Agglomeration during reduction of MoO 3 Ravi K. Enneti ⁎, Thomas A. Wolfe Research and Development, Global Tungsten and Powders Corp, Towanda, PA 18848, United States abstract article info Article history: Received 22 July 2011 Accepted 5 September 2011 Keywords: Agglomeration Material buildup Molybdenum trioxide Calciners Reduction Surface area Molybdenum powder is manufactured in a two step process starting from MoO 3 . The first step reduction of MoO 3 to MoO 2 is carried out in rotary calciners. Agglomeration of powder occurs during this reduction stage resulting in several manufacturing issues. The evolution of agglomeration during the reduction of MoO 3 was investigated in the current study. As-received MoO 3 and MoO 3 milled for 0.5 h were used as the starting powders. The powders were reduced at 550 °C, 650 °C and 750 °C in a hydrogen atmosphere. The starting and reduced powders at various temperatures were analyzed using BET surface area, XRD, and SEM techniques. The surface area of the reduced powders was monitored for quantifying the degree of ag- glomeration. The surface area was found to be minimum for the samples reduced at 650 °C. SEM observations confirmed the agglomeration of powders during reduction process. XRD analysis showed complete reduction of MoO 3 to MoO 2 at 650 °C and 750 °C. The agglomeration of the powders was either due to melting of eutec- tic formed between MoO 3 and Mo 4 O 11 or due to partial melting of MoO 3 . The reduction of MoO 3 is recom- mended to be completed at a low temperature to prevent agglomeration of the oxide powders. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Molybdenum powder is manufactured in a two step process start- ing from molybdenum trioxide (MoO 3 ) [1–4]. The first step is carried out at lower temperatures and involves transformation of MoO 3 to MoO 2 . The first step reduction is exothermic in nature. The MoO 2 is further reduced to Mo powder during the second step process. The second step reduction is endothermic in nature. A schematic showing the two step reduction process used in manufacturing of molybde- num powders is shown in Fig. 1. The first step reaction follows the re- action path [5] MoO 3 →Mo 4 O 11 →MoO 2 : In production the first step is usually carried out in rotary reactors and the second step in pusher type of furnaces [1–3]. Agglomeration/- buildup of the powders on the walls of the calciners is a major issue during the first step reaction. Powder agglomeration results in mate- rial bridging inside the tube of the rotary furnace and forms rings that block the material flow. A typical example of agglomeration of pow- ders during reduction of MoO 3 in a rotary calciner is shown in Fig. 2. The objective of the current study is to understand the evolu- tion of agglomeration/material buildup of powder during the first step reduction of MoO 3 . The results from the study will assist in selecting processing parameters to minimize agglomeration of the powder. 2. Experiment The as-received MoO 3 powder was milled for 8 h in a plow-type Littleford, FDM-130-D mill. The maximum changes to the powder characteristics (increase in surface area) occurred during the first 0.5 h of milling. The as-received and MoO 3 powder milled for 0.5 h were selected for further analysis. The characteristics of these pow- ders are summarized in Table 1. The as-received and milled MoO 3 powder was reduced in hydrogen atmosphere at 550 °C, 650 °C and 750 °C. The reduction experiments were carried out in Lindberg/Blue tube furnace. A heating rate of 10 °C/min was used for reducing the powder at various temperatures. The oxides were held at the reduc- ing temperatures for 15 min. The initial weight of the powders for all the reduction experiments was 70 g. The weight of the powders after reduction was measured to quantify the weight loss during re- duction. An AG 204 Mettler Toledo scale was used to measure the weight of the powder before and after reduction. The dew point of hydrogen was identified to have a significant effect on the progress of the first stage reaction [5]. As the current study is focused on iden- tifying the mechanisms for agglomeration/buildup of powder in calci- ners, a constant dew point of -40 °C was maintained for all the reduction experiments. A constant dew point results in providing similar atmospheric conditions for reduction in all the experiments. The surface area of the powders was measured using the Beta Scien- tific Corporation 4203 equipment. The XRD analysis to identify the phases in the reduced powder was carried out using a Rigaku Int. Journal of Refractory Metals and Hard Materials 31 (2012) 47–50 ⁎ Corresponding author. Tel.: + 1 570 268 5252. E-mail address: ravi.enneti@globaltungsten.com (R.K. Enneti). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.09.004 Contents lists available at SciVerse ScienceDirect Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM