Matthew J. King

Chapter 21 – Byproduct and Waste Streams

The processing of byproduct streams is a significant activity at copper concentrators, smelters, and refineries. The treatment of several common byproducts is the subject of this chapter. MoS 2 occurs in economic quantities in many porphyry Cu deposits. It is recovered to MoS 2 flotation concentrates by: floating MoS 2 and Cu–Fe–S minerals together in a bulk Cu–Mo concentrate, then; depressing Cu–Fe–S minerals and floating MoS 2 in a Mo–Cu flotation separator plant. Typical Mo recovery to final MoS 2 concentrate is ∼70% (80% to the bulk concentrate and 90% of that to the final MoS 2 concentrate). Other byproducts from copper production include the slime from electrorefining, dust recovered from bag houses and electrostatic precipitators, and smelting slag. All of these can be treated to recover their copper content, and the remaining material is also valuable. The slime, in particular, contains significant levels of gold, silver, selenium, and tellurium. The recovery of these metals has a significant impact on the profitability of copper production facilities.

HYDROMETALLURGICAL COPPER EXTRACTION: INTRODUCTION AND LEACHING

This chapter focuses on the hydrometallurgical processing of copper oxide and chalcocite ores. Hydrometallurgical extraction accounts for about 4.5 million tonnes of cathode copper per year (about 20% of total primary copper production). Most of this is produced by heap leaching. Heap leaching consists of trickling H 2 SO 4 -containing lixiviant uniformly through flat-surfaced heaps of crushed ore, agglomerate, or run-of-mine ore. Oxide ores are leached by H 2 SO 4 without the need for oxidation. Chalcocite (and, to a much lesser extent, bornite and covellite) needs to be oxidized and leached by H 2 SO 4 -containing solutions in the presence of oxygen and/or Fe 3+ . Leaching of sulfide ores at economic rates is made possible by indigenous bacteria, which increase the mineral leaching kinetics by several orders of magnitude. The rate determining kinetics is that of diffusion in the saturated zones of the ore. The bacterial activity is maximized at a pH of 1.5–2, a temperature of ∼30°C, and an adequate O 2 supply. The rate of leaching and the overall copper recovery are maximized by optimizing the conditions for diffusion control: crush size, acid curing, agglomeration, heap permeability, lixiviant composition, aeration, and bacterial activity.

Production and consumption

Worldwide, about 200 million tonnes of sulfuric acid are produced per year. Sixty percent comes from burning elemental sulfur. The remainder comes from the SO2 in smelter, roaster, and spent acid regeneration furnace offgases. Sulfuric acid is produced around the world. China is the largest producer. By far, the largest use of sulfuric acid is for making phosphate fertilizers, e.g., ammonium phosphate. Other large uses are for making other fertilizers and chemicals of all sorts. Sulfur acid price averaged about

Production of H 2 SO 4 (ℓ) from SO 3 (g)

50 per tonne from 2003 to 2007, spiked to over

Three catalyst bed acid plant

400 in 2008, and settled at

Costs of sulfuric acid production

150 per tonne in 2012.

Dehydrating air and gases with strong sulfuric acid

The final step in sulfuric acid manufacture is the production of H2SO4(l) from SO3-bearing gas. The H2SO4 is made by sending strong sulfuric acid down around ceramic saddles in a packed bed while blowing SO3 gas up through the bed. SO3 in the ascending gas reacts with H2O(l) in the descending acid to produce strengthened sulfuric acid, i.e., SO3ginSO3,O2,N2gas+H2Olin98.5mass%H2SO4,1.5mass%H2Osulfuricacid→80-110°CH2SO4linstrengthenedsulfuricacid~99mass%H2SO4 The strengthened acid is mostly diluted and sold. Some is recycled to the dehydration and absorption towers. Most sulfuric acid plants are double contact plants. They efficiently oxidize their feed SO2 to SO3 and efficiently make the resulting SO3 into H2SO4(l). Single contact plants are simpler and cheaper, but their exit gases contain more SO2.

SO 3 gas recycle for high SO 2 concentration gas treatment

This chapter evaluates how SO3, CO2, SO2, and O2 concentrations in feed gas, catalyst bed pressure, and catalyst bed input gas temperature affect maximum catalytic SO2 oxidation efficiency. Only input gas temperature has a significant effect. Cool input gas (but warm enough for rapid catalytic SO2 oxidation) gives highly efficient SO2 oxidation. Warmer input gas gives less efficient oxidation. The chapter also evaluates the books assumptions. The major assumptions of no heat loss and nonattainment of equilibrium cause small offsetting effects.

After-H 2 SO 4 -making SO 2 oxidation

Study estimate investment and production costs for sulfur burning and metallurgical sulfuric acid plants are provided. Spent acid regeneration type acid plants are expected to have slightly higher investment costs than sulfur burning type acid plants. Use of stainless steels in the construction of acid plants means that their investment costs are often closely linked to the price of stainless steel. The use of fabrication facilities located in China and India has lowered acid plant investment costs. Production costs for sulfur burning acid plants are driven mostly by sulfur and electricity prices. Credits for by-product electricity are significant. They appreciably lower the production costs of sulfur burning acid plants.

Minimizing sulfur emissions

SO2-bearing gas must be dry before it goes to catalytic SO2 oxidation. Otherwise, the SO3 made by catalytic oxidation will react with the gass H2O(g) to form corrosive liquid sulfuric acid in cool flues and heat exchangers, especially during shutdowns. This problem is avoided by dehydrating (i) sulfur burning air and (ii) scrubbed metallurgical/spent acid furnace off-gas by contacting these gases with strong sulfuric acid. Dehydration is represented by the reaction: H2Og+H2SO4linstrongacid→H2SO4·H2Olinslightlyweakenedacid Industrially, the process is carried out in brick-lined stainless steel towers packed with ceramic saddles. Acid descends around the saddles where it meets and reacts with ascending H2O(g)-laden gas.