Table of Contents
Stainless Steel Offers Many Advantages for containing strongly acidic leaching reactions in copper recovery operations. Hydrometallurgical techniques for copper extraction have been known probably since 1752, but the dissolution of copper by leaching and the recovery from solution by precipitation or electrowinning did not become significant in the US until the mid-1960s. Today, hydrometallurgy accounts for a large percentage of US primary copper production and virtually all copper producers now use or plan to install some form of leaching and recovery operation.
The environment of principal concern in copper leaching is a solution containing up to 20% sulphuric acid (by weight) and various concentrations of copper sulphate and other metal sulphates. Temperatures range from ambient to boiling. Acid corrosion in this environment can demand constant maintenance—one of the largest plant operating costs. Failure of components through corrosion can render a substantial portion of any plant temporarily inoperative. As a result, a growing number of leach plant engineers are expressing a preference for the use of stainless steel in leach plant pumps, piping, valves, heat transfer equipment, tanks, and various other structures because of its proven success in handling corrosion problems. Table 1 shows the compositions of the AISI stainless steels most important in these applications.
Corrosion resistance of stainless steels
The chromium in stainless steels imparts a passive chromium oxide film to the metal surface, with an accompanying resistance to corrosion. As service conditions become more severe, successively higher chromium percentages are required to promote formation of this passive film and preserve its integrity under a broad range of conditions. The addition of molybdenum to stainless steel also improves corrosion resistance, while addition of nickel improves both fabricability and corrosion resistance.
An especially useful property of stainless steels is that they withstand those oxidizing conditions that are most harmful to ordinary steel and to many nonferrous metals. However, some concentrations of relatively pure sulphuric acid, and reducing environments in general, are not conducive to the formation and retention of the passive film. The data in Fig. 1 show how an increased percentage of nickel and the addition of copper will increase the range of concentrations and temperatures within which stainless steel will exhibit satisfactory resistance to sulphuric acid.
Oxidizing salts, such as cupric and ferric salts, when dissolved in significant amounts in sulphuric acid solutions, exert a corrosion-inhibiting effect on the solubility of stainless steels in that acid. Such salts also help maintain the passive film past the normal range of resistance. To illustrate the effect, a laboratory test exposed specimens to a cupric sulphate/sulphuric acid solution at 150° F. The results of this test are shown in Table 2.
Kiefer and Renshaw investigated the inhibiting effect of metal sulphates on the solubility of stainless steels in sulphuric acid. Various amounts of ammonium, sodium, manganese, iron (ferrous), nickel, tin (stannous), and copper (cupric) sulphates were added to 5% and 30% sulphuric acid. AISI steel types 304 and 316 were tested in these solutions at 100°, 150°, 175°, and 200° F. Figs. 2 and 3 show typical data obtained in 5% sulphuric acid at 175° F. Some of the findings:
- The presence of any of the sulphates materially lowers the corrosion rate.
- As temperature increases, the inhibiting effect of the sulphate’s becomes less pronounced.
- The sulphates of metals above nickel in the Standard Electromotive Series vary only slightly in their inhibiting effect, and none provides complete inhibition. Sulphates of metals above iron have little effect.
- The valence state of the sulphate is very important; higher valence sulphates are extremely effective even at low concentrations.
Such laboratory data is well supported by extensive, successful use of stainless steels in leach and recovery plants throughout the western US. One plant uses equipment constructed of about 300,000 lb of AISI 316L stainless steel in leaching, solvent extraction, and electrowinning operations, and the plant manager reports no problems with stainless after three years of almost continuous operation. Such reports are typical among operating and maintenance engineers in many areas.
Picking the right stainless steel
Pitting corrosion may occur in stainless steel equipment even though its general corrosion resistance is excellent. Pitting results from a highly localized breakdown in the passive oxide film, and may occur in the presence of chlorides. (Unless chloride levels are relatively high, such attack is not likely.) In addition, accumulation of solids on metal surfaces is conducive to pitting and should be avoided where practical. In general, AISI 316 stainless, which contains 2-3% molybdenum, is more resistant to pitting attack than AISI 304, and its greater cost is frequently justified for copper recovery applications.
Intergranular corrosion is another problem. When some 300 series stainless steels are heated to 800-1,500° F, some of the chromium and carbon in the metal combine to form chromium carbide, which precipitates at the grain boundaries. Chromium carbide formation causes localized depletion of chromium at the grain boundaries, which can lead to intergranular corrosion in acidic environments. Hot copper sulphate/sulphuric acid solutions, for example, strongly promote intergranular corrosion. Stainless steel in which chromium carbide has formed is referred to as having been “sensitized.” This condition most commonly occurs when heat treatment of the material is not performed after weld fabrication.
Intergranular corrosion can be avoided by heat treating equipment after weld fabrication, by using low-carbon stainless steels (such as types 304L and 316L), or by employing stabilized stainless steels. In low-carbon grades containing a maximum 0.03% carbon, there simply is not enough carbon available to produce a damaging amount of chromium carbide. In stabilized stainless steels, the carbon preferentially combines with the stabilizing agent (titanium or columbium), thereby preventing precipitation of chromium carbide.
Crevice corrosion may occur at contact surfaces of flanged joints and incompletely welded attachments. In these areas, corrosive media penetrate between the surfaces to form a stagnant area or pocket, resulting in a depletion of inhibiting copper salts and entrapment of corrosion products. The contact area may become sufficiently anodic to adjacent areas to cause localized breakdown of the passive film and active corrosion. This typically results in failures under gaskets at flanged joints, and under sludge or slimes settling in tanks.
Type 316 stainless is more resistant to crevice corrosion than type 304. However, metallurgical composition is not the only factor influencing crevice corrosion. Design and fabrication details are important, and attention to a few fundamentals can avoid crevice corrosion:
- Provision should be made for free and complete drainage of equipment, avoidance of crevices, and ease of cleaning and inspection.
- Butt joints with complete weld penetration should be used wherever possible.
- If lap joints with fillet welds are necessary, the welds should be continuous on the process side.
Specific uses of stainless in copper recovery
In the precipitation recovery method, copper is precipitated from solution by contact and cementation with metallic iron, such as shredded scrap detinned cans, and the spent solution is reused for further leaching. The precipitation reaction is:
One western plant uses cone-type precipitators developed by Kennecott Copper Corp. Into each tank, 14-ft dia and 20 ft high, an inverted 10 x 10-ft cone is mounted. The annular space between the inner cone and the tank is covered by a heavy-gauge type 304 stainless screen and holds about 15 tons of iron scrap. Pregnant leach solution is pumped up through the scrap, while the copper precipitate settles down through the stainless screen into the annular space, to be discharged intermittently.
Precipitates from the cones are pumped to a filter plant where they are dewatered in a 48-in. all-stainless filter press equipped with stainless steel filter screens. The tanks containing the cones are wood, reinforced with ¾-in. -dia type 304 stainless bar hoops. A 4-in.-dia type 304 stainless pipe carries 92% sulphuric acid from the acid plant to the point where it enters the tailing water from the precipitation plant.
The plant has 34 multiple-stage, vertical turbine pumps handling leach water. Pump bodies and impellers are AISI 304 (cast), and the 11-ft-long pump shafts are precipitation-hardened stainless steel. Except for some minor erosion-corrosion problems with the pumps, the plant has had very little trouble with the type 304 steel.
Good copper recoveries can also be achieved by vat leaching of copper oxides and silicates with dilute sulphuric acid. In one vat leaching plant, crushed ore is placed in 110 x 100 x 18-ft concrete vats and leached with up to 10% sulphuric acid solution. Most piping, pumps, valves, clarifier arms, and even the 8-yd clamshell bucket at the plant are fabricated of AISI types. 316 and 316L stainless steels. According to the maintenance superintendent, service has been virtually trouble-free.
In the slimes leach tank and clarifier at the plant, fine particles of silicate are leached and settled. The steel “rake” that scrapes the tank bottom is constructed of type 316 stainless structural shapes. The slimes are so abrasive that, for the most part, rubber-lined pumps are used here— even the valves are rubber pinch valves. However, stainless gate valves back up the pinch valves.
This plant originally used PVC-lined piping to a great extent, until leaks developed at the pipe ends. Since the PVC pipe cost little for 12-in.-dia, 40-ft-long sections, every effort was made to salvage end-damaged pipe. However, the repair cost was prohibitive, so as the pipe failed, replacement was made with type 316 stainless pipe. All pumps and valves in the leach area are now stainless.
The clamshell bucket used for loading leach vats was originally mild steel, requiring wear-plate replacement every week and complete rebuilding every 60 days. After success in extending wear-plate life to five weeks by switching to type 304 stainless plate, the plant purchased a new bucket made entirely of stainless steel. Now repairs are needed only twice a year, at a fraction of the original cost.
Liquid ion exchange, otherwise known as solvent extraction is not a new process; however, its application in copper leaching is relatively new. The process utilizes a special reagent that has an affinity for copper ions in a weak acid solution and a low affinity for other metal ions. The reagent operates on a hydrogen ion exchange cycle, which proceeds as follows: The reagent, carried by an organic medium (kerosene), is intimately contacted with aqueous leach solution in the extraction system. Hydrogen ions are exchanged for copper ions, thus regenerating the sulphuric acid in the leach solution, while the copper is extracted. The copper-containing organic medium passes to the stripping system where it is contacted with aqueous copper sulphate in the presence of sulphuric acid. Copper ions are exchanged for hydrogen ions, thus regenerating the reagent, while the enriched copper sulphate solution is essentially free of impurities and ideally suited for electrowinning.
One of the first solvent extraction plants in the US, in operation since 1970, produces about 40,000 lb per day of 99.9% pure copper via this method. The plant was built using types 316 and 316L stainless for all wetted surfaces in the extraction area. There have reportedly been no problems with corrosion. Mix tanks at the plant are constructed of 3/16-in. type 316L stainless plate, while the 8 x 9-ft extraction tanks and the 7 x 9-ft strip tanks, plus all interconnecting pipes, are constructed of 0.1406-in. type 316L stainless sheet. All valves are ball or butterfly type made of 316L stainless with Teflon seats. Turbine pumps are single- or multiple-stage with stainless bowls and internal pans. Many smaller fittings, such as pipe tees, are forged from type 316 steel bar.
The enriched copper sulphate solution produced by solvent extraction must undergo electrowinning to produce high purity copper. Common practice in electrowinning is to electroplate starling sheets on 3/ 16-in.-thick hard-rolled copper or titanium blanks. However, titanium is expensive and copper blanks are subject to both corrosion at the solution line and mechanical damage from tools used to pry loose the starting sheets. In addition, copper blanks require the application of a release agent to facilitate sheet removal.
To avoid these problems, plants are using AISI 304 or 316 stainless steel for starting blanks, and most expect a 20- to 30-year service life. These blanks have an AISI 2B surface finish.
Stainless steels have also been used extensively in electrolytic copper refining equipment in tankhouses, cell acid purification and recovery, sulphate production, and in the handling of slimes. Most miscellaneous equipment, such as tankhouse tools, sheet flopping racks, anode and cathode wash boxes, and crane hooks-plus a multitude of fasteners everywhere-can be made of type 304 stainless.
However, types 316 and 316L are specifically indicated for surfaces in contact with hot solutions (200° F and higher), some of which have acid contents of up to 15% by weight. It bears repeating that low-carbon grades should be used in welding construction.
Considerable data are available showing the resistance of stainless steels to various solution conditions in electrolytic copper refining. Operating conditions that must be considered include temperature, velocity, aeration, galvanic action, and stray currents. The paper by Schillmoller and LaQue provides information on many of these subjects; however, in the absence of specific information, corrosion tests should be conducted to permit selection of the proper alloy.
Strength and hardness factors
Stainless steels are by far the strongest of the metals used in copper hydrometallurgy, a fact suggested by the stress-strain curve in Fig. 4, comparing stainless with cast iron and aluminum. Note that the curve for stainless steel continues beyond the breaking point of the other metals, emphasizing its superior strength and ductility.
Stainless steels do not exhibit a distinct yield point; their stress-strain curves show a gradual transition from a straight line to a curve. It has become customary to consider the yield strength of stainless as the point where the stress-strain curve intersects the “0.2%. offset” line (Fig. 5).
The AISI series 300 stainless steels are hardenable by cold working. This is usually done in the steel mill by cold rolling, a process that not only hardens the metal but also increases both its yield strength and tensile strength.
In addition to the standard annealed condition, some AISI series 300 steels are available in four standard tempers—¼-hard, ½-hard, ¾-hard, and full-hard. As an example of the increase in strength resulting from cold rolling, type 301 annealed has a yield strength of 40,000 psi; at ¼-hard temper, the yield strength is 75,000 psi; and at ½-hard temper, the yield strength is 110,000 psi. Type 304 stainless also work hardens, but to a lesser extent than type 301.
Even at high strengths, stainless steel retains sufficient ductility to permit forming, although additional allowance for springback is required. A recently introduced “dead soft” fully annealed stainless steel, which has a maximum yield strength of 35,000 psi, is also available for use where ease of forming is of prime importance.
Sulfuric Acid Resistant Alloys