Flowsheets

Extractive metallurgy processes, like any chemical process industry, involves sequences of operations that include:

• Materials streams (solid, liquid and gas)
• Handling equipment (e.g. piping, pumps, etc.)
• Processing equipment (e.g. reactors, separation equipment, etc.)
• Suitable storage infrastructure (e.g. tanks, stockpiles, etc.)

The flowsheet is a diagrammatic depiction of these steps. Generally, ores fall into certain general classes, like sulfides, or oxidized sulfides (which may have been converted to oxide/hydroxide minerals, silicates, carbonates and others), etc. However, each ore is also unique in terms of its mineral composition and physical properties. Thus each ore presents unique opportunities for profitable extraction and poses unique challenges (which ultimately cost money). It is imperative that the chemical, mineralogical and physical properties of the ore be well understood. This informs the decisions to be made on how to process it. At the end of the day, a flowsheet is developed with detailed information on each step in the process.

As a starting point in the development of a process the raw material must be carefully examined to determine the products that may be produced. Often a single metal is the main concern. However, numerous by-products may also be foreseeable. Depending on the ore, location and economic factors numerous classes of products may be sold. These include the ore itself, a concentrate of the ore, impure metal, refined metal and compounds. An example of the latter is NiSO4·6H2O which is sold as a by-product of platinum group metal production, even though the nickel content of the ore may greatly exceed that of the PGM’s! (There might not be enough nickel present in the ore to justify the expense of a full scale nickel plant. Then a nickel compound is sold instead.) Products may have a number of potentially applicable process options. And, a given process may be able to handle feed from a variety of sources, such as different mines or recycling. Hence the goals of an extractive metallurgy process are to:

1. Identify the suitable final products.
2. Identify the processes to be used to obtain the products.

These aspects are summarized in the diagram in Figure 9. To illustrate, a lead-zinc ore may be processed to produce several products such as lead metal, zinc metal, zinc dust (e.g. for paints) and zinc oxide (ZnO, for chemicals manufacture). Or, an ore may yield a variety of metals such as zinc, lead and relatively small amounts of precious metals (as produced at Teck in Trail, B.C.). Similarly a single metal product may be formed into a number of end products, e.g. copper as cathode (1 ´ 1 m sheets), rod, billets, wire, ingots etc. A single product may be obtained from a number of sources. INCO’s nickel plant in Sudbury, Ontario processes concentrates from various locations around the world. Finally there may be more than one possible means by which a product may be produced. For instance gold may be produced from ores by simple gravity separation, or leaching with sodium cyanide (NaCN). Sometimes, in fact, an ore may need to be treated by more than one process to obtain the metal. Gravity separation of gold is often practiced prior to cyanidation. Its cheaper and faster and the best way to recover the larger particles of gold (these may react too slowly with cyanide).

Generalized Metallurgical Flowsheet

A generalized metallurgical flowsheet is shown in Figure 10. There are three fundamental operations included. These are leaching, solution purification and metal production. Increasing levels of complexity may be seen from left to right.

In the simplest processes (at least as far as number and type of steps) the ore is simply crushed to form small rocks or pebbles without any further physical treatment (Number (1) in the figure). Heap leaching and in situ leaching fall into this category. Heap leaching involves piling crushed ore onto a base and passing solution through it to dissolve minerals. In situ leaching is essentially the same thing, but with fractured or porous rock remaining where it was found, i.e. without mining per se.

Often the ore needs to be reduced to small particle size; number (2). This involves crushing and the grinding to smaller sizes. This greatly increases the surface area and exposes or liberates the mineral grains. (See notes on Mineral Processing for definitions of mineral exposure and liberation.) Particle sizes as low as ~10 mm may be required. Particle size requirements vary greatly. The ground ore is then leached and the solution proceeds to purification etc. Cyanidation of gold ores often requires this kind of processing.

As per number (3) in the figure, it may be necessary or advantageous to perform a physical separation on the ground ore to obtain a concentrate. The concentrate may then be leached. The advantage is that much less material has to be processed (a concentration process may discard >90% of the gangue minerals in some cases). There are numerous physical separation processes. The choice depends on properties of the ground ore and the requirements of the downstream processes. An example of a hydrometallurgical process that utilizes a concentrate is oxidative, high pressure leaching of zinc sulfide in autoclaves. Zinc sulfide mineral is selectively removed from the ore without chemical modification. This concentrate is leached to dissolve zinc ions. The process is rapid compared to heap leaching.

A concentrate may be sent first for intermediate pyrometallurgical processing to convert the concentrate minerals into other compounds that are better suited to leaching. This is indicated by number (4) in the diagram. An example of this is roasting of ZnS concentrates to form ZnO. The latter oxide product is very easily leached with dilute sulfuric acid under moderate conditions (in contrast to the more severe conditions needed to leach ZnS directly). It’s a matter of economics as to which method is employed for recovery of zinc from ZnS.

Finally, the concentrate may be subjected to an entirely pyrometallurgical treatment to form matte or impure metal. This corresponds to number (5) in the figure. The impure metal may be processed further to obtain pure metal, very often by a hydrometallurgical route. Impure metal, for instance, may be electrolytically dissolved (it is made to be the anode in an electrolysis cell) and plated at a cathode to obtain purified metal. Matte may need to be chemically leached prior to metal production, or may be refined into pure metal by electrolysis, as just mentioned. Hydrometallurgical refining of metal as described here also involves leaching, but by electrochemical means (forced corrosion). The figure indicates how hydrometallurgy and pyrometallurgy may both be involved to varying degrees in the extractive process.

Generalized Hydrometallurgical Flowsheet

The general hydrometallurgical flowsheet illustrates the principal conceptual stages in most hydrometallurgical processes. It is shown in Figure 11 below. The basic steps are defined below.

Leaching. Leaching is any chemical process that dissolves minerals into a solution; in hydrometallurgical, an aqueous solution. A mineral may be wholly dissolved, as in:

CuO s + H2SO4 aq = CuSO4 aq

or (an) element(s) of interest may be dissolved, e.g.:

CuS s + Fe2(SO4)3 aq = CuSO4 aq + 2FeSO4 aq + S s

In the latter case solid CuS is oxidized by ferric sulfate to form soluble copper sulfate in solution and solid sulfur; only part of the CuS was dissolved. Note that separated minerals or the whole ore may be leached. An example of the former is leaching of a nickel sulfide concentrate with oxidizing reagents (e.g. Cl2). An example of the latter is NaOH (caustic) leaching of whole bauxite ore, containing Al(O)OH. Note then the arrows running from ore to mineral separation to leaching and from ore to leaching, illustrating both possibilities in general.

Solid-Liquid Separation

In most cases, after the leaching stage a soli-liquid separation is required. This separates the leach solution (usually bearing the valuable metal compounds) from the residual, unleached solids. This can be done by filtration or using a sedimentation procedure (that allows solids to fall to the bottom and clear solution to overflow from the top). This is often the most challenging step in a hydrometallurgical process. It requires careful testing to ensure it works properly at full scale. Note that in some cases the valuable metals reside in the leach residue and leaching is a preliminary step required to destroy unwanted minerals. An example is when very fine gold metal is dispersed in a pyrite matrix (FeS2). Then high-pressure and high-temperature oxidation (by O2) is used to leach the pyrite, leaving a solid residue that contains the gold (which may be recovered by another hydrometallurgical process).

Solution Purification

The leach solution often has to be purified to remove deleterious impurities that would interfere with production of pure metal. A great variety of chemical processes are practiced for this purpose. As an example, zinc ores are comprised mainly of ZnS. This is subjected to mineral separation and the concentrate may be leached to form an acidic solution of ZnSO4 and numerous impurities, such as Co+2, Ni+2, Cd+2, Cu+2, As and Sb compounds, etc. The concentrations of these must be lowered to very low levels prior to metal recovery, or else zinc metal cannot be recovered. Various chemical reactions are employed for the purpose. For example, adding zinc dust causes Co+2 to be reduced to solid Co metal:

Co+2aq + Zn s = Co s + Zn+2aq

which precipitates and can be easily separated from the solution. Obviously, Zn+2 entering the solution is not problematic.

Metal or Metal Compound Production

The final step is to make a pure metal precipitate, or a metal compound precipitate. This can be sold. Whether a pure metal or a compound is sold depends on the economics of the process and the desired form of the metal on the market. Note that sometimes solution purification may be bypassed and metal or a compound is produced directly from the leach solution. However, it is rare that a pure metal may be formed without intermediate solution purification.