B: molten metal and slag ladle
C: working platform
Once the solidification has occurred, the Ferro-alloy is sent to the crushing plant to be sorted and then stored. Derived fines are recycled to charge the furnace or to fit the ladles with a refractory lining.
• the high viscosity of the slag in the crucible prevents the Coke (characterized by alveolar structure) to perform its reducing action with the correct gas-solid exchange. lt follows a not entirely satisfactory ore-metal transformation and an excessive consumption of Coke compared with the results obtained;
• in consequence of a higher ore consumption, the produced slag is increased;
• the process, from an energy point of view, is inherently inefficient: the cyclical emptying of the crucible involves thermal downturns and electrical conductivity drops, because the charge of Fe-Cr is highly resistive. In these conditions, the furnace suffers of constant power loss;
• the separation of the metal from the slag occurs outside the crucible and in more stages: this circumstance does not allow to obtain a well emulsified metal thus promoting the occlusion of the reduction gas, resulting in porosity of the solidified metal. Under these operating conditions we obtain a spongy metal, which is polluted by the occluded slag. Furthermore, a part of the metal is lost because of occlusion in the slag;
• while material handling generates airborne pollutants which are harmful to operators, handling processes (subsequent to the tapping) increase the occupational risk factors;
• the formation of metallic micropowders, due to the handlig and the separation of the solid phases, drastically worsens the microclimate in the workplace.
B: Ferrochrome runner
C: slag runner
D: Ferrochrome ingot mould
E: slag ladle
F: drilling machine and capping machine
The reformulation of the slag chemistry involves a dedicated and adaptable parametric approach on the basis of production campaigns, whose aim is the perfect separation of the metal from the slag already in the furnace.
The plant design update involves:
• sizing of the crucible in terms of diameter and volume;
• optimal recalculation of electrodes wheelbase as a function of the transformer power;
• careful selection of a new refractory lining suitable for the containment and preservation of the material in the liquid state;
• designing of a new casting pit layout to allow the separated tapping of the two liquids phases.
This step allows to optimize the chemical reduction process, to reduce energy consumption and to maintain a well-defined amount of liquid material in the crucible, in order to ensure continuity of casting in the long term and simplification of the production process.
• the Coke performs its reducing action under thermodynamic ideal conditions, resulting in a more efficient reduction process. Thus the double level ensures efficiency in the ore-metal transformation and a lower consumption of metallurgical coke;
• in consequence of a lower consumption of ore, the produced slag is decreased;
• the process is energy efficient: the permanence of a certain amount of liquid in the crucible stabilizes the system thermodynamics and keeps the inside thermal conductivity constant. The use of the double level allows the furnace to operate in steady state condition;
• the separation of the metal from the slag is clear and it occurs already in the furnace, thanks to the prolonged chemical-physical action of the slags and the double level action. The correct homogenization and degassing of the Fe-Cr ensures a clearly improved quality of the Ferro-alloy (compact and well emulsified), thus eliminating Ferrochrome losses due to occlusion in the slag;
• the post-casting operations are deleted: this circumstance reduces occupational risk factors and improves the microclimate, because the formation of the metallic micropowders due to the handling is eliminated.