is managed to be reduced as a metallic component in the ferroalloy. According to the invention, by means of the nickel amount to be added into the ferroalloy it can advantageously be adjusted the reduction degree of metal components in the ferroalloy and simultaneously be achieved a ferroalloy containing the desired nickel content, as ferrochromenickel alloys having different nickel contents. Ferrochromenickel alloys containing desired nickel contents can be used for instance for the production of different stainless steels, as austenitic or duplex stainless steels. In the method according to the invention it can be used as a nickel-containing raw material at least partly nickel oxide, at least partly nickel ore and/or nickel concentrate or at least partly a nickel-containing intermediate product achieved by the leaching and/or by precipitating of nickel ores and/or nickel concentrates. The nickel-containing raw material is fed into a smelting process together with ferrochrome raw material. Before feeding into a smelting furnace the nickel- containing raw material is pretreated either so that sintered pellets are formed from the nickel-containing material together with the ferrochrome raw material or so that the nickel-containing raw material is pretreated separately to chromite pellets. It is possible to carry out the pretreatment of the nickel-containing raw material also so that one part of the nickel-containing raw material to be fed into the smelting furnace is pretreated together with chromite pellets and one part of the nickel-containing raw material is pretreated separately to chromite pellets. Thanks to different pretreatments the nickel-containing raw material to be fed into the smelting furnace and promoting the reduction of different metal components can be for instance partly nickel-containing hydroxide intermediate product, partly sulphidic or lateritic nickel concentrate.

The nickel-containing raw material to be utilized in the method according to the invention is advantageously a nickel-containing hydroxide intermediate product from mines or other hydrometallurgical processes, which intermediate product is precipitated from solutions of lateritic and/or sulphidic nickel ores and/or nickel-containing concentrates of sulphidic ores. This kind of nickel-containing hydroxide intermediate product is for instance a nickel-containing intermediate product from pressure leaching, atmosphere leaching or heap leaching of lateritic or sulphidic nickel ores or nickel concentrates as well as a nickel- containing precipitated product of solvent extraction solutions, stripping solutions or refining solutions received from solvent extraction processes or ion exchange processes of nickel-containing materials. In the method of the invention it can as a raw material be used also carbonate or sulphate nickel materials. Further, a sulphidic nickel concentrate itself and a hydrometallurgically precipitated nickel sulphide intermediate product are suited for the nickel-containing raw material of the method. According to the invention, the amount of the nickel-containing material to be fed into a smelting furnace is adjusted in the range of 5 - 25 weight %, preferably 10 - 20 weight %, from the total mass of the pretreated material to be fed into the smelting furnace. When adjusting the amount of the nickel- containing to be fed into the smelting furnace it is considered the achievement of the energy-economically favourable reduction conditions and/or the production of a prealloy, ferrochromenickel, suitable the production of favourable stainless steel in each case. Using a small addition of nickel- containing raw material, the reduction degree remains low, in which case it is created a ferroalloy with low nickel content, ferrochromenickel. This kind of ferroalloy with a low nickel content is a favourable prealloy especially to the production of duplex stainless steel grades. Using a greater addition of nickel- containing raw material the reduction degree increases and also the nickel content in the smelting product is greater. This kind of ferrochromenickel with a greater nickel content is favourable to use to the production of austenitic stainless steel grades having a high nickel content.

In the pretreatment of nickel-containing raw material to be fed into a smelting furnace in accordance with the method of the invention it is advantageously considered the composition and the microstructure of the nickel raw material. If the nickel-containing raw material is for instance a nickel-containing intermediate product of mines or other hydrometallurgical processes precipitated from solutions of nickel-containing solutions, which intermediate product requires to carry out as a pretreatment among others calcination at a higher temperature, the pretreatment of the nickel-containing raw material is carried out together with the production of chromite pellets and sintering of pellets. Instead, if the nickel-containing raw material of the method according to the invention is material, as for instance nickel oxide, nickel ore and/or nickel concentrate, which does not require in addition to a possible drying any other essential pretreatment at a higher temperature, then the nickel-containing raw material is possible to feed into a smelting furnace with the feeding of chromite pellets. The microstructure and composition of the nickel-containing raw material can also be such that it is advantageous to pretreat the raw material separately from chromite pelletizing and to feed the nickel-containing raw material into sintering of chromite pellets before feeding into a smelting furnace. In the method according to the invention it is used advantageously a smelting furnace which is provided with a preheating equipment so that the feed going into the smelting furnace is conducted through the preheating equipment into the smelting furnace. According to the invention the pretreated nickel-containing raw material is conducted also into the preheating equipment wherein the nickel-containing will come at the latest into contact with other material to be fed into the smelting furnace. In the smelting furnace the nickel-containing together with chromite pellets are smelted to ferrochromenickel having a desired composition, which ferrochromenickel can be utilized in accordance with its composition advantageously for instance in the production of austenitic or duplex stainless steels.

When according to the invention smelting of the nickel-containing raw material is carried out advantageously in a closed submerged arc furnace, carbon monoxide gases generated in the reduction and smelting can be utilized in one hand for instance in the sintering of chromite pellets and in possible other pretreatment and preheating, in another hand for instance in different steps of the production path of stainless steel produced from the smelting product, ferrochromenickel. The method according to the invention is described in more details by means of the appended example.

EXAMPLE From a chromite concentrate containing iron and chromium and an intermediate product containing nickel it was formed a mixture, into which mixture it was added as a binder 1 ,2 weight % bentonite and 3 weight % slag forming material, flux, either limestone or wollastonite. In the table 1 it is presented the contents of chromium, iron, nickel, carbon and sulphur as weight % in mixtures, into which was added 10 weight % (Test 1 ) and 20 weight % (Test 2) nickel hydroxide. Further, in the table 1 it has as a reference material (REF) a mixture, into which mixture nickel hydroxide was not added.

Table 1

The mixtures containing a binder and representing each material of the table 1 were pelletized and sintered. A part of sintered pellets was fed representatively into a smelting furnace with a slag former and a reducing agent.

The materials according to the table 1 were smelted, and in the table 2 it is presented the contents of chromium, iron, nickel, carbon and silicon in smelting products in question and further the recovery of the metal components, chromium, iron and nickel, into the smelting product. The carbon content is composed in accordance with the composition and the equilibrium of the metal alloy. The feed batch has carbon so much that carbon is some enough also for the reduction of silicon into the smelting product. The feed alloy has silicon oxide in raw material and in production bulk supplies.

Table 2 For one part of sintered pellets it was made in the laboratory scale thermogravimetric measurements in order to monitor the reduction degree of the metal components, chromium, iron and nickel, of pellets in the conditions representing the smelting process at different temperature zones with the maximum temperature of 1550 °C. In the table 3 it is presented the results of the thermogravimetric measurements for the reduction degree of chromium (Cr _met /Cr _t ot), iron (Fe _me t/Fe _t ot) and nickel (Ni _met /Ni _to t) at the temperatures of 1400 °C and 1550 °C.

Table 3

The addition of the nickel-containing raw material into pellets increases the reduction degree of chromium and iron at the temperature of 1550 °C substantially, chromium more than 15 % and iron more than 70 % simultaneously when the reduction degree of nickel increases near to 100 % with the Test 2 nickel content. The increase of the reduction degree for all metal components, chromium, iron and nickel in sintered pellets by means of the addition of a nickel-containing raw material simultaneously decreases the need of coke used as reducing agent in the achievement of the reduction conditions of the smelting process.