When we discuss roasting, basically, it is a process of metallurgy where the ore is converted into its oxide by heating it in the presence of excess air above its melting point. While calcination is the process mostly used in the oxidation of carbonates, roasting is a method that can be used for converting the sulphide ores. Whereas, during roasting, the non-metallic and moisture impurities in the form of volatile gas are released. The roasting process contains the solid-gas thermal reaction that includes reduction, oxidation, chlorination, sulfation, and also pyro hydrolysis.
However, roasting that involves the sulphides is a major source of air pollution , and the major drawback of this process is, it releases a large amount of metallic, toxic, and acidic compounds as well, which causes harm to the environment. An example of roasting can be given as when Zinc sulphide is converted into zinc oxide. A few of the major differences between calcination and roasting are tabulated below..
Calcination is a process where the air might be supplied in limited quantity, or the ore is heated in the absence of air. Roasting includes heating of ore lower than its melting point in the presence of oxygen or air. Calcination involves the thermal decomposition of carbonate ores. Roasting is mostly carried out for sulfide minerlas.
Moisture is driven out from ore during calcination. Roasting does not involve dehydration of ore. Book Chosen Science. Subject Chosen Science. Book Store Download books and chapters from book store. Currently only available for. Class 10 Class Metals and Non-metals. Carbonate ores and hydrated ores are treated in 'calcination'.
After this process, the cooled material is now named the product pellet. Schematic of the straight grate process [ 14 ]. In the process of obtaining the product pellet, the principle is that the material be heated at a recrystallization temperature according to the compound type.
If this temperature level is exceeded too much, liquid phase formation is occurred and undesirable sintering process starts. In this study, siderite ore was obtained from the Deveci district of Hekimhan Malatya Province. Figure 6 shows the pictures of the raw siderite ore and the calcined siderite ore used in the study. The elemental analyses of raw siderite ore is given in Table 5.
Raw siderite on the left and calcined siderite on the right. Its density is 2. Bentonite, a volcanic mineral formed by the decomposition of volcanic ash in situ and composed of montmorillonite clay mineral of large size which absorbs water and is used commercially in drilling mud, catalyst, paint, plastic filling works [ 15 ]. In the microwave experiment part, sucrose—C 12 H 22 O 11 Merck The raw siderite ore used in the study was sieved and weighed. The results of the sieve analysis are given in Figure 7.
Cumulative undersize graph of raw siderite. The raw siderite ore was both directly grounded and calcined before grinding operation. During this calcination process, CO 2 is removed by the effect of temperature, causing the capillary cracks in the ore, as a result fragmentation and crumbling is occurred. This situation is evident in the cumulative undersize graph given in Figure 8 showing that calcination process slightly decreased the particle size of the siderite.
Comparison of raw and calcined siderite cumulative undersize graphs. Grinding operation was done by a laboratory-type ball mill not only to examine the grindability properties of the siderite ore but also to produce fine fractions required for pelletizing.
Figure 9 shows the undersize graphs of raw siderite ore grounded at different times. Cumulative undersize graphs of raw siderite ore that milled at different times. As milling media, four different sizes of balls were charged with diameters of 20, 25, 30 and 40 mm and milling parameters are given in Table 6. The raw siderite was also calcined and then subjected to grinding.
The cumulative undersize curves of calcined siderite ore are given in Figure Cumulative undersize graphs of calcined siderite ore, which were milled at different times. Aforementioned, the cracking occurs in the siderite structure due to CO 2 escaping from its body during the calcination process. A similar situation is also in the grinding process. The calcined siderite was ground much easier than raw siderite when the raw and calcined siderite were grounded at the same conditions 90 min as shown in Figure Cumulative undersize graphs of raw and calcined siderite at equidistant times.
As can be seen from the above graph, the size fraction of both raw siderite and calcined siderite is quite different with each other after the same milling operation. Although there are minor differences in the small size fraction, the raw and the calcined siderite are not noteworthy in scale.
However, it is not possible to mention about fractions of large grain size from the same situation. The value of P 80 for the raw siderite is 4. P 80 value of the calcined siderite is about 43 times smaller than that of the raw siderite. Thus, the grinding effect of the calcination process increases as the size increases. Minimizing the size of raw materials such as ores and rocks also seriously damages the machinery and milling equipments, as well as the enormous energy consumption.
It takes 3. This demonstrates the importance of energy efficiency in size reduction processes [ 10 , 16 ]. In this respect, the production of the fine material required for the production of the pellet should be done after the calcination step. In this way it will be possible to save a great deal of energy and extend the life of grinding machines.
This is only one of the advantages to be achieved as a result of making changes in the pellet production process. Thermogravimetric analysis TGA was used to determine the weight loss of raw siderite exposed to temperature and the result is given in Figure Thermogravimetric analysis TGA of siderite ore.
In the same graph, the effects of size were also investigated. In the calcination experiments, weight loss of the siderite ore was found to be According to above graphs, it was found that the CO 2 in the siderite was completely removed from the body in terms of weight loss calcination loss: It is therefore unnecessary to keep the calcination time longer than 15 min. Also at this point it was found that the material grain size had no effect on the calcination process except when it has a heat capacity that is too high to slow the heat transfer too much.
The large particle size is also advantageous because it easily provides a comfortable circulation of hot air flow. Calcination experiments were also carried out in a microwave oven. Microwave oven application offers great advantages especially for short heating and calcination time [ 17 ]. In microwave heating method, In the microwave heating method, sucrose was used as thermal auxiliary to heat siderite ore. The calcination results, showing the weight loss versus time, of the electrical furnace and the microwave furnace are given in Figure Apart from the partial reduction in the magnetic susceptibility balance, the microwave oven has no disadvantages over the electrical furnace.
On the contrary, higher weight loss was achieved. The weight loss versus time obtained from the electrical furnace and the microwave oven. Calcination procedures which were carried out using the electrical furnace and the microwave oven were achieved. The results showed that necessary weight losses and high iron content in the calcined samples were obtained and the calcination process in electrical furnace and microwave oven caused the hematite transformation which was provided the magnetic properties.
Comparison of the XRD patterns Figure 15 showed that by addition of sucrose the siderite can be calcined by using microwave oven in 3 min. XRD patterns of the raw and the calcined siderite. The mixture conditions are given in Table 8. Figure 16 indicates the raw pellets which were obtained from the raw and calcined siderite.
The pellets obtained from the raw and calcined siderite. Examples of product pellets which were obtained using the electrical chamber furnace are given in Figure After heating process, there was no visual difference observed between the pellets made with the raw siderite and the calcined siderite.
The products pellets. Trigonal crystals were observed in the SEM images, indicating the recrystallized hematite minerals. It can be said that the pellet heating process is performed at the proper temperature. SEM images of the pellets. The data obtained in the test result are presented in Table 9.
The compressive strengths of the obtained pellets are shown on the graph in Figure As it can be seen in the graph pellets obtained from calcined ores have more strength than that obtained from uncalcined ore.
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