Ammonium salts are ionic compounds composed of ammonium ions and acid radical ions, generally obtained through the reaction of ammonia and acid. They are generally colorless crystals and are easily soluble in water. Ammonium and sodium ions are isoelectronics, and the radius of the ammonium ion (143 pm) is similar to that of the potassium ion (133 pm) and rubidium ion (147 pm). Therefore, the properties of ammonium salts are also similar to alkali metal salts, and are often isomorphic with potassium salts and rubidium salts, and have similar colors, solubility, and crystal forms.
Manganese is an essential metal resource for the development of human society. Due to its special versatility, it has been widely used in the fields of industry and metallurgy. It is the most used element in steel except iron. The content of manganese in limonite is usually above 35%, but it's processing and utilization methods have hardly been reported so far. To meet the global demand for manganese resources, an efficient and environmentally friendly technology should be developed to extract manganese from a wider variety of manganese oxide minerals.
In the metallurgical processing of complex minerals, sulfate roasting is widely used as a common pyrometallurgical technology. The principle of sulfate roasting is to convert the valuable metals in complex minerals into sulfate or ammonium sulfate, and then realize the effective extraction of metals by water leaching. Sulfuric acid roasting often uses sulfuric acid as an additive, and the mass fraction of sulfuric acid is usually required to be above 80%, which will cause serious corrosion to the equipment. The principle of ammonium sulfate roasting is the same as that of sulfuric acid roasting, but it is less corrosive to equipment, and the ammonia gas volatilized during the roasting process can regenerate ammonium sulfate. In recent years, studies on the extraction of valuable metals from various minerals or solid wastes using ammonium sulfate roasting technology have been widely reported. In conclusion, ammonium sulfate roasting technology is an efficient and environmentally friendly method, and is considered to be a promising process.
Recently, some researchers used two-stage ammonium sulfate roasting method to selectively extract manganese from brown manganese ore. The detailed process conditions were studied, the roasting mechanism was discussed, and the ammonia gas released during the roasting process was used to precipitate manganese from the leaching solution. In addition, the researchers also conducted a preliminary evaluation of this process. The process is short, simple and easy to operate, and realizes efficient separation and recovery of manganese in brown manganese ore without adding any strong corrosive reagents. This study is of great significance for realizing the efficient utilization of various manganese oxide minerals.
As the calcination temperature increased from 350°C to 400°C, the extraction rate of Mn increased significantly from 49.23% to 93.28%, and the extraction rates of Fe and Al also increased significantly. At the calcination temperature of 450℃, the extraction rates of Mn, Fe and Al reached 96.87%, 93.98% and 90.04%, respectively. However, when the temperature was higher than 450℃, the extraction rates of the three metals all decreased, and the extraction rate of Fe decreased most obviously. The decline of Mn extraction rate may be due to the fact that the calcination temperature far exceeds the complete decomposition temperature of ammonium sulfate (about 427 °C), resulting in the rapid decomposition of part of ammonium sulfate into various gases and volatilization, without participating in the sulfation reaction. The decreased extraction yields of Fe and Al are due to the decomposition of some of their sulfate or ammonium sulfate salts into their respective oxides. With the increase of the second-stage calcination temperature, the extraction efficiency of Mn changed little, but the extraction rate of Fe and Al decreased gradually. As the temperature increased from 550°C to 650°C, the extraction rates of Fe and Al decreased from 23.49% and 47.35% to 1.36% and 4.73%, respectively.
The results of phase analysis show that the roasting temperature in the first stage is 450°C, and the addition of ammonium sulfate roasts the (Mn2O3)2MnSiO3, MnO2, Fe2O3, Ca3SiO5 in the brown manganese into (NH4)2Mn2(SO4)3, (NH4)3Fe (SO4)3, NH4Fe(SO4)2, Fe2(SO4)3, CaSO4, SiO2, release NH3, H2O and SO3 at the same time. After the first stage of calcination, without adding any reagent, the calcination temperature was increased to 650° C. for the second stage of calcination. At this time (NH4)2Mn2(SO4)3 is decomposed into MnSO4, (NH4)3Fe(SO4)3, NH4Fe(SO4)2 and Fe2(SO4)3 are decomposed into Fe2O3, and NH3 and SO3 are released at the same time. Due to the difference in water solubility between MnSO4 and Fe2O3, the selective leaching of manganese can be achieved by water immersion, and the amount of ammonium sulfate is less than that of direct roasting at 650 °C. Through elemental mapping and energy spectrum analysis, it is shown that the strip-like and long-sheet-like structures are CaSO4·2H2O and CaSO4, the irregular block-like structure is SiO2, and the small particles adhering to each other are Fe2O3. EDS analysis shows that the Mn content is only 0.4%, while the Fe content is significantly higher than that of the ore, indicating that the selective extraction of Mn has been achieved.
The effect of solution pH on Mn precipitation rate was studied under the conditions of precipitation time 60 min and precipitation temperature 70℃. The results showed that when the pH increased from 8.5 to 10.0, the precipitation rate of Mn increased from 90.05% to 99.48%. It is finally determined that the complete precipitation of Mn can be basically achieved under the conditions of solution pH of 10.0, precipitation temperature of 70°C, and precipitation time of 60 min. The precipitate is mainly composed of Mn3O4 and MnOOH, but the degree of crystallization is very poor, and the diffraction peaks are disordered. The sample is brown, which is close to the color of Mn3O4. This is because divalent manganese is easily oxidized, and it contacts oxygen to form Mn3O4 during the drying process. Afterwards, by adjusting the calcination temperature and time, manganese oxide products such as Mn3O4 or Mn2O3 can be obtained, which can be used as raw materials for the production of lithium-ion batteries. The final filtrate is neutralized with sulfuric acid to reduce its pH value to about 5.5, and then evaporated and crystallized to obtain ammonium sulfate product, which can be directly used in the ammonium sulfate roasting process, thereby realizing the recovery of ammonia and reducing the cost of the entire process.
The reducing agent used in traditional manganese oxide ore reduction roasting is usually coal or coke, and the reduction temperature is required to be about 900°C. Although it has been proposed in recent years that microwave heating reduction can effectively reduce the reduction temperature, the selective extraction of Mn cannot be achieved, which brings troubles to the subsequent recovery of manganese from the leachate. This process achieves the selective extraction of Mn at a calcination temperature of 650 °C, which is 250 °C lower than the traditional reduction roasting-acid leaching process. In general, compared with the traditional reduction roasting-acid leaching process, this method has the advantages of good selectivity, high extraction rate and low environmental impact. In addition, the volatilized NH3 and SO3 can be absorbed, then evaporated and crystallized to regenerate ammonium sulfate, which is used in the roasting process, thereby reducing the production cost. Due to the selective extraction, the manganese in the leaching solution can be directly converted into Mn(OH)2 precipitates, which can then be calcined to obtain various manganese oxides.
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Reference
- High-efficiency and environment-friendly separation and recovery of manganese from braunite via the ammonium sulfate roasting-water leaching process: Behavior and mechanism.
Chemical Engineering Journal, 466, 143218.
