Question 2 of 6: Ten hydrometallurgical/electrochemical term pairs

Problem 2 — Ten hydrometallurgical/electrochemical term pairs (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Hydrolysis vs. Hydration. Hydration is the electrostatic association of water molecules (as intact H2O dipoles, via ion-dipole attraction) around a dissolved ion, forming a hydration sheath (solvation sphere) — no chemical bonds are broken. Hydrolysis is a chemical reaction in which a dissolved metal ion reacts with water and breaks an O-H bond, releasing H+ (or OH-) and forming a hydroxo- or oxo-complex, e.g. $\\text{M}^{n+}+\\text{H}_2\\text{O}\\rightleftharpoons \\text{M(OH)}^{(n-1)+}+\\text{H}^+$. Difference: hydration is purely physical (solvation), hydrolysis is a proton-transfer chemical reaction that changes the ion's own speciation and lowers solution pH.

b) Speciation diagram vs. Distribution diagram. A speciation diagram plots the concentration (often log-concentration, as in Problem 3) of every dissolved AND solid species of an element against a single master variable (usually pH or Eh) at one FIXED total concentration. A distribution diagram (fractional-distribution or $\\alpha$-diagram) plots the FRACTION of total dissolved metal present as each species (fractions sum to 1 at every pH) – it deliberately discards the absolute concentration axis to show only relative speciation. Difference: a speciation diagram carries absolute concentration information (and can show solid-phase stability fields), while a distribution diagram is normalized and shows only relative proportions among the dissolved species.

c) Cementation vs. Precipitation. Cementation is a specific electrochemical (redox) metal-recovery method: a more active (less noble) metal, added as scrap or powder, is oxidized and dissolves while simultaneously reducing a less active dissolved metal ion onto its surface as solid metal, e.g. $\\text{Fe(s)}+\\text{Cu}^{2+}\\rightarrow \\text{Fe}^{2+}+\\text{Cu(s)}$. Precipitation is the broader term for forming any insoluble solid from solution (metal hydroxide, sulfide, carbonate, etc.) by adding a reagent that exceeds the solid's solubility product – no electron transfer between two metals is required. Difference: cementation is always a redox (electron-transfer) reaction between two metals; precipitation is a general solubility-driven phenomenon that need not involve any change in oxidation state (e.g. Ni(OH)2(s) forming from Ni2+ + OH-, Problem 3, is precipitation, not cementation).

d) Anode vs. Cathode. The anode is the electrode at which OXIDATION occurs (electrons leave the electrode into the external circuit). The cathode is the electrode at which REDUCTION occurs (electrons arrive from the external circuit and are consumed). Difference: this oxidation/reduction assignment is the same in both a galvanic and an electrolytic cell; what differs (part e) is which electrode carries the + or - terminal sign.

e) Galvanic cell vs. Electrolytic cell. A galvanic (voltaic) cell converts a spontaneous chemical reaction ($\\Delta G<0$) directly into electrical energy; it generates its own EMF and drives current without an external power source. In a galvanic cell the cathode is the positive terminal and the anode the negative terminal. An electrolytic cell (as used throughout electrowinning in this paper) consumes electrical energy from an external DC source to force a non-spontaneous reaction ($\\Delta G>0$) to proceed. In an electrolytic cell the anode is connected to the positive terminal and the cathode to the negative terminal (Problem 5b). Difference: the sign of $\\Delta G$ (spontaneous vs. forced) and the terminal polarity are reversed between the two, even though oxidation still occurs at the anode and reduction at the cathode in both.

f) Solubility vs. Dissolution. Solubility is an equilibrium (thermodynamic) property: the maximum concentration of a substance that can remain dissolved in a solvent at equilibrium, at a given temperature/pH/Eh (e.g. the Ksp-controlled curves of Problem 3). Dissolution is the kinetic PROCESS of a solid entering solution over time; its rate depends on surface area, agitation, temperature, and the driving force (how far the bulk solution sits below the solubility limit). Difference: solubility is a fixed equilibrium endpoint (a number); dissolution is the time-dependent process of approaching that endpoint, and can be fast or slow independent of the equilibrium solubility value.

g) Saturation vs. Supersaturation. A saturated solution holds exactly the equilibrium (solubility-limit) concentration of a solute at a given temperature – it is in stable equilibrium with the solid phase (undissolved solid, if present, neither grows nor shrinks). A supersaturated solution holds MORE dissolved solute than the equilibrium solubility limit – it is metastable, and will spontaneously precipitate solid (given a nucleation site) to relax back toward saturation. Difference: saturation is a stable equilibrium state; supersaturation is a metastable, higher-concentration state that is thermodynamically unstable with respect to precipitation.

h) Filtrate vs. Raffinate. The filtrate is the liquid that has passed through a filter medium, having been physically separated from suspended solids (a solid/liquid separation product, e.g. Problem 1's S/L separation step). The raffinate is the liquid phase LEFT BEHIND after a liquid/liquid solvent-extraction step has removed the desired solute into the organic phase – i.e. the depleted aqueous phase (often recycled to leaching, Problem 1). Difference: filtrate results from solid/liquid separation (filtration); raffinate results from liquid/liquid separation (solvent extraction) and specifically denotes the depleted, extracted-from phase.

i) Ion exchange vs. Solvent extraction. Ion exchange (IX) uses a solid resin (or zeolite) bearing fixed, exchangeable ionic functional groups; target ions from solution swap places with the resin's own mobile counter-ions as the solution percolates through a solid resin bed. Solvent extraction (SX) uses a water-immiscible organic liquid containing an extractant that selectively complexes the target metal ion and partitions it from the aqueous phase into the organic phase, which is then physically separated (settler) and stripped. Difference: ion exchange is a solid-phase (resin-bed) process suited to dilute solutions and requires no liquid/liquid phase separation; solvent extraction is a liquid/liquid process, generally preferred at higher concentrations, and requires mixer-settler equipment to separate the two liquid phases.

j) Electrowinning vs. Electrorefining. Electrowinning (EW) deposits a metal, by electrolysis, FROM A PURIFIED AQUEOUS LEACH SOLUTION onto an inert (or starter-sheet) cathode; the anode is typically inert (e.g. Pb-alloy or Ti, evolving O2 or Cl2, as in Problem 4). Electrorefining (ER) purifies an already-metallic, impure anode (cast from smelter/leach product) by dissolving it electrolytically and re-depositing pure metal on the cathode, leaving insoluble impurities behind as anode slime. Difference: electrowinning's anode is inert and its feed is a solution; electrorefining's anode IS the impure metal being purified, and the anode itself dissolves as the process runs (both use the same basic electrolytic-cell wiring of Problem 5b).