Research Progress on Ammonium Molybdate

Our country has abundant molybdenum resources, which is one of the six major advantageous mineral resources in China. It is also a major producer and trader of aluminum resources, holding an important position internationally. Therefore, it is crucial to pay high attention to the safety of molybdenum resources in our country and to develop and utilize molybdenum products rationally. About 30% of production manufacturers still use traditional processes, mostly small factories, while approximately 50% use improved traditional processes, and about 20% adopt oxygen pressure cooking and alkaline pressure cooking methods. In contrast, foreign countries generally use advanced processes such as pressure alkaline leaching, extraction, or ion exchange, with most of the ammonia and acid gases generated during production not being recovered and reused, leading to environmental pollution. However, foreign countries generally recover and treat both ammonia and acid gases for reuse. Currently, the ammonium molybdate sold in the market mainly consists of ammonium heptamolybdate (AHM), ammonium dimolybdate (ADM), and β-type ammonium tetrathiomolybdate, especially with ammonium heptamolybdate being favored for its good water solubility, while ammonium dimolybdate has a single composition, uniform particle size, and good fluidity.

1. Production of Ammonium Molybdate

The common process for producing ammonium molybdate involves roasting molybdenum concentrate, nitric acid leaching, ammonia leaching, purification, and acid precipitation or evaporation crystallization. Experiments were conducted using molybdenum concentrate as raw material to produce ammonium molybdate, and the optimal process conditions were found to be: roasting time of 2.4 hours at a temperature of 636°C; leaching time of 3.69 hours at a temperature of 91.1°C; purification with ammonium sulfide added at 1.05 times the theoretical amount; precipitation at pH=7.5, adding ammonium chloride and reacting for 30 minutes before adding sulfuric acid, with the endpoint pH at 0.5; ammonia solution at a temperature of 80-90°C, with the amount of ammonium tetrathiomolybdate reaching saturation. Research was conducted on preparing ammonium molybdate from amorphous molybdenum ore, resulting in ammonium molybdate products with a molybdenum content close to 60% through crushing, grinding, oxidative roasting, sodium carbonate solution leaching, ammonium chloride precipitation, filtration, washing, and drying.

1.1 Ammonium Heptamolybdate (AHM)

Ammonium heptamolybdate, also known as ammonium paramolybdate, is formed by dissolving tetrathiomolybdate in ammonia water to create an ammonium molybdate solution. After the solution becomes clear and transparent, it is filtered, and through ammonia solution recrystallization, the purpose of purification and impurity removal is achieved. The separated crystals can be obtained by drying or low-temperature drying. The main factors affecting the formation of ammonium paramolybdate crystal type and purity include solution density, pH value, and temperature. If the solution density is too low, ammonium heptamolybdate crystals cannot form; if the density is too high, filtration becomes difficult. If the pH value and temperature are too low, the solution does not reach the appropriate density, which in turn affects product purity and clarity. Meanwhile, the optimal endpoint conditions for ammonia solution are: density of 1.40-1.45g/cm³, pH value of 6.0-7.0, and temperature of 75-85°C.

The quality of ammonium molybdate is related to the performance of the final molybdenum production products. Experimental research on the quality of ammonium heptamolybdate was conducted by controlling the stirring speed of ammonium heptamolybdate crystallization and drying process conditions, transforming tetrahydrate ammonium heptamolybdate into monohydrate ammonium heptamolybdate, thus improving the quality of ammonium heptamolybdate. Considering the improvement of the drying method, after microwave drying for 90 seconds, the dehydration rate of ammonium paramolybdate reached 99.98%, and experiments concluded that the main factor for microwave drying is the drying time.

1.2 β-type Ammonium Tetrathiomolybdate (β-AQM)

Ammonium tetrathiomolybdate exists in several crystal forms: hydrated, anhydrous α-type, anhydrous β-type, and micro-powder type. The crystals of ammonium tetrathiomolybdate produced by the acid precipitation method are mostly in the form of hydrated ammonium tetrathiomolybdate before drying. It is a metastable crystal that can lose crystallization water at any time during the crystallization, filtration, washing, and drying processes, transforming into α-type and β-type ammonium tetrathiomolybdate. The α-type ammonium tetrathiomolybdate has poor processing performance, and the molybdenum powder produced is prone to clumping, making it difficult to screen. The β-type ammonium tetrathiomolybdate produces molybdenum powder in a lotus root slice shape, which is easy to screen and has superior processing performance.

1.3 Ammonium Dimolybdate (ADM)

The production of ammonium dimolybdate is similar to that of ammonium paramolybdate, both being produced through the evaporation crystallization of ammonium molybdate solution, but the control conditions differ. Generally, the production of ammonium dimolybdate requires a higher concentration of ammonium molybdate, with an ammonia to molybdenum ratio (NH₃:Mo) of 1:1 or higher. The main influencing factors for ammonium dimolybdate production include the concentration of ammonium molybdate solution, pH value, stirring speed, evaporation temperature, and time. Under conditions of a molybdenum solution density of 1.3-1.35g/cm³, a pH value of 8-9, lower evaporation temperature, and longer evaporation time, pure ammonium dimolybdate can be obtained, with the crystal particles being large and uniform.

To produce ammonium dimolybdate with regular shape, uniform particle size, and high purity, the first step is to add ammonium tetrathiomolybdate and ammonia water into pure water at 70°C, resulting in an ammonium molybdate solution containing MoO₃ at 400g/L and a pH value of 5.5-6.5. The solution is then heated to 80-90°C, filtered, and the filtrate is placed in a crystallization tank for stirring. When a small amount of ammonium dimolybdate crystals appear in the solution, cooling is initiated until room temperature is reached, allowing ammonium dimolybdate crystals to precipitate.

In response to the current issues of unreasonable structure in traditional domestic equipment, which can lead to inconsistent product crystal types, modifying traditional equipment has a significant impact on improving the quality of ammonium dimolybdate products. Factors affecting the particle size distribution of ADM include solution concentration, evaporation temperature, cooling rate, impurities, solid materials formed on the tank walls during evaporation concentration, and stirring speed during crystallization. Therefore, modifying the structure of the evaporation tank can adjust and control product particle size, ensuring consistent product shape and uniform particle size distribution.

To obtain regular large particle size ammonium dimolybdate, research has been conducted on the production practices of regular large crystal ammonium dimolybdate, suggesting that adding varieties and controlling evaporation rates can yield regular large particle size ammonium dimolybdate.

2. Treatment of Ammonium Molybdate Waste Liquid

In the production of ammonium molybdate, some waste liquid that needs to be treated will inevitably be generated. Wastewater containing sulfuric acid can be neutralized with lime to meet discharge standards; extraction methods can recover aluminum from the acid mother liquor, and then ammonia water can be used to neutralize the extract, which can be concentrated and crystallized to produce ammonium chloride as fertilizer. Currently, the main methods for treating acidic wastewater from aluminum ammonium sulfate to recover aluminum include neutralization hydrolysis, sulfide precipitation, and activated carbon adsorption, which can effectively meet wastewater discharge standards, but there are issues such as high reagent consumption and the potential for secondary environmental pollution from the waste residue. The treatment of wastewater from ammonium molybdate production should not only consider end-of-pipe treatment to meet discharge standards but should also adopt a comprehensive treatment plan to recover and utilize the molybdenum elements and ammonium salts. Some have used ion exchange methods for comprehensive treatment of wastewater from ammonium molybdate production. The results show that the recovery rate of molybdenum in the wastewater reaches 86% to 92%, with ammonium nitrate purity exceeding 99%, and the removal rate of other heavy metals is above 98%, demonstrating good environmental and economic benefits. Considering that a single ion exchange method cannot effectively achieve the recovery of molybdenum resources, it is believed that an ideal process flow for treating the waste acid from ammonium molybdate production should include filtration, ion exchange, neutralization, and precipitation.

3. Outlook

Compared to foreign countries, the quality of domestic ammonium molybdate products is unstable, with excessive impurity content and incomplete product varieties and specifications. Currently, domestic manufacturers producing ADM and AHM all use traditional batch production methods and employ evaporation crystallization techniques. During the evaporation crystallization process, due to the unreasonable structure of traditional evaporation crystallization equipment, and the tendency of traditional powdered ammonium molybdate to agglomerate due to high nucleation supersaturation or excessive nucleation rate, the product particle size distribution is too wide and uneven, resulting in significant differences in product quality compared to similar foreign products. Therefore, it is necessary to upgrade traditional equipment. Improving the quality of ADM and AHM products is currently the primary task for domestic manufacturers of ADM and AHM. At the same time, it is recommended that the government increase support for enterprises to promote technological advancement and environmental protection, and that enterprises actively engage in industry-university-research collaboration to develop high-tech products.