Granulated blast furnace slag is a waste of metallurgical production. To be more precise, this is the raw material that remains after the smelting of cast iron in blast furnaces.
What is slag and where does it come from?
In order to produce raw materials such as cast iron, certain raw materials must be loaded into the blast furnace. Iron ore, flux stone, coke became such materials. At the exit from the furnace, only two substances are obtained - this is cast iron and slag. The composition of blast furnace slag includes substances such as quartz, aluminum oxides (processing from iron ore), calcium and magnesium oxides (waste from flux stones). The molten slag obtained at the outlet of the blast furnace unit may have a temperature of 1500 degrees Celsius, and in some cases more. Naturally, cooling of the material is required. Currently, plants use only 4 main types of blast furnace slag processing.
- The first method involves cooling the feed with ordinary cold water. This operation is also called swelling of slag.
- In the second method, cooling of the substance using air is used.
- The third way is crushing.
- The fourth is grinding.
Main components of the material and processing results
It is important to note here that the choice of the processing method for blast furnace slag will determine what kind of substance will be obtained in the end and what specific properties it will have. Each method gives unique qualities to raw materials. Another important observation is that the main oxides that make up the slag - magnesium, calcium and aluminum oxides, do not occur in nature in free form. In other words, they do not exist, and they can only be obtained by processing slag and iron ore in a blast furnace. In addition to the basic oxides, the composition of the substance includes manganese, iron and sulfur compounds, and quartz.
For example, if you resort to cooling the waste material with air, then the composition of blast furnace slag will be approximately as follows: silicates and aluminosilicate materials, such as volastonite, melilite and mervinite.
How is the granulation process going?
The process of granulation of the substance is carried out by the method of rapid cooling of molten raw materials. It is important to add here that in some cases a process of mechanical crushing of either still liquid slag or already half-solidified may be added. The main goal of this operation is to process large pieces of material into smaller grains, which will be more convenient to process in the future, as well as improve quality such as hydraulic activity. This is a very important operation, since it is this property that is most important, since slag can be used as a constituent component for slag cements, and can also act as an additive to Portland cement.
You must understand that as such, special production of blast furnace slag does not exist. It is obtained only during the processing of ore and other substances in a blast furnace.
Granulation Methods
You need to know that for the granulation of production waste, that is, slag, there are several methods, as well as devices that perform this operation. Depending on what humidity of the slag is obtained in the end, and the plants are divided into types. There are installations for wet as well as for semi-dry granulation.
If the wet granulation method is carried out, then the slag loaded into the slag ladles in a hot form is fed to reinforced concrete pools filled with water. After that, from the ladles it merges into these pools through special gutters. A small advantage of the method is that the pools are divided into several sections. This allows the cooling process to be carried out almost continuously. When incandescent material is poured into one section, already cooled blast furnace slag can be discharged from the other at the same time. For the shipment of raw materials at the plants, there are clamshell cranes that feed it directly to open-type railcars. After that, the slag is delivered either to the warehouse or immediately to the consumer.
Moisture contents
It is worth noting that the moisture content in the material depends on the total weight of blast furnace slag, which goes through the granulation process. In other words, the more porous the grain structure, the more moisture remains in them. The formation of pores in hardened granular slag is due to the action of gases. These volatile substances are contained in the molten raw materials and begin to evaporate when the temperature of the slag decreases, i.e. when it is cooled. However, it is important to note here that the cooling and solidification of the material occurs so quickly that the gas does not have time to evaporate from the slag. It remains inside in the form of small bubbles, which create the porous structure of the grains. The porosity, and because of this, the moisture content of the slag, also directly depends on those conditions that are used directly in the process of cooling the slag. That is, not only the final result, but also the structure itself depends on the choice of method.
Semi-dry method
Semi-dry slag is obtained using the method of mechanical crushing, as well as throwing into the air previously cooled, but at the same time, not yet hardened slag. Thus, the structure of the material is much denser, which significantly increases the physical weight by about one and a half times, when compared with the weight that the slag would have if it passed through wet granulation, having the same initial amount of substance. If we talk about such a characteristic as humidity, then for wet raw materials this value is in the range from 15 to 35%, rather rarely 10%. While dry granulation is accompanied by slag moisture ranging from 5 to 10%. If we compare volumetric weight, then with a wet processing method it will be in the range from 400 to 1000 kg / m, and with a dry method it will be in the range from 600 to 1300 kg / m. It should be added that with increasing temperature of the blast furnace the total weight of blast furnace slag will decrease.
It is important to add that most plants use wet granulation, despite the fact that the method has several disadvantages:
- High fuel consumption for drying the material before it breaks.
- The performance of slag drying equipment is low.
- Freezing wet slag in wagons, warehouses and other storage places in the winter.
Blast furnace slag GOST 3476-74
This standard describes all technical requirements, test methods and acceptance rules, as well as methods for storing and transporting blast furnace slag. One of the clauses of the state document concerns the moisture content of raw materials. It is said that the percentage of humidity should correspond to that agreed upon by the customer and supplier. In other words, we can say that the customer chooses the method of granulating the material.