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Classe : Terminale Enseignement : LV (anglais) et enseignement technologique

GOLD EXTRACTION

Extrait du BOEN

CONNAISSANCES CAPACITÉS

Première CBSV

En fonction du pH du milieu et du pKA du couple, une espèce d’un couple acide/base prédomine.

- reconnaître l’acide ou la base dans un couple acide/base conjugué- écrire l’équation d’une réaction acide-base à partir des deux couples acide/base mis en jeu

Terminale Chimie du tronc commun STL – STI2D

Réactions acide-base et transferts de protons. Solutions acides, basiques. pH.

- Définir les termes suivants : acide, base, couple acide-base.

- Écrire une réaction acide-base, les couples acide-base étant donnés.

Terminale CBSV

L’Homme exploite les voies métaboliques particulières d’organismes vivants dans certains procédés industriels

Exploiter des ressources documentaires

Terminale Sciences physiques et chimiques en laboratoire

Complexe, ion ou atome central, ligand

Réaction de formation d'un complexe

Complexes inorganiques

- Reconnaître dans un complexe : l'ion ou l'atome central, le ou les ligands.

- Écrire l'équation de la réaction associée à la synthèse d'un complexe.

- Extraire des informations pour illustrer des applications des complexes

Compétences transversales et attitudes

• Mobiliser ses connaissances • Rechercher, extraire, organiser des informations utiles  • Raisonner, argumenter, démontrer 

Type de ressource

• Activité documentaire • Réinvestissement et/ou approfondissement 

Résumé du contenu de la ressource

Cette séquence sur différents procédés d'extraction de l'or aborde des problèmes environnementaux ainsi que des thèmes relatifs à la chimie des solutions aqueuses. Un lien est fait avec les organismes vivants par l'intermédiaire d'un procédé de biolixiviation.

Cette activité se divise en 3 parties :

Partie 1 : Gold extraction process using mercury (durée 1 h)

Partie 2 : Gold extraction process using cyanide leaching (durée 2 h)

Partie 3 : Bioleaching of refractory gold ores (durée 2 h)

Les documents utilisés sont des documents vidéos ou de courts textes.

Mots clés de recherche : gold, mercury, cyanide, complexation, pollution, bacteria, bioleaching

Provenance : TRIBOULET Philippe (philippe.triboulet@ac-dijon.fr)

GOLD EXTRACTION PROCESS USING MERCURY

Located in Peru at 5,500 m above sea level, La Rinconada is the highest gold mine in the world.

Watch this video to see how gold can be extracted and what the drawbacks are of this process.

http://www.youtube.com/watch?v=POosyASNQnQ&feature=relmfu (0 s - 4:25 s)

Answer the following questions :

Question 1 : Which other metal is used in this process. Circle the correct answer.

Silver mercury arsenic

Question 2 : Match each word on the left with a word on the right.

Poudre crushOr releaseDégager goldChauffer oreBroyer powderMinerai heat

Question 3 : Put the following sentences into the correct order to describe the process

the powder is added to a mixture composed of water and mercurypure gold is obtained and mercury evaporatesthe gold contained in the powder fixes to the mercury the ore is crushed to powdergold and mercury are separated by heatinga solid mixture of gold and mercury is obtained by pressing the mercury through the clothe

Question 4 : Fill in the blanks with the correct word from this list : melts – evaporates – condenses

A flame is used to separate gold and mercury : we observe that mercury ------------------- .

The mercury vapour ----------------- when it comes into contact with cold air.

Water is recovered when the snow ---------------.

Question 5 : Explain in a few lines why people in Rinconada drink polluted water

Question 6 : What are the consequences for people in Rinconada ? Circle the correct answer(s).

domestic violence thefts of pure gold

accumulation of wealth for the miners nervous system attack

heart attack environmental pollution

Question 7 : Fill in the blanks.

The merchant buys one gram of gold at the price of ---------------

For every ton of ore, ------------- g of mercury contaminates the ecosystem.

The solid mixture is composed of ------------ g of mercury for every gram of gold.

If two tons of gold a year are produced, ------------ tons of mercury are released into the atmosphere.

GOLD EXTRACTION PROCESS USING CYANIDE LEACHING

1/ Cyanide chemistry

Cyanide ion is a singularly charged anion consisting of one carbon atom and one nitogen atom.

When solid sodium cyanide dissolves in water, sodium ion and cyanide ion are present in the solution. The cyanide ion can combine with hydrogen ion to form hydrogen cyanide. Hydrogen cyanide and cyanide ion form an acid-base pair (pKa is equal to 9,3).

The concentrations of both species depend on the pH value. At a pH value of 10,3 or more, there is little hydrogen ion present and the concentration of hydrogen cyanide becomes very low.

Cyanide ion is a toxic substance and may be lethal if ingested. But the most hazardous compound is dissolved hydrogen cyanide ; this gas is very volatile and easily inhaled. It may kill within minutes.

A complex is a soluble species which has a metal ion at its centre with a number of other molecules or ions surrounding it. The molecules or ions surrounding the central metal ion are called ligands.

Cyanide ions form complexes with gold ions that are very stable (direct reaction) and so do not dissociate (reverse reaction). Among these complexes are the following complexes :

[Au(CN)2]– and [Au(CN)4]–

Answer the following questions :

Question 1 : Give the formulas for cyanide ion and hydrogen cyanide.

Question 2 : Indicate the acid and the base of the acid-base pair.

Question 3 : Write the balanced equation of the dissolution of sodium cyanide.

Question 4 : Write the balanced equation of the dissociation of the acid.

Question 5 : The control of pH in a cyanide solution is critical from a safety point of view : which pH value do you recommend ?

Question 6 : Write the balanced equation of the formation of complexe [Au(CN)4]–.

2/ Cyanides use in gold extraction

After the ore has been excavated, it is ground to a size of less than one millimetre. Then it is transported to the tanks of the leaching plant. This powder is fed into large mechanically stirred tanks where a compressor introduces air. These tanks contain a cyanide solution. Lime, a base, maintains pH near 10,5 in the cyanide solution.

The cyanide solution dissolves the gold contained in the solid particles. A generally accepted mechanism is described by Elsner reaction :

4 Au(s) + 8 CN– (aq) + O2 (aq) + 2 H2O (l) = 4 [Au(CN)2]–

(aq) + 4 HO– (aq)

The next main steps of the process consist of the adsorption of gold on activated carbon and of the electrowinning. Pure gold (99,9 %) is obtained after refining.

The most significant risk from use of cyanide solutions in gold mining is possible leakage into soil and groundwater. The gold cyanidation remains a big concern.

Answer the following questions :

Question 1 : Complete the table with the translation.

TankLeakageRefiningGroundwateractivated carbonis fed

Question 2 : Explain both functions of lime.

clues : think to the safety and to the chemistry of cyanide.

Question 3 : Explain why air is fed into the tank

clue : think to the composition of air

BIOLEACHING OF "REFRACTORY" GOLD ORES

Bioleaching is a method used for the extraction of precious and base metals from hard to treat ore with the aid of bacteria. For instance Thiobacillus ferrooxidans is highly effective for decomposing sulfide minerals.

The process is based on an interaction between microorganisms and sulfide minerals. For example, bacteria catalyse the breakdown of the mineral pyrite (FeS2) by oxidation. The microbial oxidation process occurs at the cell membrane of the bacteria. The electrons pass into the cells and are used in biochemical processes to produce energy for the bacteria.

“Refractory” gold ores, are ores that do not allow the recovery of gold by direct cyanide leaching. This problem occurs when gold ores contain sulfide minerals such as pyrite and arsenopyrite. For gold cyanidation, the cyanide solution must come into direct contact with gold particles. With many sulfide ores, the ore cannot practically be ground down fine enough to expose these particles. The goal of bioleaching is to remove enough of the sulfide so that at least a small portion of all gold particles is directly exposed to the cyanide solution at the next step of the process.

Today new gold mines mostly consist of ores containing sulfide so if the traditional cyanide leaching process is used it will lead to poor gold recovery. Bioleaching pre-treatment increases the yield at least 90 %.

Question 1 : Question 1 : Complete the table with the translation.

CellSulfide EffectiveRecoveryRemoveYield

Question 2 : Why do electrons appear at the membrane ?

Question 3 : Arsenopyrite is a mineral. Which chemical elements is this mineral composed of ?

Question 4 : Thiobacillus ferrooxidans is a bacteria which is easily found in these open mines. What are the advantages of using bacteria instead of a chemical reagent for pre-treatment ?

The next documents give information about bacteria and the bioleaching process.

Invisible miners part 1http://www.youtube.com/watch?v=1wO3blbL8sM(3:40 s - 4:02 s)(4:25 s - 4:58 s)

Question 1 : Which kind of chemical reaction is involved in the "work" of the bacteria ?

Question 2 : Why do the bacteria need energy ?

Question 3 : What is the source of energy for the bacteria ?

Question 4 : Indicate a range of pH values for the life of these bacteria.

Invisible miners part 2http://www.youtube.com/watch?v=gUvFbBwtGlQ&feature=related(3:02 s - 3:52 s)(6:20 s - 7:35 s)

Question 1 : What is the application of bioleaching in Europa ?

Question 2 : What are the qualities requested for this type of bacteria ?

Question 3 : Is it possible to say that mankind and the bacteria share the same goals ?

Question 4 : Can we afford to let the bacteria take their time ?

Question 5 : What is the main advance in bioleaching when processing in a tank ?

Question 6 : Justify the title of the video "Invisible miners".

Invisible miners part 3http://www.youtube.com/watch?v=xGeGyy2fqrc&feature=related(0 s - 1:10 s) (7:00 s - 8:30 s)

Question 1 : What was the main fear of the researchers concerning the bacteria in Nordic countries ?

Question 2 : What was the result of the study ?

Question 3 : What is the main condition for success in bioleaching ?

Question 4 : Which parameters have to be controlled for optimizing the growth of bacteria ?

Question 5 : How is it possible to know if the tank contains "healthy" bacteria ?

Question 6 : What are the consequences for the process if the bacteria die ?