Preparation of molybdenum trioxide by pyrolysis of ammonium molybdate

May 05, 2022

Ammonium paramolybdate is a series of industrial isopoly acid ammonium salt molybdenum mixture, which mainly include: ammonium molybdate, ammonium molybdate and ammonium paramolybdate four.

The following table lists several common ammonium molybdate salts.

Table common ammonium molybdate

name	Molecular formula	Dehydration temperature (°C)	Conversion temperature (°C)	Conversion product
Ammonium paramolybdate	(NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O	90° off a crystal water	230	Ammonium tetramolybdate
Ammonium tetramolybdate	(NH ₄ ) ₂ MoO ₁₃	130° off the rest of the crystal water	315	Molybdenum oxide
Ammonium molybdate	(NH ₄ ) ₂ MoO ₄ ·2H ₂ O	120		Molybdenum oxide

The thermal dissociation reaction of ammonium paramolybdate and its conditions are as follows:

(NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O	90~130°C	(NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O+4H ₂ O↑
	→

(NH ₄ ) ₆ Mo ₇ O ₂₄	150~250°C	(NH ₄ ) ₂ Mo ₄ O ₁₃ +NH ₃ ↑+2H ₂ O↑
	→

(NH ₄ ) ₂ Mo ₄ O ₁₃	280~380°C	4MoO ₃ +2NH ₃ ↑+H ₂ O↑
	→

In industrial production, this series of reactions is carried out in the same rotary furnace. The furnace temperature is maintained at 450~500 °C. The furnace temperature is low, and the thermal dissociation of ammonium paramolybdate is not complete; the furnace temperature is high, and the vapor pressure of molybdenum trioxide after dissociation rises, which is lost due to sublimation. The heating of the rotary kiln is usually achieved by a resistance wire wound around the furnace.

The molybdenum trioxide produced by the thermal dissociation of ammonium paramolybdate is extremely pale yellowish green, which can basically meet the requirements of high purity molybdenum trioxide. This process requires high quality of the raw material, ammonium paramolybdate, and the impurities in the raw material often enter the calcined molybdenum sand - high purity molybdenum trioxide product. Therefore, when the raw material contains high impurities, it must be purified by impurity removal until it reaches the requirement, and then enters the thermal dissociation process.

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