The traditional method of adding oxidant

(1) The function of crushing open-pit iron ore before alkali leaching is to increase the contact area and speed up the reaction;

So the answer is: increase the contact area and improve the reaction rate;

(2) Due to the hydrolysis of sodium silicate and sodium metaaluminate in the solution, SiO2-+H2O? HSiO3-+OH-、HSiO3-+H2O？ H2SiO3+OH-、AlO2-+2H2O？ Al(OH)3+OH-, lowering pH is beneficial to the shift of hydrolysis equilibrium to the positive reaction direction. When pH is adjusted to 7 ~ 8, they can be completely hydrolyzed to form precipitate.

So the answer is: H2SiO3, Al (OH) 3;

(3) In acidic environment, CrO42- is transformed into Cr2O72-, that is, 2CrO42-+2H+? cr2o 72-+H2O；

So the answer is: 2CrO42-+2H+? cr2o 72-+H2O；

(4) The reason why KCl is added in operation ⑤ is that the temperature has little effect on the solubility of sodium chloride, but it has great effect on the solubility of potassium dichromate, which can be obtained by double decomposition reaction;

So the answer is: temperature has little effect on the solubility of sodium chloride, but it has great influence on the solubility of potassium dichromate, and potassium dichromate can be obtained through double decomposition reaction;

(5) (1) According to the reaction: I2+2S2O3-= 2i-+S4O62-, when the last drop of sodium thiosulfate solution is added dropwise, the iodine reaction is complete and the blue color of the solution fades, reaching the titration end point.

So the answer is: when the last drop of sodium thiosulfate solution is added, the blue color of the solution fades and does not change color for half a minute;

② the reaction formula is cr2o72-+6i-+14h+= 2cr3++3i2+7h2o; I2+2S2O32-=2I-+S4O62- The relation of available reactions is Cr2O3 2-~ 3I2 ~ 6S2O32-, calculated according to the relation.

Cr2O72-~3I2~6S2O32-

1mol？ 3mol？ 6 mol

n？ 0. 1200×40× 10-3 mol

Then the amount of substances containing potassium dichromate in 250ml is n = 0.1200× 40×13 mol 6×10, and the purity of potassium dichromate in the obtained product is 01200× 40×10.

So the answer is: 94.08%.