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Tellurium Recovery during Dore Processing

Recycling PM Scrap to DORE Metal : Theory : Oxydation

  • All metals except Precious Metals can be oxidized

  • Blowing air or oxygen into or above a metal bath containing PM's will result in all non PM's to combine with oxygen and to form slag layer of oxides on top of the metal

Recycling PM Scrap to DORE Metal : Theory : DORE Refining

  • DORE refining consists in removing the slag layer and keeping on blowing air or oxygen until no layer anymore is formed

 

  • Then all impurities have been removed and the metallic bath consists of PM’s only

Recycling PM Scrap to DORE Metal : Impurities easy to oxidise ... or not !

  • The reality is more complicated :

  • Part of the impurities are easy to oxidize and to eliminate : Arsenic / Antimony / Lead / ..

Recycling PM Scrap to DORE Metal : Impurities almost precious & precious metals not completely precious

The reality is more complicated :

  • Other impurities are almost precious : Bismuth is difficult to oxidize, also Copper

  • Finally, all PM’s are not always even precious : Rhodium and secondary PGM’s can be oxidized

  • Other impurities can make compounds with PM’s : Selenium and Tellurium can combine as Silver Selenides or Tellurides

Recycling PM Scrap to DORE Metal : Volatile Impurities

Also, volatile metals can evaporate depending on their activity (or content) in the metal bath :

  • Gold melting temperature : 1064 C = 1947 F

  • Silver melting temperature : 962 C = 1764 F

 

  • DORE Furnace final temperature : 1150 C = 2102 F ?

Boiling temperatures :

  • Arsenic : 616 C = 1141 F

  • Selenium : 685 C = 1265 F

 

  • Tellurium : 988 C = 1810 F

Recycling PM Scrap to DORE Metal : Practically : Fluxes

  • Remember « Blowing air or oxygen into or above a metal bath containing PM’s will result in all non PM’s to combine with oxygen and to form a slag layer of oxides on top of the metal »

=> Add slagging fluxes to make the slag layer liquid

Recycling PM Scrap to DORE Metal : Which fluxes? Lead

  • Lead will easily oxidize to form Litharge = PbO = Lead Oxide melting at 888°C = 1630F

 

  • The Litharge can collect and dissolve most impurities

  • Silica will form Lead Silicate glass with low viscosity at furnace temperature

Recycling PM Scrap to DORE Metal : Other fluxes? Carbonates - Borates - Phosphates

  • Borax or Sodium Borate Na2B4O7 is known to dissolve metallic oxides

  • Same for Phosphates

  • Sodium Carbonate decomposes at furnace temperature, allowing the resulting Sodium Oxide to form complex salts, for example Sodium Selenate

 

Recycling PM Scrap to DORE Metal : Copper Anode Slimes

Complex By Product :

  • Silver : 5% to 15%, with Gold, Platinum, ..

  • Copper : 2% up to 30% (if undecopperized)

  • Lead : 10% to 20%

  • Arsenic, Antimony, Tin : 1% to 5%

  • Bismuth, Tellurium : 1% to 5%

  • Selenium : up to 20%

  • Barium Sulfate + Silica : 10% to 30%

Recycling PM Scrap to DORE Metal : Melting and first Slagging of Copper Anode Slimes

  • Melting with Fluxes : 

Borax / Sodium Phosphate / Sodium Carbonate

  • First Slag :

  • Collecting Refractory Oxides

  • Collecting Metal Oxides showing a higher Affinity for 

  • Metal Bath :

Silver settles with part of Lead + Copper

Oxygen than Lead

Recycling PM Scrap to DORE Metal : Refining and second Slagging of Copper Anode Slimes

  • Blowing of Oxygen:

Resulting in Lead / Copper / Selenium / to oxidize

  • Refining Slag 

Collecting Metal Oxides showing a lower Affinity for Oxygen

than Lead

  • Metal Bath :

Silver DORE

Recycling PM Scrap to DORE Metal : Particular case : Tellurium

  • in the Silver Alloy extracted from Lead Refining

  • in Anode Slimes produced during Copper Electrorefining

Tellurium is an impurity in Precious Metals Concentrates :

Recycling PM Scrap to DORE Metal : Tellurium occurrence in Precious Metals Operations

Tellurium is an impurity during Precious Metals Processing :

  • Tellurium is perturbing the PM recovery.

  • Tellurium is accumulating in By Products of the PM process.

  • Tellurium is increasing PM operating costs :

  • increased volume of By Products

  • increased charges for the Re-processing

  • increased PM loss in By Products

Recycling PM Scrap to DORE Metal : The price of Tellurium is close the price of Silver

  • Silver : $ 30/ to (November 2024) = $ 1,000 / kg

  • Tellurium November 2024 : $ 80 / kg

  • Tellurium December 2011 : $ 300 / kg

Recycling PM Scrap to DORE Metal : Preliminary Tellurium Concentration prior to Extraction

  • During Precious Metals Processing, Tellurium can accumulate in the Furnace together with Silver, thanks to :

  • use of correct oxygen potential in the Furnace

  • adjusted slag composition

  • use of other metals, Copper, Lead, Silver to collect Tellurium as Telluride (Cu2Te and similar)

Recycling PM Scrap to DORE Metal : Recovering Tellurium makes sense

  • Extracting Tellurium will result in :

  • reduced PM losses and processing costs

  • potential additional gross value corresponding to Tellurium : 0.8 million $ at current market prices (November 2024)

 

  • The cost for recovering Tellurium is limited :

  • No investment needed, use of existing equipment

  • No additional charges except assaying

Recycling PM Scrap to DORE Metal : Controled Tellurium Extraction during Silver Refining

 

  • The Silver Refining Process must be monitored in order to identify the several Refining Steps :

 

  • Adjusting oxygen potential of the Furnace

  • Controling of the slag fluidity

  • Monitoring of the slag composition

  • As a result, Tellurium will concentrate in the Refining Slag with contents between 5% and >15%, depending on the Tellurium activity

Recycling PM Scrap to DORE Metal : Electronic Scrap : Printed Circuit Boards

 

Printed Circuit Boards consist of

  • 15% Copper

 

  • > 100ppm Gold

  • 500ppm Silver

 

  • Traces Tin, Lead, Nickel, Iron, Aluminium, Ceramics

  • Balance : Plastic

 

Recycling of Printed Circuit Boards in a Silver DORE Furnace

At furnace temperature :

  • The plastic fraction is burning

  • Aluminium, Iron, Ceramics are dissolving in the Slag

  • PMs, Tin, Copper, .. are collected in the Metal Phase

Recycling PM Scrap to DORE Metal : Precious Metals Catalysts

Precious Metals Catalysts consist of :

  • A Ceramic Substrate, Zeolithe, Silica, Alumina, ..

  • Coated with Silver, Gold, Platinum, .. Rhodium

  • Possibly with organics

Recycling of Precious Metals Catalysts in a Silver DORE Furnace

At furnace temperature :

 

  • Precious Metals are collected in the Metal Phase

  • No need to melt the substrate, only the coating must be recovered

Recovery rate for Precious Metals Recycling

The recovery of Precious Metals is a direct function of :

  • DORE Furnace temperature

  • Bath movement / Furnace rotation, allowing to increase the contact surface between charge and collector

  • Fluidity of the Slag, allowing metal druplets to decantate and settle down

  • Furnace Oxygen Potential

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