The production technology of magnesia‑carbon bricks is not complicated. However, to produce high‑quality products, each process link must be strictly monitored. Among them, mixing, forming and drying are particularly important in production.
1. Crushing
Crushing is the process of processing bulk materials into fractions with desired particle sizes. It is an indispensable procedure in refractory production. Simple as it is, it greatly affects the stability of product quality. Meanwhile, crushing equipment consumes large power, suffers heavy wear and high maintenance frequency, which leads to high servicing costs. Proper crushing management stabilizes product quality and realizes energy‑saving & consumption reduction.
In magnesia‑carbon brick production, crushing is mainly used to prepare raw materials of various particle sizes. It increases the specific surface area of materials and damages crystal lattices to produce defects, so as to accelerate physical and chemical reaction rates.
2. Weighing
Weighing means combining different raw materials and particle fractions according to product formula design. Weighing methods vary depending on the type and state of raw materials.
Weight batching is commonly adopted for magnesia‑carbon bricks due to its high accuracy, with error generally no more than 2%. Common weight‑batching equipment includes manual weighing scales, automatic weighing scales and weighing trolleys. Select proper equipment according to practical requirements and automatic control level.
3. Mixing
The purpose of mixing is to homogenize material compositions and maximize contact surfaces among different materials. Kneading of refractories is a homogenization method accompanied by extrusion, kneading and degassing. Similar to powder mixing, refractory kneading proceeds step‑by‑step. It becomes more complex owing to differences in material components, particle sizes, binders and additives.
There are many types of magnesia‑carbon bricks. Formulas vary with service positions, mainly including magnesia grade, graphite dosage, as well as types and dosages of additives. For instance, for the slag‑line zone of steel ladles, higher‑grade graphite with increased addition is required to improve slag resistance and thermal‑shock stability. If carbon content is lower than 10%, a continuous carbon network cannot form inside bricks. Carbon properties cannot be fully exerted, which impairs slag resistance and thermal‑shock resistance. Excessively high carbon content not only brings production difficulties but also causes easy oxidation of bricks. Therefore, carbon content in magnesia‑carbon bricks is generally controlled within 10%‑20%.
To make magnesia grains evenly coated by graphite, follow this mixing sequence: granular materials first, then resin, next graphite, and finally fine powders and various additives. Graphite has low density and tends to float with large dosage, which makes uniform mixing difficult. Additives are added in tiny amounts. Long‑time high‑intensity stirring is required for full homogenization. Nevertheless, binders will volatilize and dry during stirring. Over‑long mixing time will peel off graphite and fine powders coated on particles, so mixing duration must be strictly controlled.
4. Forming
Forming is an essential process for shaped refractory products. Under combined force of pressing equipment and moulds, refractory mixtures are turned into green bodies with specific shape and strength.
Various forming methods are available for refractories. Magnesia‑carbon bricks adopt semi‑dry pressing. Semi‑dry forming has moderate requirements for mixtures with simple procedures. Due to low moisture content of mixtures, high pressure is required to achieve tight bonding between particles. Under external force, granular materials rearrange, expel gas and bond to generate strength and form shaped green bodies. Applied pressure is the dominant factor for semi‑dry forming. Within a certain range, pressure directly determines comprehensive performances of magnesia‑carbon bricks. Higher pressure brings higher bulk density, lower porosity and greater strength.
High‑performance magnesia‑carbon bricks share the features of high bulk density and low porosity. Bricks with open porosity below 4% show very low erosion rate.
Forming aims to densify the microstructure of magnesia‑carbon bricks. High pressure is required for its semi‑dry forming process. Given fine particle size and high graphite content of mixtures, forming shall comply strictly with operation regulations; otherwise cracks or delamination will occur. Apply pressure step‑by‑step from low to high: low‑pressure for degassing with slow descent, high‑pressure for holding with slow lifting.
If conditions permit, use vacuum brick press. Vacuumize mixtures inside mould cavity before pressing to skip degassing during compaction. Even with faster low‑pressure pressing speed, few cracks or delamination will occur. It is especially suitable for high‑carbon magnesia‑carbon bricks prone to delamination.
5. Drying
Factory personnel usually call magnesia‑carbon brick heat‑treatment “drying”, while this term is not precise. Drying mainly refers to moisture removal. Apart from water evaporation, magnesia‑carbon brick heat treatment involves a series of physical‑chemical changes, hence it is defined as heat treatment.
Heat‑treatment temperature directly influences brick performance. This process is essentially phenolic resin curing. Temperature and holding time determine whether resin is fully cured. Within a certain range, full‑curing time is inversely proportional to temperature, not in a linear relation.
Experimental researches show that magnesia‑carbon bricks reach optimal density only within a specific temperature range. Density drops if temperature goes below this range, and declines sharply when temperature exceeds it. Repeated tests prove that the reasonable temperature window is 200℃‑250℃.
