- THE MAGAZINE
The abrasive industry in China began in the 1940s, when what became known as Grinding Wheel Factory No.1 was started by the Japanese. During the next two decades, the industry expanded with the assistance and transfer of technology from East Germany.
This expansion was a part of the Chinese government’s First Five Year Program. Each new factory was given the name Grinding Wheel Factory with a sequential number. The term “Grinding Wheel Factory” was all-inclusive, and any factory could be assigned to produce one or more of the following: raw materials for bonded and coated abrasives, bonded abrasives, coated abrasives, refractories and superabrasives. Grinding Wheel Factories No.1 through No.7 were established, each with its own mission and area of specialization. In addition, numerous other small manufacturers of abrasive materials were formed throughout China.
One product still produced by several of the Grinding Wheel Factories today is fused brown aluminum oxide (BFA), which is the primary abrasive material used in the production of grinding wheels and coated abrasives, as well as an excellent refractory material.
BFA is produced by melting and reducing calcined bauxite in an electric arc furnace. The calcined bauxite contains varying amounts of impurities, such as iron oxide, silica and titania, which must be removed during the melting process. The concentration of the impurities is decreased to predetermined levels to ensure the optimum properties of the BFA in each intended application. Excess impurities are removed in the electric arc furnace by melting the calcined bauxite with additions of carbon and iron. The carbon reacts with the oxygen in the impurities to form carbon monoxide gas, and the impurities are reduced to their corresponding metals.
The metals, being denser and insoluble in the molten BFA, separate and settle to the bottom of the furnace. The excess iron combines with the silicon metal to form a ferrosilicon alloy, which precipitates out of the melt and forms a pool at the bottom of the furnace.
BFA Production in ChinaTwo designs of electric arc furnaces are in operation in China today: the stationary or batch type, referred to in North America as the Higgins furnace, and the continuous or tilt pour, which is similar to a steel melting furnace. The trend worldwide has been to switch from the stationary electric furnace to the tilt pour. The tilt pour type electric arc furnace typically has a higher throughput and more consistency in BFA quality.
At least three of the major Chinese producers are using tilt pour electric arc furnaces to produce BFA, and at least one other is considering the switch to tilt pour. A leading producer of BFA in China, Mountain Brand, formally known as Grinding Wheel Factory No.7, is believed to have been using tilt pour furnaces for many years. Mountain Brand began production in 1972 and reported in 1986 that production had reached 40,000 metric tons (MT). During a tour of North America in 1986, Mountain Brand representatives predicted a capacity of 100,000 MT by 1990. It is believed that Mountain Brand has reached its anticipated capacity level.
One of the most significant expansions in the Chinese BFA production was the installation of three 6-meter diameter electric arc tilt pour furnaces during the last decade. These state-of-the-art furnaces with computer controls were designed and furnished by Whiting Equipment Company Ltd. of Canada. Only one other furnace of this size producing BFA is known to be operating in the world. This unit is operated by Washington Mills Electro Minerals in Niagara Falls, Canada.
One of the Whiting furnaces shipped to China is operated by White Dove, formerly Grinding Wheel Factory No.2. The other two units are owned and operated by the Taiyuan Twin Towers Aluminum Oxide Co. in Taiyuan, Shauxi Province. The output of each furnace is estimated to be between 25,000 and 30,000 MT per year. The furnace capacity is limited by the size of the 10 MVA transformers that were supplied with the furnaces. An increase in transformer capacity would significantly increase their output.
To put the potential output of these furnaces into perspective, let us first consider that the output of BFA in North America has dwindled from a high of 220,000 MT in 1966, to 160,000 MT in 1995, to less than 100,000 MT today. The decline in production has been offset by the use of sol gel based aluminas produced in North America and imports of BFA from China.
CapacityThe estimated current production capacity of several BFA producers in China is shown in Table 1. The two companies with the 6-meter diameter furnaces have the potential of significantly increasing their output. Each six-meter diameter furnace installed in China has a volume capacity of more than 110 MT. The usable melt capacity is estimated to be between 25% and 50% of the total capacity. Twenty-five metric tons or 25% of the volume could be poured every four hours if sufficient electrical power were connected to the furnace to increase its melting rate.
To produce 25 MT every four hours, the furnace would require 16 MVA of connected power. Therefore, each furnace is capable of being modified by adding transformer capacity to increase its output to at least 48,000 metric tons per year based upon operating 320 days a year. Impediments to increasing the output of these furnaces are the current world demand for BFA, current world pricing of BFA, the cost of the transformer retrofit, and the capacity of the existing electrical distribution network.
QualityThe quality of the BFA from China can be characterized from excellent to poor. Most producers are striving to compete on both quality and price. Table 2 presents the chemistry of a high-quality BFA from China.
CostThe major costs in producing BFA are calcined bauxite, electrical power, graphite electrodes and maintenance. China probably has advantages in all cost areas. Some BFA producers are within 100 kilometers of their bauxite source, thus eliminating the cost of transporting the bauxite thousands of miles to the producing site.
The reduction of a bauxite to 1 MT of BFA typically requires 2,200 kWh. The cost of electrical power can vary. However, China’s northern region has an abundance of coal for generating competitively priced electrical power, and China’s southern region has low-cost hydroelectric power. Electrode consumption in the production of BFA is estimated to be 11 kg of high-quality electrode per metric ton of BFA. There are less expensive domestically produced electrodes. These electrodes can be of poor quality, and their use in the production of BFA results in increased electrode consumption; however, the lower cost of the electrodes should still represent a cost advantage.
Domestic labor and material costs are well below costs in North America and Europe and also represent a competitive advantage. Maintenance costs, however, could escalate if operators of large furnaces were required to make major repairs with parts supplied from North America or Europe.
There are shortcomings to operations in China that impact the producer and purchaser of BFA from China, especially the packaging and transportation within China. Bags and “super sacs” typically supplied by many Chinese producers are difficult to open, and debris from the bags can contaminate the product. Some manufacturers are moving towards the adoption of bagging systems using double-walled paper bags with a plastic liner—a method favored by many Western users of BFA.
Transportation from the producer to the port, storage at the port, and transfer onto the ship could also be improved. Keeping the BFA dry and free from cross contamination with other materials being handled at the port can be a major concern. One improvement that would help to maintain the integrity of the shipment would be the use of dedicated covered rail cars for transporting the BFA.
Quality Considerations When Specifying BFAExperience 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.