Fluidized-bed roasting of molybdenite concentrates

Details Fluidized-bed roasting of molybdenite concentrates Fluidized-bed roasting of molybdenite concentrates

1997-08-07

2001-02-20

Beck, Shrive (Department: 1764)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Group vib metal

C266S172000, C423S659000

Reexamination Certificate

active

06190625

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed generally to fluidized-bed roasting and specifically to fluidized-bed roasting of molybdenite concentrates.
BACKGROUND OF THE INVENTION
Molybdenum oxide, particularly MoO
3
, is widely used as a raw material in the manufacture of stainless and low alloy steels, pure molybdenum metal, superalloys, catalysts, and specialty chemicals. Molybdenum oxide is commonly produced from molybdenum sulfides, particularly molybdenum concentrates, that are obtained by grinding copper or molybdenum ores and concentrating the sulfides contained therein.
To convert molybdenum sulfide to molybdenum oxide, molybdenum sulfide is typically air roasted in a multiple-hearth roaster according to the following overall reaction:
MoS
2
+3.50
2
→MoO
3
+2SO
2
.
In multiple-hearth roasters, a static bed of feed material containing the molybdenum sulfides is calcined by an oxidizing gas (air) at temperatures ranging from 550 to over 700° C. To avoid severe fusion of the bed, operators periodically mix the bed manually to maintain bed porosity and permeability at desired levels.
In designing a more efficient roaster for molybdenum sulfide concentrates, there are a number of important considerations. By way of example, the roaster should be continuous and produce a uniform quality, low-sulfur molybdenum oxide product. To reduce the capital and operating costs of gas handling and acid plants, it is desirable to minimize the use of excess air and thereby produce a roaster off-gas containing a relatively high concentration of SO
2
. Second, the roaster should be capable of automated operation to provide reduced operating costs. In other words, the roaster design should allow automated process control at operating conditions that will allow long operating times without downtime for cleaning and maintenance. Third, the roaster should recover heat energy released during sulfide oxidation as steam for useful purposes. Fourth, the roaster should have relatively few moving parts to improve system reliability, simplify system operation and decrease downtime and maintenance costs. Fifth, the roaster should provide for substantially uniform distribution of heat of reaction throughout the bed. The existence of temperature gradients in the bed can create hot zones where high temperature can cause partial fusion and sintering of the bed and volatilization of molybdenum oxide. Sixth, the roaster should have little, if any, refractory lining. Refractory lining can cause molybdenum loss (via molybdenum penetration into the refractory lining) and product contamination. Seventh, the roaster should eliminate the formation of hard crusts of molybdates and oxides. Such crusts can erode moving parts (e.g., rabble arms and teeth in conventional multiple-hearth roasters) and increase operating costs through increased labor to clean the roaster. Eighth, the roaster off-gas should have little, if any, dust entrainment to minimize product loss and downstream gas cleaning costs. Ninth, the roaster should be capable of handling feed materials comprising impurities, such as calcium, copper, iron and rhenium without operational problems. Finally, the roaster should operate at a temperature low enough to retard the volatilization of molybdenum trioxide and later condensation of the molybdenum trioxide in cooler pipes. Such condensation can cause operational and maintenance problems and reductions in product yields.
SUMMARY OF THE INVENTION
It is an objective of the present invention to provide a continuous roasting apparatus and method that can produce a uniform quality molybdenum oxide product at relatively low operating and capital costs. Related objectives include providing a roasting apparatus that is amenable to automatic process control; recovers energy released during sulfide oxidation; has few moving parts; has no refractory lining; can eliminate the formation of hard crusts of sintered molybdates and oxides, minimizes dust entrainment in the roaster off-gas, can handle feed materials comprising impurities without operation problems and can retard the volatilization of molybdenum trioxide.
These and other objectives are addressed by the present invention which provides a fluidized bed, roasting apparatus including:
(a) a chamber (e.g., a fully enclosed housing) for containing a bed of a feed material including molybdenum sulfides, such as molybdenite concentrates, the chamber having a feed port for introducing the feed material including molybdenum sulfides into the bed and a discharge port for removing the product from the bed;
(b) fluidizing means (e.g., a blower and connected ductwork) for contacting a fluidizing gas such as air with the bed to convert the molybdenum sulfides into molybdenum oxides, such as MoO
3
, and sulfur oxides, such as SO
2
.
(c) cooling means (e.g., a cooling tube containing a cooling fluid such as water) for removing the heat generated by the roasting reaction and thereby maintaining the bed at a desired temperature.
Because the conversion of molybdenum sulfides to molybdenum oxides is strongly exothermic, the cooling means closely controls the fluidized bed temperature. The cooling means can include a heat exchange fluid, such as water/steam, for recovering energy released during sulfide oxidation. The cooling means enables precise control of temperature at a set point which retards fusion and/or sintering of the materials in the fluidized bed, eliminates the need for a refractory lining, and retards volatilization of molybdenum trioxide. Preferably, the cooling means maintains the fluidized bed temperature below the sublimation and melting points of the molybdenum oxide and molybdate compounds that can form due to the presence of impurities. More preferably, the maximum fluidized bed temperature is less than about 580° C.
The roasting apparatus of the present invention has numerous advantages relative to conventional multiple hearth roasters. By way of example, the roasting apparatus of the present invention can be continuous, have a high throughput and relatively low capital and operating costs. The roasting apparatus can be amenable to automated process control, can handle feed materials comprising impurities such as calcium, copper, iron and rhenium without operation problems, and can have relatively few moving parts and therefore a high degree of system reliability and simplified system operation.
To eliminate gas channeling which causes pockets of unfluidized solids in the bed and to provide for a relatively high degree of fluidized bed porosity and permeability, the roasting apparatus can include vibratory means (e.g., a vibrator) for vibrating the bed during roasting. Preferably, the minimum amplitude of vibration of the vibratory means is about {fraction (1/16)}″ and the maximum amplitude is about ½″
To inhibit entrainment of finely-sized feed material in the roaster off-gas, the chamber can have an expanded section at its upper end to provide for decreased velocity of the roaster off-gas. Entrained particles will return to the bed in response to the decrease in fluidizing gas velocity from the increased area of flow and decreasing gas temperatures due to heat losses from the upper roaster walls. Additionally, the apparatus can include gas/particulate separator means (e.g., a cyclone) to separate any remaining entrained particles from the roaster off-gas.
To provide for substantially uniform fluidizing gas (e.g., air) distribution and heat of reaction throughout the bed, the roasting apparatus can include a distributor plate located below the fluidized bed for distributing the fluidizing gas very uniformly across the bottom portion of the bed. Preferably the distribution plate has a plurality of pores for passage of the fluidizing gas, with the maximum distance between the centers of adjacent pores being about ¼ inch.
To provide for plug flow of the feed material as the material moves through the roasting apparatus, the apparatus can include one or more baffles to divide the chamber into a number of re

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