Pyrochlore ores

Niobium minerals, especially columbite, are also associated with other valuable minerals, such as tantalum, zircon and rare earth minerals. Pyrochlore and a mixture of pyrochlore and columbite have different origins, and therefore, beneficiation of pyrochlore and columbite are different from that of the mixed tantalum niobium ores. In actual plant practice, the treatment process is significantly different from that used for mixed niobium tantalum ores. This is due to the fact that the beneficiation process is largely determined by the nature of gangue minerals present in the ore. In most cases, the beneficiation process applicable for pyrochlore ore cannot be successfully applied for beneficiation of tantalum/ niobium ores. 

The reaction of chlorine gas with a mixture of ore and carbon at 500—1000°C yields volatile chlorides of niobium and other metals. These can be separated by fractional condensation (21—23). This method, used on columbites, is less suited to the chlorination of pyrochlore because of the formation of nonvolatile alkaU and alkaline-earth chlorides which remain in the reaction 2one as a residue. The chlorination of ferroniobium, however, is used commercially. The product mixture of niobium pentachloride, iron chlorides, and chlorides of other impurities is passed through a heated column of sodium chloride pellets at 400°C to remove iron and aluminum by formation of a low melting eutectic compound which drains from the bottom of the column. The niobium pentachloride passes through the column and is selectively condensed the more volatile chlorides pass through the condenser in the off-gas. The niobium pentachloride then can be processed further. 

The reaction of finely ground ores and an excess of carbon at high temperatures produces a mixture of metal carbides. The reaction of pyrochlore and carbon starts at 950°C and proceeds vigorously. After being heated to 1800—2000°C, the cooled friable mixture is acid-leached leaving an insoluble residue of carbides of niobium, tantalum, and titanium. These may be dissolved in HF or may be chlorinated or burned to oxides for further processing. 

The niobium-tantalum weight ratio in North American pyrochlores is 100-150 1, while in Russian ores the ratio varies between 8 1 and 1 20 [21 ]. 

Literature on flotation of gold, PGMs, rare earths and various oxides is rather limited, compared to literature on treatment of sulphide-bearing ores. As mentioned earlier, the main problem arises from the presence of gangue minerals in the ore, which have flotation properties similar to those of valuable minerals. These minerals have a greater floatability than that of pyrochlore or columbite. In the beneficiation of oxide minerals, finding a selectivity solution is a major task. 

There are two major types of pyrochlore-containing ores pegmatite ores and carbonatites. This classification is based on the mineral composition of these ore types. The main waste minerals contained in the pegmatite ores include quartz and nepheline. This ore type also includes granites, where pyrochlore is represented in a coarse crystalline form. Granites are composed of cryolite and topaz as the main gangue minerals. 

Carbonatite ores are mainly composed of calcite, dolomite and phosphates as the main gangue minerals. The beneficiation process for pegmatites containing pyrochlore mostly includes gravity preconcentration. Such deposits are common in Africa (Kongo, Madagascar). 

The major minerals contained in pyrochlore-containing ores are pyrochlore, columbite and sometimes ilmenorutile to a lesser extent. Table 22.1 shows pyrochlore minerals present in pegmatite and carbonatite ores. 

The gangue composition of the various carbonatite ores varies considerably. Calcite-dolomite content in some ores ranges from 30% (Niobec, Canada) up to 70% (Panda Hills, Africa). From a mineralogical point of view, pyrochlore usually occurs in crystallized form, as well as octahedron form. Pyrochlore occurs in considerable range of colours, varying from translucent white to opaque black appearance with glassy surfaces. The Nb205... 

Pyrochlore minerals contained in pegmatite and carbonatite ores... 

The treatment process and flotation properties of pyrochlore are very much dependent on the gangue composition of the ore. The selective flotation of pyrochlore from carbonatite ore is not possible since calcite and dolomite have similar flotation properties as pyrochlore. In addition, in the presence of carbonates, the stable pH required for flotation of pyrochlore (i.e. 5.0-5.5) cannot be maintained. 

In the case of carbonatite ores, a beneficiation process involves preflotation followed by reactivation and flotation of pyrochlore. In the case of pegmatitic ores that contain silicates, biotite, albite and limonite, as the gangue minerals, direct flotation of pyrochlore can be achieved with a variety of different collectors. 

The successful flotation of pyrochlore from carbonatite ores depends on a number of factors ... 

In actual plant practice, by removing the calcite-dolomite, the pyrochlore in the flotation feed is significantly upgraded. In some ores, which assay 0.4% Nb205 in the feed after calcite-dolomite preflotation, the pyrochlore assays in the pyrochlore flotation feed is over 1.2% Nb205. 

Extensive studies have been carried out using orthodihydroxybenzene, known as catechol (commercial name). This reagent has improved the rate of fine pyrochlore flotation and also has a beneficial effect on selectivity. Research work with this reagent was conducted on carbonatite ore from Canada. 

In the majority of cases, amines are used as pyrochlore collectors during treatment of carbonatite ores. Aliphatic mono amines, aliphatic diamines, condesates of capritic acid and partially neutralized diamines are the principal collectors for pyrochlore. Tallow diamine acetate (Duomac T, Akzo Nobel, USA and Canada) is also used as a pyrochlore collector. 

Effect of different amines on pyrochlore flotation from St. Honore Niobec ore... 

There are several operating plants treating pyrochlore-containing ores from carbonatite and pegmatite ores. Operating plants that treat carbonatite ores described in this chapter include St. Honore Niobec, Canada, and OKa, Quebec, Canada. The operating plant that treats pegmatite ore is Araxa (Brazil). 

Bulatovic, S., An Investigation into Recovery of Pyrochlore from St-Honore Niobium ore (Canada), Report of Investigation, 2003. 

The results showed that amines normally used for pyrochlore flotation did not work for flotation of Ta/Nb. Therefore, collector selection is very dependent on the type of niobium minerals present in the ore. 

The ore used in this example contained a mixture of pyrochlore and columbite as the major niobium minerals. The tantalum is mainly associated with columbite. The major gangue minerals present in this ore were soda and potassium feldspars with small amounts of mica and quartz. Beneficiation of this ore using cationic flotation, normally employed for flotation of niobium, was not applicable for this particular ore, since most of the mica and feldspar floated with the niobium and tantalum. The effect of amine on Ta/Nb flotation is illustrated in Figure 23.9. The selectivity between Ta/Nb and gangue minerals using a cationic collector was very poor. 

From disseminated ores contained in mineral lenses, the recovery of bastnaesite and monazite is accomplished using flotation. The flotation properties of bastnaesite and monazite are similar to the gangue minerals contained in the bastnaesite and monazite, such as calcite, barite, apatite, tourmaline, pyrochlore and others, which represent difficulties in selective flotation. However, in recent years, significant progress has been made in the flotation of both monazite and bastnaesite [2,3]. 

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