Quality Considerations When Specifying BFA

Experience in dealing with BFA from China and elsewhere has revealed a need to adequately specify the quality of the product desired. Defining the quality requires an understanding of the product’s intended application, as well as an understanding of the BFA production process as it can influence the quality of the BFA.

BFA is produced from calcined bauxite by reducing its oxide impurities to their metal state by reacting them with a carbon source. It is important in the manufacture of BFA to have control over the amount of reduction that occurs and to reduce only the amount of impurities necessary to meet the chemical specifications of the BFA being produced. Both over- and under-reduction will result in poor product performance.

The process requires proper blending of the furnace feed and control over the amount of reduction in the furnace melt by adjusting the electrical power and duty cycle of the applied power. To ensure proper blending of the furnace feed, chemical analysis of all starting materials (bauxite, iron and carbon source) must be made on a periodic basis. The fusion or melting process is monitored by taking dip samples of the melt and analyzing them for the levels of titania, iron and silica. Over-reduction of the titania must be avoided.

A rapid chemical analysis of the dip sample is necessary if adjustment of the melt is to be made before the furnace is tapped. Producing a consistent BFA tends to be easier for companies using the larger tilt pour type furnaces. Producers with smaller furnaces are faced with having to test additional samples required to yield the same output as a producer with a larger furnace.

Poor control of the furnacing conditions of BFA can result in several quality problems for the BFA purchaser. BFA is produced under reducing conditions, and it is possible to form reduced compounds such as carbides and sulfides. Aluminum carbide and aluminum oxycarbide will start to form in BFA if the melt is over-reduced during furnacing. Sulfides can form if a source of excess sulfur gets into the furnace during the reduction process. Many BFA products are used in an oxidizing atmosphere, where the oxidation of the carbide and sulfide compounds can cause serious problems. The oxidation of these carbides and sulfide compounds can result in the formation of voids or bubbles on the surfaces of parts made from BFA. Parts having these voids or bloated surfaces are usually unusable, and the manufacturer of the part suffers considerable monetary losses.

Consumers of BFA may be unaware that their problems with voids and bloating can be caused by these compounds. Because of this, only a few purchasers of BFA specify the concentrations of carbon and sulfur. Once carbides or sulfides have contaminated BFA, their removal can only be accomplished by high-temperature roasting of the BFA in an oxidizing atmosphere. This is an expensive process, and few manufacturers have the necessary equipment.

The source of the carbon for reducing the impurities in the calcined bauxite is either coke or coal. In North America, metallurgical coke has been used exclusively in the manufacture of BFA for more than 40 years. Coke is low in sulfur since most of the sulfur present is burned out during the coking process. In China, coal is used in the production of BFA because it is readily available and inexpensive, but coal can be a source of sulfur contamination. The amount of sulfur present in coal varies considerably from one mine to another and can even vary within different zones of the same mine. It is very difficult to burn out the sulfur in the BFA furnace during normal processing conditions.

The use of coal also has one other possible problem. Coke, with its high surface area, tends to react fast and burn up during furnacing of BFA. Coal, on the other hand, with its lower surface area, will react more slowly and may not completely burn up during furnacing. This often results in small particles of carbon being trapped in the BFA.

Entrained carbon particles in a BFA can cause voids and bloating. Some Chinese companies may be using very coarse sized coal, which reacts even slower during BFA furnacing. Depending on the application, it may be important to consider the carbon and sulfur levels in the BFA being purchased.

The oxidation and burnout of particles of free iron or ferrosilicon can also cause voids or bloating in BFA products. During crushing, BFA can become contaminated with iron particles as a result of crusher wear part erosion. Particles of ferrosilicon become entrained in the BFA during its manufacture in the arc furnace. Ferrosilicon is the principle by-product in the production of BFA, but most of it is separated from the BFA in the furnace.

In North America, the ferrosilicon is allowed to accumulate in the bottom of the furnace. Depending on the size of the furnace and the chemistry of the bauxite mix being processed, several dozen BFA batches will be poured from above the ferrosilicon layer before it has filled most of the furnace. When the ferrosilicon layer accumulates to an unacceptable level, the furnace operator makes a separate pour called a deep pour and pours out most of the ferrosilicon into a pit or rail car lined with sand. In China, the operator makes two pours to lower the level of the ferrosilicon. The first pour is all BFA, and the second is made up of both BFA and ferrosilicon.

Both iron particles and entrained ferrosilicon can be removed after crushing by passing the BFA over a magnetic picker. However, two possible circumstances can result in the metal particles not being removed with this process. The first is when a non-magnetic steel has been used in the wear plates of the crushing system. Manganese-based toughened steels are non-magnetic and are commonly used to make crusher wear parts. These steel particles cannot be easily removed by low intensity magnetic pickers. The second source of metal particles is caused by poor control of the BFA furnacing conditions. When the concentration of the silicon in the ferrosilicon alloy is allowed to exceed 17% silicon, the alloy becomes nonmagnetic, and this nonmagnetic ferrosilicon is also difficult to remove with low-intensity magnetic pickers.

Both nonmagnetic and weakly magnetic iron and ferrosilicon particles can be removed by high-intensity magnetic pickers, but with a high loss of acceptable product. Problems with free iron and non-magnetic particles could be avoided by having a procedure in place to determine the amount of acid-extractable iron present in the BFA.

The BFA purchaser should specify the concentrations of alkali and alkaline earth oxides that are acceptable in the intended application. In abrasive applications, these oxides react with alumina to form beta-aluminas, which reduce the toughness of the BFA. In refractory applications, these oxides can increase the formation of low melting glass phases and reduce the hot strength. Chinese bauxites are available with acceptable levels of soda, potash, calcia, and magnesia, but they may demand a premium price.