The Self-Reduction during the Thermal Decomposition of an Ammonium Molybdate

Abstract

In the hydrometallurgical process of molybdenum using ammonia solution, ammonium paramolybdate tetrahydrate (APT: (NH₄)₆Mo₇O₂₄·4H₂O) is produced as an intermediate product after a crystallization step. ATP is then thermally decomposed at a high temperature to give MoO₃, which is reduced by hydrogen gas in a two-stage process to reduce molybdenum metal powder as the final product. If APT is pre-dried at an appropriately low temperature to remove the crystal water corresponding to 4 mol per mol of APT, it changes into (NH₄)₄Mo₅O₁₇, and the content of residual ammonia, which can be utilized as a reductant, in the ammonium molybdate increases. In this regard, the self-reducing potential of (NH₄)₄Mo₅O₁₇ was examined in this study through the effectiveness analysis of the residual ammonia component as a reductant for the primary hydrogen reduction step. In a series of experimental work on the thermal decomposition of (NH₄)₄Mo₅O₁₇ in an inert atmosphere, a maximum self-reduction degree of 18% was achieved. Based on this result, it can be expected that the metal powder can be manufactured in a more effective way than conventional processes in terms of hydrogen consumption and reaction time.

1. Introduction

Molybdenum is regarded as one of the key metals in Republic of Korea due to its high melting point, high strength at high temperatures, high thermal conductivity, and adequate resistance to corrosion [1,2]. Although there is an operating molybdenum mine in Republic of Korea, the self-sufficiency ratio of molybdenum in Republic of Korea remains under 2% [1]. Most molybdenum resources such as sulfides are imported, and therefore, there have been attempts to improve the molybdenum production technologies and processes in Republic of Korea [3].

In the production of molybdenum metal powder of a high purity, ammonium paramolybdate tetrahydrate (APT: (NH₄)₆Mo₇O₂₄·4H₂O) is the first intermediate, and then it is roasted and converted into molybdenum trioxide, MoO₃, which is the second intermediate. When the ammonium molybdate is heated, it decomposes in a stepwise procedure and gives off NH₃ and H₂O, leaving MoO₃ as the final product. The stepwise decomposition of ammonium molybdate is reported in the literature as follows: (NH₄)₆Mo₇O₂₄·4H₂O → (NH₄)₄Mo₅O₁₇ → (NH₄)₂Mo₄O₁₃ → (NH₄)₂Mo₁₄O₄₃ → (NH₄)₂Mo₂₂O₆₇ → MoO₃ [4,5,6].

According to the analytical results of TGA, DTA, and XRD from the literature, the stepwise decomposition was observed in the range of 110~380 °C, and finished around 400 °C. Although the steps of the thermal decomposition might be changed by heating conditions, they are not as affected by the oxygen potential, especially in the early part, as shown in Figure 1. If APT is pre-dried at an appropriately low temperature to remove the crystal water corresponding to 4 mol per mol of APT, it changes into (NH₄)₄Mo₅O₁₇, and the content of residual ammonia, which can be utilized as a reductant, in the ammonium molybdate increases. Although the self-reduction by the residual ammonia was reported briefly in the U.S. Patent 2,385,843 [7], no satisfactory investigation on the self-reduction of (NH₄)₄Mo₅O₁₇ has been reported yet. Therefore, the self-reducing potential of (NH₄)₄Mo₅O₁₇ was extensively examined in this study through the analyses of the effectiveness of this residual ammonia component in terms of decreasing hydrogen consumption and reaction time.

2. Materials and Methods

High-purity APT and MoO₃ powders of reagent grade (99.9 wt%) were used in this study. The APT was dried using an ON-11E natural convection oven (Jeio Tech. Co. Ltd., Daejeon, Republic of Korea) at a temperature range of 70 to 100 °C. Then, 10 g of the sample was loaded in an alumina boat and placed in the middle of a horizontal tube furnace (ID 60 × L 1000). Ar and H₂ gases were used in the self-reduction stage and in the primary reduction stage, respectively. The Ar gas flow rate was fixed at 0.3 L/min, and H₂ gases at 0.1 or 0.2 L/min. The temperature was fixed at 575 °C because it was the lowest temperature in this study to achieve 100% conversion of MoO₃ → MoO₂, and the residence time range was from 30 to 60 min. The conversion rate was calculated by the weight loss, and the phase change and the morphology were characterized by X-ray diffraction (XRD, D/Max 2500, Rigaku, Tokyo, Japan) and by scanning electron (SEM, MIRA-3, Tescan Co., Brno, Czech Republic) at the Eco-friendly Shipbuilding Core Research Support Center, respectively.

3. Results and Discussion

3.1. Thermal Decomposition of Ammonium Molybdates

3.1.1. Drying of APT

As APT, (NH₄)₆Mo₇O₂₄·4H₂O, is thermally decomposed in a stepwise manner, the weight and the composition of ammonium molybdate change by the equivalent amount of gaseous loss, as can be seen in

Table 1. Weight and composition changes in ammonium molybdates in stepwise thermal decomposition.

Compound	H2O/NH3	NH3/O	O/Mo	NH3/Mo	H2/O	Weight Loss (%)
(NH4)6Mo7O24·4H2O	0.67	0.25	3.43	0.86	0.50	0.00
(NH4)4Mo5O17	0.00	0.24	3.40	0.80	0.47	6.67
(NH4)2Mo4O13	0.00	0.15	3.25	0.50	0.31	11.09
(NH4)2Mo14O43	0.00	0.05	3.07	0.14	0.09	16.35
(NH4)2Mo22O67	0.00	0.03	3.05	0.09	0.06	17.11
MoO3		0.00	3.00	0.00	0.00	18.45

. In the first step, which can be referred to as the drying stage, (NH₄)₆Mo₇O₂₄·4H₂O is transformed to anhydrous ammonium molybdate, (NH₄)₄Mo₅O₁₇, and all of the water molecules, including the crystal ones, are removed together with a small amount of ammonia, as presented by Equation (1):

5 (NH₄)₆Mo₇O₂₄·4H₂O → 7 (NH₄)₄Mo₅O₁₇ + 2 NH₃ + 21 H₂O

(1)

From Table 1, it is clear that (NH₄)₄Mo₅O₁₇ is the most favorable state of ammonium molybdate compared to other anhydrous ammonium molybdates in terms of the ratios of NH₃/O and H₂/O. Based on the data, experimental work was carried out to obtain the optimal conditions for the production of (NH₄)₄Mo₅O₁₇. The identification of (NH₄)₄Mo₅O₁₇ was performed by the analyses of XRD and weight loss.

The results are shown in Figure 2, Figure 3 and Figure 4. When (NH₄)₆Mo₇O₂₄·4H₂O was dried at 90 °C, the crystal water could be completely removed after 720 min. When it was dried at 100 °C, the transformation was observed after 60 min, and a considerable amount of MoO₃ phase was observed after 120 min. An optimal temperature, therefore, can be found in the range of 90~100 °C. The SEM images for the phase transform are shown in Figure 4. Entangled fiber networks were observed in the structure of (NH₄)₄Mo₅O₁₇ after the transformation.

3.1.2. Self-Reduction of (NH₄)₄Mo₅O₁₇

After the transformation of (NH₄)₆Mo₇O₂₄·4H₂O into (NH₄)₄Mo₅O₁₇, the contents of residual ammonia and hydrogen, which can be utilized as reductants, in the ammonium molybdate increased, as shown in Table 1. The reactions related to this utilization can be listed as follows:

(NH₄)₄Mo₅O₁₇ → 5 MoO₃ + 4 NH₃ (g) + 2 H₂O(g)

(2)

3 (NH₄)₄Mo₅O₁₇ → 15 MoO₂ + 5 N₂(g) + 21 H₂O(g) + 2 NH₃(g)

(3)

12 MoO₃ + 2 NH₃(g) → 3 Mo₄O₁₁ + N₂(g) + 3 H₂O(g)

(4)

15 MoO₃ + 10 NH₃(g) → 15 MoO₂ + 5 N₂(g) + 15 H₂O(g)

(5)

This utilization was thermodynamically examined using Outokumpu HSC Chemistry 6 software, and the results are presented in Figure 5, which shows the equilibrium molar amounts of molybdenum oxides and gaseous components when the reaction “5 MoO₃ + (0~4) NH₃ (g) + 2 H₂O(g)” takes place at 575 °C with a stepwise increase in internal NH₃ from 0 mol to 4 mol. Based on the graphs shown in Figure 5, it is theoretically expected that MoO₃ can be completely converted to MoO₂ at 3.4 mol of internal NH₃, which means MoO₃ can be completely reduced to MoO₂ by the internal ammonia without the need for any external supply of ammonia. Experimental work was also carried out to examine the self-reduction, and the results are presented in Figure 6 and Figure 7. The highest degree of reduction achieved in this case was about 18% based on the analysis of weight loss, as follows:

$$\mathrm{Reduction} \mathrm{Degree} (\%)=\frac{\mathrm{Removed} \mathrm{oxygen} \mathrm{amount}}{{\mathrm{Initial} \mathrm{oxygen} \mathrm{amount} \mathrm{in} \mathrm{form} \mathrm{of} \mathrm{MoO}}_3 }$$

(6)

Unlike the theoretical expectation, a perfect reduction degree could not be obtained in this study, and this difference can be ascribed to the technical difficulties in keeping conditions of standard state in the experiment. Practically, the NH₃ produced during the thermal decomposition cannot have sufficient time to react with Mo oxide powder before leaving the reactor, because it passes through the powder layer as soon as it is produced. The SEM images for the phase transform are shown in Figure 6. The material exhibited crystallinity, and several shapes of crystal, plates, needles, and irregular bulk shapes were observed after the self-reduction. Molybdenum trioxide (MoO₃) usually exhibits three polymorphic structures, orthorhombic (α-MoO₃), monoclinic (β-MoO₃), and hexagonal phase (h-MoO₃), among which the former is the only thermodynamically stable phase [8,9,10]. The changes in particle size and morphology of MoO₃ powder with sintering temperature have been reported in the rage of 100 °C to 700 °C [11]. Meanwhile, molybdenum dioxide (MoO₂) crystallizes in a monoclinic cell as a distorted rutile crystal structure, and the crystal structure of the metastable phase Mo₄O₁₁ can be found either as a low-temperature monoclinic η-phase or high-temperature orthorhombic γ-phase [12,13,14]. MoO₃, Mo₄O₁₁, and MoO₂ phases in Figure 7 were not identified. The only differences between orthorhombic and monoclinic crystals are the size and the distortion, and thus, they were not easy to distinguish. In addition, there have been reports that MoO₂ is likely to retain the general morphological habit of the MoO₃ from which it was fully reduced in solid state [15,16]. Moreover, the size and the morphology change with the sintering temperature.

3.2. Direct Hydrogen Reduction of Ammonium Molybdates

In a conventional process, (NH₄)₆Mo₇O₂₄·4H₂O is first roasted to produce MoO₃, which is then reduced by hydrogen gas in two stages: MoO₃ → MoO₂ and MoO₂ → Mo. MoO₃ is easily reduced by either hydrogen or other reducing gas, and several studies have reported morphology changes during the reduction [17,18,19,20,21]. In the process proposed in this study, (NH₄)₆Mo₇O₂₄·4H₂O is first dried to (NH₄)₄Mo₅O₁₇ and then is directly reduced, without roasting, by hydrogen gas in two stages, so that the ammonia constituent in (NH₄)₄Mo₅O₁₇ can be utilized as a reductant in the first stage to produce MoO₂. The experimental results carried out for this purpose are presented in Figure 8, Figure 9 and Figure 10. In the experiments, (NH₄)₄Mo₅O₁₇ was reduced by hydrogen gas of two flow rates, 0.1 L/min and 0.2 L/min at 575 °C, which is a typical temperature for the first stage, and the conversion rates of (NH₄)₄Mo₅O₁₇ were compared with those of MoO₃. As can be seen in Figure 8 and Figure 9, the conversion rates of (NH₄)₄Mo₅O₁₇ were found to be higher than those of MoO₃, especially in the time range of 20~30 min. By the time the conversion rate (i.e., the reduction degree) of (NH₄)₄Mo₅O₁₇ had reached 100%, that of MoO₃ reached 80% at the hydrogen flow rate of 0.2 L/min. This phenomenon also supports the self-reduction of (NH₄)₄Mo₅O₁₇ by the internal ammonia. The morphology changes during the direct reduction are presented in Figure 10. As the direct reduction proceeded, the kernel shape of the ammonium molybdate transformed into platelet shapes of molybdenum oxides such as Mo₄O₁₁ and MoO₂, and finally all the ammonium molybdates were changed into MoO₂ of platelet shapes. This appearance agrees with the model suggested by Dang et al. for hydrogen reduction of MoO₃ to MoO₂ at 773 to 829 K [22]. The SEM images of the internal structure of the directly H₂-reduced samples are shown in Figure 11. SEM images of that for (NH₄)₄Mo₅O₁₇ could not be obtained because of technical difficulties. As can be seen, as the direct reduction proceeded, more pores and cracks were observed within the internal structures. This aspect is similar to what was observed during the hydrogen reduction of MoO₃ [3].

4. Conclusions

Ammonium paramolybdate tetrahydrate (APT: (NH₄)₆Mo₇O₂₄·4H₂O) was pre-dried at an appropriately low temperature to remove the crystal water corresponding to 4 mol per mol of APT, and then it was transformed into (NH₄)₄Mo₅O₁₇, of which the residual ammonia content increased. The utilization of the ammonia content as a reductant was extensively examined for the first stage of hydrogen reduction to produce MoO₂. The results can be summarized as follows:

(1)

As the direct hydrogen reduction of (NH₄)₄Mo₅O₁₇ proceeded, the kernel shape of the ammonium molybdate was transformed into platelet shapes of molybdenum oxides such as Mo₄O₁₁ and MoO₂, and finally all the ammonium molybdates were changed into MoO₂ of platelet shapes. The conversion rates of (NH₄)₄Mo₅O₁₇ were found to be higher than those of MoO₃, especially in the time range of 20~30 min. By the time the conversion rate of (NH₄)₄Mo₅O₁₇ had reached 100%, that of MoO₃ reached 80% at the hydrogen flow rate of 2 L/min.

(2)

The best reduction degree obtained in this case was about 18% based on the analysis of weight loss. Although a perfect degree could not be obtained in this study due to the technical difficulties to keep conditions of standard state in the experiment, the effectiveness of this residual ammonia content in (NH₄)₄Mo₅O₁₇ was confirmed in terms of decreasing the hydrogen consumption and reaction time.