September/October 1996
U.S. Zinc transforms galvanizer’s byproducts, zinc scrap, and primary zinc metal into a broader range of zinc products than just about any of its competitors.
Here’s how...
By Eileen Zagone
Eileen Zagone is an associate editor of Scrap.
Back in the late 1950s, Gulf Metals Industries began a transformation that would eventually change its business: The Houston-based processor of nonferrous scrap opened a new division—known as Gulf Reduction Corp.—to produce zinc metal from scrap. “We wanted to focus and saw the opportunity to do so with zinc,” says Jerome Robinson, chairman.
And focus the company did. Today known as U.S. Zinc Corp., the firm’s business is dedicated solely to zinc, enabling it to become one of the country’s key players in this sector. In fact, U.S. Zinc now recycles more than 100 million pounds of zinc scrap and galvanizer’s residues each year and is considered the largest zinc dust producer in the world and the second-largest producer of zinc oxide. Plus, the firm continues to produce zinc metal.
Production takes place at four different facilities. Zinc metal and zinc dust are made in Houston at the division still known as Gulf Reduction Corp. Zinc oxide, meanwhile, is produced at facilities located in Chicago; Hillsboro, Ill.; and Millington, Tenn.—all of which go by the name of Midwest Zinc.
The company also includes a zinc scrap and byproduct trading affiliate with offices in Los Angeles known as Western Zinc; and Pittsburgh; Hilden, Germany; and Mississauga, Ontario, all known as MetalChem. Started in the 1980s to help the firm keep up with changes in the zinc market and foster healthy relationships with galvanizers and other suppliers, the trading affiliate has a close relationship with U.S. Zinc’s production facilities, but not an exclusive one. In fact, only about 30 percent of the zinc drosses, skimmings, and scrap it purchases are sold to Gulf Reduction or Midwest Zinc. Likewise, the melting operations obtain their feedstock from sources beyond the trading affiliate.
Galvanizer’s Skimmings Turn to Zinc Metal
U.S. Zinc’s entree into scrap consumption, its zinc metal operation, is a true example of closed-loop recycling: Zinc galvanizer’s skimmings purchased from around the country are melted and refined into pure zinc metal and sold back to galvanizers for reuse.
The operation consumes about 2.5 million pounds a month of raw material—primarily top skimmings from hot-dip galvanizers—which is shipped in drums or loose to the Houston facility by rail or truck. Once inside the 150,000-square-foot plant, the skimmings, which look a lot more like rough, gray gravel than any type of metal, are segregated into bins with like material and labeled with the details of their particular metallurgical characteristics.
While every galvanizer produces skimmings with different qualities and characteristics, each galvanizer tends to generate skimmings with fairly consistent qualities so U.S. Zinc usually has a good idea of the chemistry of each shipment, says Ed Schlotzhauer, vice president of sales. Nevertheless, he notes, because there’s always the possibility that a particular load will exhibit unexpected physical characteristics, and these differences could affect the end product, all shipments are analyzed in the company’s on-site laboratory to aid sorting.
This early analysis also helps U.S. Zinc identify contaminants in the raw material—such as iron, other metals, and vermiculite (used in some galvanizing operations to contain and insulate the molten zinc)—and remove them prior to further processing.
Once the skimmings are segregated and fairly contaminant-free, workers transfer the pebbly feedstock into a ball mill—a churning device that crushes the material and air-separates it to remove oxides and any remaining particles of other metals and contaminants. The isolated zinc is then graded and put into stacked and labeled bins.
Next stop is the facility’s electric induction furnace, which operates around the clock, seven days a week. Ground material is continually fed into the furnace, where it’s melted at temperatures of about 900ºF, transforming it to molten metal.
The molten metal is then transferred to a 135,000-pound capacity liquation furnace, or holding furnace, that separates alloyed iron that may have made it into the furnace from the pure zinc. The process used here takes advantage of gravity—and the fact that zinc melts at a much lower temperature than its galvanizing partner, iron. Since iron is heavier than zinc, it falls to the bottom of the holding furnace, where it is captured, reclaimed, and when appropriate, recycled.
As the final step in the process, which produces about 1.5 million pounds of zinc metal every month, the refined zinc left in the holding furnace is analyzed through an inline spectrometer that checks for impurities before being poured into one of three different sizes of molds: 55-pound slabs, 500-pound blocks, and 2,400-pound “jumbos.” As the cast chunks cool, they are removed from the molds and placed in gleaming stacks to await shipment by rail or truck to galvanizers in the region.
Bottom Dross Produces Zinc Dust
The Houston facility’s other product, zinc dust, begins life primarily as bottom dross from hot-dip galvanizers. Arriving at the plant by rail or truck in quantities totaling about 3 million pounds a month, this gray, solid dross is sorted according to type before recovery begins.
Its first shot at melting takes place in oven-sized caldrons into which a flame is introduced long enough to melt the material. From there, the molten metal is transferred to a retort furnace, where it spends the next 18 hours being heated to its boiling point—about 2,100ºF—producing a zinc vapor that rises into condenser boxes above the furnaces. As the vapors cool, pure zinc dust particles (called raw dust at this point) are formed and fall into trays at the bottom of the condenser boxes for easy collection.
Iron and other metals and contaminants that made it into the retort furnace do not vaporize along with the zinc, and are gathered from the furnace and, where appropriate, shipped to others for recycling.
The zinc dust operation also consumes primary zinc metal to keep up with customer demand and meet stringent quality requirements for particular zinc dust specifications. Processed separately from the bottom dross, the primary zinc is melted in U.S. Zinc’s new electro-thermal furnace rather than the retort furnace. The rest of the process for making dust out of zinc metal is similar to making dust out of bottom dross, however, with the metal melted to produce vapors that turn to raw dust.
Regardless of the original feedstock, once raw dust is collected from the condensers it is vacuumed or conveyed into the facility’s sizing system (which is enclosed to prevent contamination) and run through a series of screens that sort out larger particles of zinc metal. The 100-percent zinc dust is then run through homogenizers, where it is mixed and blended to customer specifications, and sent on to an air classifier that sorts the dust by particle size.
The 3 million pounds of zinc dust produced in this operation each month goes primarily to the coatings industry, which uses it as a corrosion-resistant additive to paints and primers. It is also used for gold and silver mining, and as an ingredient in various chemicals and lubricants.
Top Dross and Scrap Become Zinc Oxide
Last but certainly not least among U.S. Zinc’s recycling operations are its three zinc oxide facilities, which produce about 11 million pounds of oxide per month—making this the company’s largest-volume business.
About half of this oxide is made from primary zinc metal, with the remainder produced from approximately 5 million pounds of continuous galvanizer’s top dross and zinc scrap per month. Their transformation goes something like this: The different feedstocks are combined according to proprietary recipes to achieve different grades of zinc oxide, then melted together in sweat furnaces at about 900º F. The resulting molten material is then transferred to muffle furnaces that heat the zinc to between 2,100º and 2,400ºo F, its vaporization point, thus following a process much like that used to make zinc dust.
Where the process differs from making zinc dust is that oxygen is introduced into the vaporized zinc, an addition that oxidizes the particles and forms zinc oxide. Zinc oxide particles are about 80 percent zinc and 20 percent oxygen, and, although about the same size as zinc dust, the particles are much lighter due to their oxygen content.
To finalize the process, the particles are blown up into baghouses that capture and store the zinc oxide. Some of the oxide is sold in this form, but the company also further processes it into pellets and gives it surface treatments, depending on specific customer needs.
More than half of those customers are tire and rubber manufacturers, which rely on zinc oxide during vulcanization to give their products elasticity, strength, and stability. A variety of other customers use the zinc oxide as an anti-corrosive and protective coating for metals, ceramics, and electronic components and as an additive to lubricants, paints, glass, and pharmaceuticals.
Good Things Come in Many Packages
So, that’s how U.S. Zinc makes each of its three products, but these processes are only part of what the company’s executives consider their final product: the packaged material.
“Packaging is the key to customer service,” says Steve Brown, vice president. Thus, the company offers a range of zinc dust and zinc oxide packaging options to meet just about any customer request, including a variety of sizes of paper and plastic bags, low-melt bags, 2,000- to 3,000-pound bags, paint cans, and drums of varying sizes to name a few. The dust and oxide operations can also affix bar codes and customized labels on these packages as a customer service.
The firm also considers its quality control processes a key part of its customer service efforts. On-site laboratories analyze not only the raw materials coming into U.S. Zinc’s facilities, but also their end products, and each shipment comes with a certificate of analysis. “No competitors do what we do in terms of quality, quantity, scope of products, and packaging,” says Brown.
Zinc From A to Z
Even within the metal industry, zinc is one of those materials few know much about. Sure, most understand it’s a common mineral found in vitamin supplements and its zinc oxide form serves as a cream to put on noses at the beach, but these uses only scratch the surface of the metal’s common applications.
According to the American Zinc Association (Washington, D.C.), zinc is primarily used for galvanizing steel, a process in which steel is coated with zinc by dipping or running it through baths of melted zinc to protect it from corrosion, even in salty environments. In addition, zinc dust is often sprayed onto steel as a corrosion-resistant primer and used as an additive to paint for the same protection.
Zinc oxide, meanwhile is essential to making rubber, and this particular application is expected to skyrocket as more and more tires are needed for the expanding global automobile market.
The metal is also die cast into complex and enduring forms for use in electronics, automobiles, and airplanes. Plus, it’s alloyed with copper to make brass.
The medical community is looking into the role zinc plays in bolstering immunity, fetal development, treating male infertility, and treating eating disorders.
And just in case you thought zinc could do everything but cure the common cold, some say zinc gluconate lozenges could ease cold symptoms and hasten the course of colds.•