by Robert 0. HoEfland, ~rcsident and Atlantes Chemical Systems

Transcription

1 by Robert 0. HoEfland, ~rcsident and Joe Portera, Sales Manager Atlantes Chemical Systems One of i:he pr:>bl~ms with the dry proces:; for hot clip galvanizing is contamination of tile zinc aininonium chl.or ide preflux bath. Carryover from the pickle line, along with reaction of the preflux with the steel racks and with the work itself, results in a buildup of iron salts and acid. Smith Industries, Tnc. a Houston-based firm, tackled this problem by adding a continuous, automated system for treatment and removal of contaminants from the preflux. The treatment system, designed and installed by Atlantes Chemical Systems of Houston, Texas, maintains consistent purity of the preflux solution, eliminating the problems caused by contaminated flux solution. Quality and productivity are improved, and zinc wasted in the kettle as bottom dross and top skimmings is reduced, without the tedious work and wasted flux solution of previously used batch treatment methods. Smith Industries' Galvanizing Division is a custom operation, coating everything from small angles and flat bar to 18-m (60-foot) transmission poles. Separate pickle lines supply a 7-m (23 foot) and 12.8-m (42 foot) kettle. Faced in late 1976 with strong regulatory pressure to reduce airborne emissions of ammonium chloride, the Galvanizing Division converted completely to the dry galvanizing process, already being used successfully in the company's Fastener Division. Note: Airborne ammonium chloride - results from floating. the zinc ammonium chloride flux on the surface of the hot kettle "Dry process" versus "Wet process'' is galvanizing Jargon. The wet process is the process where the ammonium chloride is floated on the hot kettle surface in a molten salt form. All steel entering the kettle must pass through the layer of 1

2 molten salt which instantaneously fluxes the part. The "day process" involves fluxing the parts in an aqueous solution of zinc ammonium chloride. The parts are premitted time to dry before entering the kettle. The ammonium chloride top flux was repl-aced with an aqueous zinc ammonium chloride babh before each kettle. ~ r galva- y nizing proved to have many advantages in addition to reduced air pollution, but was also found to be very sensitive to contamination of the flux. Frequent disposal of over 45 K L (12,000 gal.) of fl-lux solution, valued at over 19.8 $/L(75 $ per gal.) was too costly to consider seriously. Widely recommended methods to add chemicals, settle, and decant the flux into another tank proved very tedious, as well as wasting large amounts of good solution. Techniques for producing a smaller volume of thicker sludge reduced the waste to sone degree, but increased the labor involved in pumping or shovelling the waste material out of the tank. After results with a small batch treatment system indicated that substantial savings could be realized, Smith management approved the installation of a continuous unit. The preflux baths contain a solution of zinc ammonium chloride tripple salt (ZnCI2-3NH4CI) at a concentration of 358 g/l (3 lb/gal.), heated to an average temperature of 66 C. (1500F.). A ph of 5 is maintained by the presence of slab zinc, which reacts with acid carried in from the pickle line. Contaminats consist-primarily of iron salts. Ferrous (Fe2+) iron is dissolved in the solution, while ferric (Fe3+) iron is mainly found as a precipitate at the bottom of the tank. For purification, the solution is pumped from the bottom of the flux bath to a continuously stirred treatment tank, where Hydrogen peroxide and ammonium Hydroxide additions are controlled by an ORP (oxidation reduction poten-.tial) and ph sensing system. The net reqult is the conversion of soluble ferrous iron to the insoluble ferric form, at a ph which maximizes precipitation of the iron while minimizing precipitation of the zinc. 2

3 The treated material passes to a second continuously stirred tank, where a polymer (Calgon L 690 E) is added to aid in precipitation of the insoluble material. After overflowing into a surge tank, the solution is pumped to a clarifier equipped with a tribe settler. The clear s~ipesnatant overflows back to the pickle line, while the sludge is compacted in a specificall-y designed acid resistant filter press. Filtrate from the filterpress flows back to the treatncnt tank, while the compacted solids are placed in drums for disposal. A central control panel electrically &ctiv?te.; mlenoid valves controlling two air-operated diaphragm pumps, the gravity feed from the clarifier, and a chemical feed from the four reagent tanks. The panel also includes a two-channel recorder for continuous monitoring of ph and ORP, the alarm system and an auxiliary air conpressor. The system is designed to turn over the flux tanks once every 24 hours, The rate of cleanup can be adjusted by increasing or decreasing the pumping rate, or by operating the system for less than 24 hours daily. This flexibility is important for a job-shop, because the rate of contamination varies considerably with the type of work being run. Benefits from the purification system have been numerous. Studies at Smith had shown that 50 % of the consumption of flux was associated with losses during the settle-and-decant purification step. It was not surprising to find that concentration of the waste solids with the filter press resulted fn a more than 40 % decrease in flux consumption. Labor associated with the automated system is confined to adding chemicals and water to the 379-L (100 gal) reagent tanks, and moving chemicals and sludge in and out of the area with a forklift. In contrast, the settle-anddecant treatment required several manhours of labor, which had to be carried out at overtime rates to avoid interfering with production. Besides the addition of large quantities of chemicals at one time, and pumping several thousand gallons of flux into another tank and back, it was necessary to remove up to 11.3 K L (3,000 gal.) of a hot, thick, corrosive, acidic sludge from th bottom of the preflux tank under severe time constraints. Elimination 3

4 of this unpopular task not only saves time, it helps employee morale. With the old process, the most efficient method of removal of the sludge was to pump the..n,ijoi:ity into a t=iiik truck for disposal as liquid waste, arid shovel the r(2rnai.nder into driiins. TJastc disposal (;OSi:S were incrca5e.d by the addition of water to the flux to facilitate pumpiiig. While the rates for solid waste disposa'l are even higher than those for liquid waste in Houston, the greatly decreased volurne still rnsnlts in a saviuys. Important as these improvelnents rire, khey do not compare with the savings realized by increased production and decreased waste of zinc. Increasing contamination levels i n the flux lead to increasing levels of dross and skims, and in turn to increasing effort for the men working the kettle, as they ateinpt to "wash" the waste products off the work. Inmersion times are increas?d, resulting in excessive zinc coatings, which are not only wasteful but undesirable from a quality viewpoint. If not corrected, hard work gives way to an increasing proportion of rework, and can lead finally to total shut-down. By operating at a consistently low level of contamination, it is possible to simultaneously improve the efficiency of zinc utilization (the proportion of the total zinc that is used on the work and not wasted), and to produce work with a thinner coating. The total savings to be realized depend, of course, on the original degree of contamination. This system does not control the composition of the flux, just the contaminants. It is possible, however, to control the ratio of zinc to ammonium in the solution by appropriate selection of the ph setting in the treatment tank. Otherwise, the zinc additions are controlled by the amount of ferrous iron buildup. These two factors will vary from plant to plant, depending on such factors as immersion times and aeration of the flux bath. For a plant using sulfuric acid, the use of a clarifier and filter press should permit substitution of calcium chloride for the barium chloride, which is considerably less expensive and simplifies disposal. I 4

5 . Before deciding on continmus treatment, company officials considered several atlernatives for reducing flux contamination. Racking systems with minimal exposed surfaces to minimize drag-in; cranes designed for rapid and effective drainage of the work; double rinse t~nk:;; c1.can rinse water; and good rinsing tcchniqllc can all work together to maintain f3 UX quality. All. these factors should be considered in the design of a new facility. For an existing plant, however, where space can bo limited dnd racking systems and cranes are alr*?ady in plac~?, continuous cl,?anup can be a si.inpic!, cqst eff%?ctfvo, ;mediate solution to the probl-em of gliix contaxindtian. The flux purification system was custom fabricated by Atlantes Chemical System of Houston, Texas, The filter press is a model A-30 also manufactured by Atlantes Chemical System. The transfer pumps are hastalloy air diaphragm units manufactured by Wilden Pump of Colton, California. All instruments and recorder are Leeds & Northrup. 5