A method of eliminating hematite structure and thermodynamic stability of a principle of natural hematite iron hematite (Fe 2 0 3) 70% iron, sometimes containing titanium and magnesium, form isomorphism, in aphanitic A mechanical mixture of Si0 2 and Al 2 0 3 is often present in the composition. The trigonal system has a lattice constant of a = 0.504 nm and c = 1.377 nm. The structure is corundum type. Hematite crystals are often rhombohedral and plate-like. Since the polycrystalline twin crystals are formed by the rhombohedron and the triangular stripes are seen on the bottom axis, the aggregates are often scaly, braided, kidney-like blocks having a radial structure, and red powder. The crystallized hematite is iron black to steel gray, cryptocrystalline or earthy, red powder. b High temperature hydrolysis of trivalent iron Hematite Fe 3+ is a product of high temperature hydrolysis. In order to clarify the conditions for the formation of hematite, the predecessors conducted extensive research. The left figure shows the relationship between the logarithmic value of ferric ions and the pH value at 25 to 200 ° C. It is obvious that the higher the temperature, the better the iron is precipitated under higher acidity conditions. The right figure below shows that at a high temperature of 200 ° C, even if the sulfuric acid concentration is as high as 100 g / dm 3 , the residual iron in the solution can be reduced to 5 ~ 6g / dm 3 . Hydrometallurgical process solutions often contain more metal sulfates. In order to remove iron from the leachate by hematite, the effects of other sulfates should be understood. When the temperature is higher than 150'C, the hydrolysis of Fe 3+ sulfate has different reaction history with the change of acidity. The process of the Iijima Smelter is as shown in the above figure, which is processed in 4 steps:
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Hematite has two crystalline forms, namely, γ- Fe 2 0 3 and α- Fe 2 0 3 . Natural hematite belongs structurally α -Fe 2 0 3, which is non-magnetic, and then the γ- Fe 2 0 3 with strong ferromagnetism. The conversion temperature of γ -type hematite is approximately 400 ° C. When heated to 400 ° C, it will change to α -type, and the magnetic properties disappear.
When iron hydroxide is precipitated from a low-temperature aqueous solution by heating, goethite (Fe 2 0 3 · H 2 0) is obtained first, followed by hydrohematite (Fe 2 0 3 · 0.5H 2 0). The gamma -type hematite is the third product of the heating process. The transition temperature of goethite and γ- Fe 2 0 3 is 160 °C. If high temperature hydrolysis is used, hematite with good filtration performance can be obtained. The isotherm of Fe 2 0 3 -S0 3 -H 2 0 at a high temperature of 200 ° C is shown in the following figure. At 200 ° C, even if the acidity of the solution is high, Fe 2 O 3 can precipitate mostly. 
[next]
When low acidity:
Fe 2 (S0 4 ) 3 +3H 2 0 ==== Fe 2 0 3 +3H 2 S0 4
When high acidity:
Fe 2 (S0 4 ) 3 +2H 2 0 ==== 2FeOHSO 4 +H 2 S0 4
The acidity required for the precipitation of trivalent iron oxide increases with the addition of copper , zinc and magnesium sulfate. This is due to the fact that sulfate acts as a buffer for the system sulfuric acid, ie SO 4 2- can combine with H + in the solution, reducing the activity of the sulfuric acid. [next]
Zinc sulfate is the most important coexisting sulfate salt when Fe 3+ is precipitated. The presence of zinc sulfate, so that precipitation increased acidity, such as when the presence of 100g / dm 3 at 200 is zinc deg.] C, the upper limit of pure iron sulfate acidity 63.7g / dm 3 moves to 106g / dm 3. The effects of magnesium and copper salts and zinc salts are the same.
B. Application of hematite in zinc hydrometallurgy The hematite method, like the yellow iron sputum method and the goethite method, is one of the treatment methods for zinc smelting hot acid leaching residue. At present, it is reported that the factories that use the hematite method to treat zinc slag include: Iijima Plant of Akita Zinc Corporation of Japan and Dartrun Plant of Ruhr Zinc Company of Germany.
Japan's Akita Smelting Company Iijima Plant was established in 1971. The first phase of the project was put into operation in January 1972. In 1985, it was expanded to an annual output of 156kt. The comprehensive utilization of valuable metals for recycling is of great significance to Japan, a country with a relatively scarce resource. The biggest purpose of the leaching residue treatment is to increase the actual yield of zinc. However, for Iijima, due to the high content of gold, silver and copper in Japan's domestic “black minesâ€, it is necessary to efficiently recycle these metals and comprehensively utilize rare metals such as crops and bismuth, as well as iron and zinc in zinc concentrates. Lead . The hematite method has the characteristics of high recovery rate of valuable metals, and even the hematite slag produced may be directly used as ironmaking raw materials, and the waste can be reduced to a minimum. [next]
(1) leaching with waste electrolyte and sulfur dioxide;
(2) separating copper and other impurities by precipitation with hydrogen sulfide;
(3) neutralizing free sulfuric acid with calcium carbonate and removing impurities such as crops, strontium , and arsenic ;
(4) Oxidation of ferrous ions with oxygen to form Fe 2 O 3 .
The zinc contained in the zinc slag is almost all in the form of zinc ferrite. It is slurried with waste electrolyte and leached with SO 2 in an autoclave (temperature 100-110 ° C, pressure 2 MPa), and stayed in the kettle for 3 h, Fe, The leaching rate of Zn, Cd, and Cu is 90% or more. The total leaching rate of Fe and Zn is higher than 95%. The slurry from the autoclave is removed from the SO 2 solution in the desorption column and sent to the copper removal tank. The amount of H 2 S can be calculated from the oxidation-reduction potential. After decoppering, the liquid contains about 20 g/dm 3 of free acid and is neutralized with two sections of calcium carbonate. The first stage is neutralized to pH=2, dehydrated by a centrifuge to produce gypsum ; the second stage is neutralized to pH=4.5, and harmful impurities such as Ga, Ge, In , As, Sb are precipitated. In order to promote the precipitation of Ga, As, air is blown in, and part of Fe 2+ is oxidized to Fe 3+ to precipitate. The neutralized liquid contains Zn 2+ , Fe 2+ , Cd 2+ , etc. in an autoclave to remove iron, steam is heated to 200 ° C, and then pure oxygen is bubbled to maintain a pressure of 2 MPa. Under this condition, Fe 2+ is oxidized. Formed into hematite. In addition to iron, three series autoclaves were used. The solution into the No. 1 kettle contains 40-50 g/dm 3 of iron, and the solution from the final section contains only 1 g/dm 3 of iron. Due to the remelting phenomenon during the rinsing and thickening process, the countercurrent flowing out of the thickener finally contains 3 to 4 g/dm 3 of iron, the precipitation rate of iron is higher than 90%, and the sulfur content of Fe 2 0 3 is 3%, including Fe 58% -60%, containing H 2 0 12%, is a relatively easy to handle iron raw material. When hematite is precipitated, there is almost no precipitation of zinc and tin in the solution, and the solution is sent to the main process to recover zinc and tin.
The process of treating zinc dross by the Iijima Hematite Process is still being improved. The copper-plating process has changed from the use of hydrogen sulfide to the addition of sulfur powder in the leaching autoclave, thereby reducing the consumption of hydrogen sulfide, and the copper sulfide grains produced are enlarged, so that the sedimentation, filtration and flotation performance are both Improved.
As early as 1972, the West Germany Dallon Electric Zinc Plant faced the problem of leaching slag. Because Luer Zinc Company did not accumulate leaching slag in the Dartrun area, it would cause harm to the environment, so it is hoped that after treatment The leaching slag can be sold in the market. Taking into account this special condition, the plant cannot use the process of reducing and leaching of the Iijima SO 2 , but by adding zinc concentrate for reduction leaching and then pressurizing. Oxygen was blown at 200 ° C and a pressure of 2 MPa, and the iron was oxidized to Fe 3+ and precipitated as hematite. The precipitate contains more than 64% iron. After sufficient washing, iron can be sold as a by-product.
The Canadian Electric Zinc Company has developed an improved hematite treatment zinc leaching process. The process is characterized by leaching and precipitation in one stage. The key technology is to treat the iron slag at 200 ° C and 1 MPa high pressure to dissolve the zinc. Iron is precipitated in the form of hematite, so it is called high temperature conversion method, also called the combination of jarosite method and hematite method.