Study On Preparation Of Iron Oxide Black Pigment And Magnetite Powder From Pyrite Cinder And Their Fundamental Theories

Pyrite Cinder is the solid waste of sulfuric acid industry. The comprehensive utilization of pyrite cinder is of important significance. This is not only due to the elimination of environmental pollution, but also the recovery of iron-rich resource. Iron oxide black pigment, magnetite powder for the production of toner, cobalt-modified magnetite powder for magnetic recording were prepared from pyrite cinder. Our study was concentrated on the oxidation precipitation of magnetite, control of size and shape, surface chemical properties of magnetite, surface modification and magnetic properties of cobalt- modified magnetite powder.Using sulfuric acid to leach pyrite cinder, pyrite was used to reduce acid-leaching solution for the first time. An aqueous ferrous sulfate solution was then obtained after purifying the reducing solution, FeSO₄ · 7H₂O was crystallized from the ferrous sulfate solution by concentrating and cooling. In the process of leaching pyrite cinder with sulfuric acid, reaction temperature has the greatest influence on the rate of iron recovery. Under the controlled conditions that sulfuric acid concentration is 50%,proportion between reactant liquid and solid 2:1, reaction at 115℃ for 4h,the rate of iron recovery is up to 90%.In the new process of reducing acid-leaching solution with pyrite, 95%Fe³⁺ is reduced to Fe²⁺ under the conditions of Fe³⁺ concentration of 1.1mol/L, proportion between reactant liquid and solid of 3:2, H⁺ concentration of 0.7mol/L,reaction at 90℃ for 3h. Due to the lower cost and being easier to industrialization, pyrite is superior to iron sheet in the reduction of acid-leaching solution.Iron oxide black pigment was prepared by air oxidation of purified aqueous ferrous sulfate. The influences of process parameters on pigment properties were investigated. The properties of as-prepared pigment attain the first level of HG/T2250-91 and surpass sample 722. In the preparation process, adding dispersants such as sodium lauryl sulfate, CMC, PVP, polyethylene glycol or citric acid , high tinting strength iron oxide black pigment was obtained. Surface chemical properties indicate that surface charge of magnetite powder changes significantly, ZPC shifts toward acidic environment, zeta potential and surface charge density increase probably due to electrostatic adsorption and chemical adsorption in the presence of sodium lauryl sulfate,CMC or citric acid, as a result, the diameter of Fe₃O₄ is decreased, size distribution narrowed, dispersibility and tinting strength improved. Inthe solution of containing polyethylene glycol or PVP, the change of zeta potential and surface charge is brought by complex bond and position obstruction repulsion, some effect is shown on the tinting strength and dispersibility of Fe3O4.For the first time, precipitation process of magnetite by air oxidation wasinvestigated in detailed. The results show that oxidation reaction is controlled byoxygen transfer process with apparent activation energy of 3.4kJ ? mor'.The reactionorder is zero for Fe( II) and around 0.5 for partial pressure oxygen. It is suggested thatoxidation reaction be completed in the liquid film around the gas bubble. Three stagesare observed during Fe3C>4 formation with alkali added gradually. In the first stage,Green Rust is formed, oxidation to Fe(III) and the associated hydrolysis to produce aproton are rapid compared to the reaction that consumes the proton generated in thisstage, and the slow step is due to the proton reacting with a solid Fe( II) species toproduce soluble Fe( II) ,as a result ,pH and Fe( II) in supernatant liquid decrease withincreasing time. In the second stage, Green Rust transforms to Fe3C>4 ,pH remainsnearly constant and Fe( II) in supernatant liquid increases with increasing time. In thethird stage, Fe3C>4 redissolves as oxidation continues, pH decreases abruptly, andFe( II ) in supernatant liquid continues increasing to the theoretical Fe( II )concentration at the beginning of the oxidation and then keeps constant during thestage. SEM photographs reveal a change in the particle morphology during theoxidation process. The initial shapeless material develops to form large thinhexagonal crystals that are shown to be Green Rust. As oxidation continues, smallspherical Fe3C>4 particles are formed in addition to, or instead of Green Rusthexagonal crystals. With further oxidation, Green Rust particles disappear and Fe3C>4particle size increases accompanied by the appearance of few needle-like particles.While alkali is added at a time, Fe35x-— 5AG^0 =-29.49KJ- mol^x. With surface modification, magnetite displays similar magnetic properties to unmodified magnetite .For the first time, acicular a -FeOOH was prepared in alkaline solution with ferrous sulfate as raw material obtained from pyrite cinder, followed by dehydroxylation at high temperature to obtain acicular a -Fe2O3. Magnetite for magnetic recording was derived by the reduction of a -Fe2C?3 under the slow streaming hydrogen. A layer of cobalt was then coated on the surface of magnetite by neutral precipitation. The influences of modification conditions on properties of the product were investigated. The results show that pH influences the composition of the product directly, the desirable CoFe2C?4 is obtained as pH value is 12.The coercivity of the particles increases with the increase of cobalt content, cobalt efficiency reaches a maximum value 5.92kA ? m'V%Co at cobalt content of 2.71wt%. The optimum conditions are as follows: pH is controlled at 12, cobalt content 2.71wt% and adding substance at 45 °C. Then let it to be in reaction for 4h. The prepared particles have a saturation magnetization of 83.6 Am2/kg,a coercivity of 51.6 kA/m and squareness ratio of 0.43,which reach Germany Bayer level.Their stability is close to CoFe- Y -Fe2O3. The nature of the coating studied by XRD, TEM and XPS, reveals thatmagnetite with cobalt modification has the similar lattice constant and squareness ratio to that without cobalt modification,the acicular morphology is retained during the coating process.The particles appear to have uniaxial magnetic anisotropy.The epitaxial growth of CoFe2O4 layer takes place at cobalt content of 2.71wt%, Co1.5Fe1.5O4 at cobalt content of 5.03wt%.Magnetic analysis reveals that coercivity enhancement is attributed to magnetocrystalline anisotropy of the cobalt ferrite and uniaxial magnetic anisotropy induced by inner...