Some Design and Economic Considerations

Hydrometallurgical processes are designed around key specifications: (a) chemical composition and physical properties of raw materials, and (b) specifications for the products. All ores are different. There are various specifications for a given product, depending on the use. This determines the value of the product. LME grade copper has <0.001% iron, i.e. not more than 10 ppm Fe by weight. The Fe:Cu ratio in the product must be <10-5:1. But, iron may be present in an ore at >10 times the copper content. An ultimate separation factor in this case of >106:1 is required. Obviously this will impact the design of the process.

Metals are broken down into two basic classes: precious metals and base metals. The precious metals are gold, silver and PGM’s. Base metals refer to the lesser value metals such as aluminum, zinc, lead copper, nickel etc. (Prices of some metals have increased so much in recent years that they could almost be reclassified as precious metals.)

Water has a high heat capacity, so where heating is required energy costs are significant. Careful design and use of heat exchangers to use waste heat can lower these costs. Hydrometallurgical processes are conducted at temperatures usually well below those in pyrometallurgical processes, and using much lower concentrations of materials. This results in lower reactions rates. Hence for a given throughput larger reaction volumes and equipment are involved, and as a result, larger amounts of metal are bound up in the plant. This amounts to an inventory cost. For a high value metal like gold this can be significant.

A rough breakdown of principle costs in a hydrometallurgical process is shown below:

Fixed costs include plant depreciation, infrastructure and inventory (of ore, metal). Operating costs include reagents, energy, labour, waste treatment, maintenance and marketing.

Reagents and their costs are a critical parameter in process selection. A high-value metal may justify use of expensive reagents. A low-value metal requires cheaper reagents. For base metals the amount of money that can be spent on reagents is a fairly small part of the total cost. This greatly diminishes the number of viable processes. However, expensive chemicals may be used, if efficient and cost effective recycling processes can be applied. For example, NaOH used in bauxite leaching to dissolve Al(O)OH is quite expensive, but most of it can be recovered for recycle:

Al(O)OH s + NaOH aq + H2O l = Na[Al(OH)4]aq

(This reaction uses high temperatures in autoclaves.)

Na[Al(OH)4]aq = Al(OH)3 s + NaOH aq

After cooling the leach solution Al(OH)3 precipitates and regenerates NaOH. (The Al(OH)3 product is calcined to form Al2O3 which is mostly used as a feed for Al metal production by electrolysis.)

Costs of reagents typically should not exceed ~10% of the process costs for base metals. Some common reagents and their approximate bulk prices are listed below. Prices may vary greatly. For example, the cost of cyanide can jump by 100% when the price of gold increases dramatically. The most common hydrometallurgical reagent is probably sulfuric acid, H2SO4. It is the cheapest strong acid. Sulfuric acid is made from elemental sulfur, which is generally abundant and inexpensive (though prices vary). In fact, sulfuric acid is the number one or two tonnage chemical produced in the world. The world is mostly an oxidizing place and the oxides of the main group elements at the right end of the periodic table are acids. (The common strong acids are derived from N, S and Cl.) Bases are also critically important in hydrometallurgy. The main ones are CaO (lime; the most commonly used), NaOH and CaCO3. The order of base strength (extent of dissociation to form OH- in water) is NaOH > CaO > CaCO3. The cost increases in the order CaCO3 < CaO < NaOH. While reagents costs in the table below are rough estimates, they do provide a sense of the relative costs.

Equipment costs must also be considered. Important factors are:

• complexity – pressure vessels cost much more than open tanks.
• materials of construction – corrosive chemicals require expensive alloys or linings.
• maintenance costs – complex equipment/corrosive chemicals result in higher maintenance costs.

A key issue in selecting materials of construction is resistance to corrosion. Leaching is often the most chemically aggressive step. Mild steel is relatively inexpensive, but is suitable only for solutions that are not very corrosive. Dilute sulfuric acid solutions can be handled with stainless steel quite safely, so long as the temperature is not too high. Chloride-based reagents are more corrosive towards steel. Corrosion resistant alloys and titanium can be used, but these are expensive. Rubber, lead and brick may be used to line steel vessels, which add to the cost. Plastics may also be used, e.g. for electrolytic cells in electrowinning plants. Table 11  provides an indication of materials required for some reagents.

Before a process is implemented an economic evaluation is required. This grades up through stages of increasing complexity, cost and accuracy as the phases of a project proceed. Stages may include a study of feasibility, which would indicate if further work is warranted for a given ore body. Bench scale (laboratory) testing to evaluate if available processes are suitable or to develop new ones (potentially a very lengthy and costly undertaking), piloting (a small scale continuous plant), possibly a demonstration plant (an intermediate scale production plant) and finally a full scale production plant. At each stage an economic assessment is done to determine if an adequate return on investment can be obtained. Important variables include:

• Revenue –  the sum of all payments received for all saleable products.
• Investment costs –  the sum of all capital costs for all plant, site preparation, ancillary plant (e.g. power plants) and off-site facilities.
• Production costs –  direct operating costs (everything required for day to day running of the plant, such as reagents, alternate feed sources, labour, energy etc.) and capital charges (interest).