Titanium slag is a titanium product obtained by melting and enriching titanium concentrate and reducing agent in an electric furnace. It can be divided into acid-soluble titanium slag and chlorinated slag according to its application. The TiO2 content of acid-soluble titanium slag is 70-80%, and the TiO2 content of chlorinated slag is generally above 85%. With the increasingly mature application technology of titanium slag, the proportion of titanium slag used in the production of sulfuric acid titanium dioxide, chlorination titanium dioxide and sponge titanium is getting higher and higher, and it has become an important basic raw material for the development of titanium industry. 

The research shows that the method of preparing metallized pellets by pre-reducing titanium concentrate on equipment other than electric furnace, and then carrying out electric furnace melting and titanium slag smelting has the advantages of stable furnace condition, short electric furnace smelting time, low smelting power consumption, etc. It is an effective way to improve the production capacity of titanium slag electric furnace and improve the technical and economic level of titanium slag smelting. 

                          Fig 1: XRD of Titanium Concentrate                                        Fig 2: The ΔGθ Reactions for 1& 2                                       Fig 3: Effect of Temperature and Time on Metallization rate
 
1. Principle of experiment 
The composition of ore phase in titanium concentrate is relatively complex. In order to better guide the smelting of titanium slag, XRD analysis of iron concentrate is first carried out, as shown in Fig. 1. As can be seen from the figure, the main phases of titanium concentrate are ilmenite and a small amount of magnetite, olivine and magnesium titanium ore. 
The main reactions occurring during the pre-reduction of titanium concentrate and smelting of titanium slag include: 
(1) FeTiO3+C=TiO2+Fe + CO (g) 
(2) FeTiO3+1. 5C=0. 5Ti2O3+Fe+1. 5CO (g) 

   As shown in Fig 2, when the temperature is higher than 700. D egree. C., both reactions 1 and 2 can proceed spontaneously, and the two reactions reach reaction equilibrium at 1200. D egree. C. Under the same conditions, when the temperature is lower than 1200 ℃, reaction 2 takes place preferentially, and when the temperature is higher than 1200 ℃, reaction 1 takes place preferentially. Considering the temperature conditions of pre-reduction and titanium slag smelting, both can occur, and reaction 1 is the main one. 

2. Experimental section 
2.1 Test Method 
Pellet preparation and drying: titanium concentrate, reducing agent, binder and water are mixed and pressed in proportion to form pellets, anthracite is used as reducing agent, organic binder is selected, the proportion of water added is 2%-5%, and wet pellets are dried at 250 ℃ for 40 minutes to obtain dry pellets. 

Reduction of dried pellets: Vertical resistance furnace is used as heating and reduction equipment, dried pellets are put into graphite crucible, carbothermal reduction is carried out according to set temperature and time, and nitrogen gas is introduced as protective gas during the experiment. After the reduction is completed, the sample is quickly taken out and cooled to room temperature according to different cooling methods. Nitrogen plays a protective role in the cooling process. 

Test and test: Sample and analyze TFe, MFe and other indexes of metallized pellets, and calculate metallization rate according to the content. The formula is: R=MFe/TFex100%, R is metallization rate, MFe is metal iron content in the sample, TFe is total iron content in the sample. 

2.2 Industrial tests 
Titanium concentrate, reducing agent and binder are evenly mixed according to a certain proportion, and enter a high-pressure ball pressing mechanism to prepare carbon pellets with internal distribution. The wet pellets enter a belt dryer for drying, and the heat is supplied by the combustion waste gas of the rotary hearth furnace. The dried pellets are screened and transported to the rotary hearth furnace process to produce metallized pellets. Materials less than 5mm under the sieve are returned to the ball pressing process. 

3. Outcome discussion 
3.1 Influence of Carbon Distribution Coefficient on Metallization Rate 
Under the conditions of reduction temperature of 1360 ℃ and reduction time of 60 minutes, anthracite is used as reducing agent with carbon distribution coefficient of 11%-21%. Thermal reduction experiments of internal carbon distribution pellets are carried out. The influence of carbon distribution coefficient on metallization rate is shown in the figure. When the carbon distribution coefficient is 11%-21%, the metallization rate can reach more than 80%, and the carbon distribution coefficient is 11%-17%. The metallization rate increases with the increase of the carbon distribution coefficient. When the carbon distribution coefficient exceeds 17%, the metallization rate shows a downward trend. The reason may be that too high a carbon ratio is not conducive to the accumulation and growth of metallic iron, and fine iron crystals have high activity and are oxidized when they are released from the furnace. 

3.2 Effect of Reduction Temperature and Time on Metallization Rate 
Anthracite is used as reducing agent with a carbon distribution coefficient of 17%, and carbon distribution pellets are prepared. Carbothermal reduction experiments of carbon distribution pellets are carried out in the temperature range of 1270-1420 ℃. The effects of reduction temperature and reduction time on metallization rate are shown in Fig. 3. 

The metallization rate of carbon-containing pellets increases with the increase of reduction temperature and reduction time. When the reduction temperature is 1360-1390 ℃ and the reduction time is more than 20 minutes, the metallization rate of pellets can reach more than 80%. If the reduction temperature and reduction time are continuously increased, the metallization rate of pellets increases slowly. When the reduction temperature is 1420 ℃ and the reduction time is 60 minutes, the metallization rate can only reach about 90%. According to thermo-calculation, when the temperature is higher than 1080 ℃, TiO2 starts to be reduced to form low-valence titanium oxides. When the reduction temperature is higher than 1390 ℃, low-valence titanium such as Ti3O5 and Ti2O3 are generated in large quantities and form solid solution with unreduced FeO, which makes the reduction reaction of iron oxides more and more difficult and the metallization rate of pellets increases slowly. 

3.3 Influence of Cooling Methods on Metallization Rate
The metallized pellets obtained by carbothermal reduction of the inner carbon pellets have high temperature and porosity, and the inner metal iron has high reactivity. At high temperature, the metal iron will undergo oxidation reaction when encountering oxygen and water vapor, and will be easily reoxidized in the cooling process after being discharged from the furnace, thus reducing the metallization rate. Necessary anti-reoxidation measures need to be taken to ensure the production efficiency and production cost of the whole process. The fixed experimental conditions are reduction temperature 1390 ℃, reduction time 30 minutes, anthracite is selected as reducing agent, and the reduction products after carbothermal reduction reaction are respectively cooled by natural cooling, sealed cooling and forced cooling to test their effects on metallization rate.