Uses of Borates in Metallurgy
Nonferrous smelting and precious metals refining
Gold refining can be traced back to Egyptian times and dated somewhere between 5,000 and 2,000 B.C. Other reported legends say the Babylonian goldsmiths of 2,000 BC used borax from the East as a metalworking flux. However it is believed that either Georgius Agricola (1494–1555) in his book “De Natura Fossilium” or Vanoccio Buringuccio (1480–1538) in his book on metallurgy, “De la Pyrotechnica,” suggested that Chrysocolla (Borax) was the fluxing agent in metal refining during their period.
- Reduce the melting point of the metal being refined
- Remove metal oxides
- Prevent oxidation of the refined metal from the furnace
- Can help to adjust the slag’s viscosity for easier removal from the molten metal
Borates fluxing value has been well known in glass, ceramic and other vitreous applications. That same fluxing action provides value in purifying nonferrous and precious metals (i.e. brass, bronze, copper, gold, platinum, silver, zinc, etc.) by reducing the melting point of the batch. While there are other additives used in the metal recovery process, our focus will be on borates.
Often times these nonferrous and precious metals come from scrap metals or from the slag portion during the primary smelting phase. It is the metal oxides and these other impurities that need to be separated from the pure metal that will generate a higher yield and value to the secondary smelter. One process is to charge the furnace with a borate compound in the bottom. The target scrap metal to be smelted is placed on top of approximately half the borax required with the remainder being added during the refining phase. The ratio of borate to metal scrap into the furnace varies with the metal to be recovered and furnace type. As the borate melts it creates a pool of fluxing material that begins to rise and cover the metal that has already begun to melt. These metal oxides and impurities found within the scrap metal rise to the surface of the melt with the help of the molten borate compound.
Sometimes silica sand at the surface of the melt helps congeal the metallic slurry and increase the viscosity so that these impurities can be scraped from the surface of the purer metal melt.
Once the impurities are removed, the purer metal may be poured from the furnace allowing it to flow into ingot molds where it cools. The above process will typically work for cupola furnaces while other furnace technology may require adding borates with the scrap metal when charging the furnace with the recycled metal scrap.
Uses of Borates for Metallurgy
Choosing the right sodium borate for secondary metal recovery
|Sodium borate hydrated (Etibor 48)||Anhydrous borax|
||@ 69% B2O3 Water content: @ 1% Energy required to melt is 1,967 BTU/lb. of B2O3 Less boron emissions up the stack due to less water in borate crystal Better throughput since anhydrous borax has already gone through dehydration Less required per ton of recycled material Less corrosive to refractory brick due to less water being emitted. Cost is higher than hydrated sodium borates|
Precious metal ore recovery (Gold)
The purpose of this section is to discuss the post ore refining process.
The gold concentrate which is sorted into acid and basic gangues plus pyrite ores is put through a process of oxidation, sulfurization and smelting to remove metal oxides and impurities and produce a pure gold material. Usually the gold is refined to its present state in ceramic crucibles. Oxygen is injected into the gold melt to help remove any potential oxides that are still found within the gold-metal concentrates. These oxides rise to the surface of the gold melt and form a slag which is removed from the surface of the melt prior to pouring into bars or more commonly called bouillons.
Anhydrous borax is typically used because of its excellent fluxing action and safety. Hydrated borates Etibor 48, Colemanite, and Ulexite have been used; however caution should be taken as hydrated materials can produce a potential explosion of water from the melt.
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Investigate these potentially helpful links:
- American Foundry Society (AFS)
- American Society of Mechanical Engineers (ASME)
- Industries (ISRI)
- International Zinc Association (IZA)
- The Historical Metallurgical Society
Brazing, Soldering & Welding Fluxes
The Sumerians, during the Bronze age (3600 – 1200 BC) in Ur (Iraq), made swords which were joined by hard soldering. A gold goblet, found in the tomb of Queen Pu-abi (2600 BC), was double-walled with a braze fillet around the periphery. The Egyptians (1200 – 500 BC) heated iron ore in a charcoal fire to reduce it to sponge iron; the particles were then welded together by hammering. This "pressure" welding or "solid-phase" welding was the first recorded instance. It is believed that borates were first mentioned (at least recorded) in the monthly journalThe Manufacturer and Builder Volume 0020 Issue 3 (March 1888). In that journal a comment was made that iron filings and borax would make a strong weld. In France at the turn of the 20th century, E. Fouch and F. Picard invented the oxyacetylene torch where borates provided value as a flux. In the 1920s automatic welding took form using a wire that was encased in an inert gas (i.e. argon, hydrogen, helium). Still other discoveries included shield metal arc welding in the 1950s using a flux coated electrode which was believe to be coated with a borate and other additives.
Borate’s value was also used to deoxidize the metal surface for steel brazing. In the area of welding a borate-based compound appeared to be used as a deoxidizer for steel production between 1910 –1920. When used with welding electrodes, borax was introduced to this technology around the 1930s when structural welding became common place.
- Clean away dissolved metal oxides and other foreign debris
- Prevent oxidation of the metal weld by covering the welded joint
- Act as a flux toward a good weld or joining of metals
Soldering is defined as the joining of metals by the fusion of filler metal between them at a temperature below the solidus temperature of the metals being joined and below 4500. This type of metal fusion uses propane torches and soldering irons to achieve the proper temperature. These are commonly used in electronics, jewelry, metalworking and plumbing. The filler metals used can include combinations of aluminum, bismuth, cadmium, lead, silver, tin, and zinc. More recently lead has been considered an environmental concern and is being regulated out of most formulations.
Brazing is a term used industrially and is defined as the joining of metals by fusion of a filler metal between them at temperatures below the solidus temperature of the metals being joined but above 45OOC.
It should be noted that the solidus temperature is the temperature metals of an alloy system become completely solidified on cooling or start to melt on heating. The filler metal will be composed of more than two alloys based on the ability of these filler metals to wet out the surface of the metals to be joined. These filler metals can be in the form of (but not limited to) a powder, ribbon, rod or wire.
The heat source for such operations comes from blanket brazing, dip, electron beam, furnace, induction, infrared, laser, resistance brazing and torch. Brazing can be applied under various gases including air, argon, hydrogen, and nitrogen.
Welding is defined as the process to join two or more metal parts by applying heat and pressure to both. This is achieved with or without a filler metal with a goal of producing a localized union across the interface of these metals through fusion or diffusion. The sources of heat for welding can include electric arc, electron beam, friction, gas flame, laser and ultrasound. Unlike soldering and brazing, welding can be performed both in air and underwater.
Another aspect to consider is arc welding where an electrode is used to provide a strong direct or alternating electrical current to fuse two pieces of metal together. Typically, the welding rod or “stick,” as it is called, will have a coating of flux on the surface or at the core of the rod itself. Borates are often found on these rods as a flux.
Uses of Borates in the Brazing & Soldering Process
Brazing and soldering is a process of joining two or more base metals where a third filler metal is heated beyond the melting point and distributed by capillary action between these two or more metals. This third metal filler is protected using a flux which helps to wet out (fuse and joining) the filler metal over these base metals and then allowed it to cool, joining these metals together.
While there is a heat source to help join the metals together there is also a flux component. The flux compounds used are composed of borates and other additives and are meant (in-part) to prevent oxides from forming while the metal surfaces that are being heated. The flux also serves to clean impurities off the metal surfaces being joined. Flux compounds are in the form of paste, liquids and powder compositions.
There are a number of flux types that could include acidic, basic, neutral, and fluoride-based compounds.
A mixture of borax and ammonium chloride is used as a flux when welding iron and steel. It lowers the melting point of the unwanted iron oxide (scale), allowing it to run off. Borax is also mixed with water as a flux when soldering jewelry metals such as gold or silver. It allows the molten solder to flow evenly over the joint in question. Borax is also a good flux for 'pre-tinning' tungsten with zinc, which makes the tungsten soft-solderable.
Sometimes a simple combination of borate compounds provides an effective flux formulation. In this case a high temperature brazing powder flux is required and the only two additives are Boric acid (70 – 90%) and Borax 10 Mol (10 – 30%).
Still other flux formulations can contain Borax 10 Mol (10%); Boric acid (45%); Potassium fluoride (35%); and water (10%) to form a paste flux.
Yet another flux formulation for cast iron applications includes Boric Acid (10-30%) Slag (45-55%) and Silica (8 – 12%).
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