1. Influencing factors of reduction roasting process 

   Fig1: Effect of roasting temperature on the DR roasting.                   Fig2: Effect of reduction time on the DR roasting.                      Fig3: Effect of external coal dosage on the DR roasting.

1.1 Effect of Roasting Temperature
As shown in the figure, the effect of roasting temperature on the effect is shown. The experimental conditions are as follows: 10% coal blending inside, 20% coal blending outside, roasting time 40 minutes. As can be seen from the figure, with the continuous increase of roasting temperature, the iron grade of reduced iron increases obviously, reaching 91.11% at 1200 ℃. The recovery rate of iron is greatly increased at 1000-1050 ℃. Then it increases slightly with the increase of temperature, but decreases at 1200 ℃. 

From the naturally cooled burnt balls after reduction roasting at different temperatures, it is found that the hardness of the burnt briquetts increases with the increase of temperature, and the higher the temperature, the more obvious the deformation of the burnt balls. The hardness of the roasted ball at 1200 ℃ is greater than that of the roasted ball at 1000 ℃, and the shrinkage deformation after firing is more obvious. It may be that the higher the temperature, the more conducive to the diffusion and condensation of metal iron generated in the roasted ball, so the roasted ball shows higher hardness and obvious shrinkage. 

The reduction roasting temperature is a very important influencing factor in the reduction reaction process. Increasing the temperature can promote the reduction of iron oxides, the gasification of carbon and the coagulation of iron particles generated in the reduction reaction. The higher the temperature, the better the reduction atmosphere, which can promote the iron particles generated in the reduction to gather to the center and gangue to gather to the shell. Therefore, the iron grade in this experiment gradually increases with the increase of temperature. However, if the temperature is too high, a large amount of Si in the raw ore reacts with FeO generated by reduction to generate ferroolivine with low melting point. At high temperature, ferroolivine will form a large amount of liquid phase on the surface of the reduced ore, thus hindering the reduction from diffusing to the inside. 
 
1.2 Effect of Roasting Time 
The roasting temperature is 1200 ℃, other conditions remain unchanged, and the experimental results of roasting time are shown in the figure. We can see that with the extension of roasting time, the iron grade and iron recovery rate of direct reduction iron increase, reaching 95.9% and 78.78% respectively at 80min.

 
The reduction of iron oxide in iron ore is carried out step by step, mainly experiencing the reduction process from high-priced iron to low-priced iron. Prolonging the reduction reaction time is conducive to the conversion of siderite to metallic iron, and promotes the continuous merger and growth of the generated iron grains. At the same time, it is also conducive to the re-reduction of ferroolivine generated by ferrous oxide and gangue, so the iron grade and recovery rate increase accordingly. Therefore, the reduction roasting time is finally determined to be 80min. 
 
1.3 Influence of Coal Dosage
The temperature is controlled at 120 ℃, the roasting time is 80min, and other conditions remain unchanged. The experimental results of the amount of external coal blending are shown in Fig. 3. We can see that with the increase of the amount of external coal blending, the iron grade in reduced iron gradually decreases and the recovery rate significantly increases. 

In the direct reduction process of iron ore, the external coal blending covered outside the roasting ball mainly plays a role in maintaining the original reduction atmosphere, reducing the escape of CO generated in the carbon gasification process, and preventing the metal iron generated by reduction from being oxidized. When the amount of external coal blending is low, less coal is covered around the roasting ball, which cannot well maintain the reduction atmosphere, thus resulting in less metal iron generated by reduction, thus resulting in lower iron recovery rate obtained by subsequent magnetic separation. However, excessive pulverized coal will affect the subsequent grinding and magnetic separation process, so the iron grade will decrease with the increase of coal consumption. Therefore, after comprehensive consideration, the dosage of external coal blending is selected to be 20%. 

The optimum roasting conditions determined by the above experiments are as follows: the dosage of internal coal blending is 10%, the dosage of external coal blending is 20%, the roasting temperature is 1200 ℃, and the roasting time is 80min.