Waste.net
Recovery of Metals
from acid solutions
Hydrometallurgy

Overview
There are a number of Hydrometallurgical approaches to refine or purify metals using wet Chemical Reduction Methods.
The use of Strong caustic PH Base stripping baths is one common process. Solvent Extraction methods to achieve Absorption or Adsorption are also employed in various applications.
One of the most commonly used methods encountered involves the use of acids to segregate metal complexes by dissolving mixed metals into solution and selectively recovering each individual metal.


Residual Materials
It is generally assumed that any metals or materials of significant value would have been eliminated by source separation or salvaged by treasure hunters who cherry pick any "low hanging fruit". We therefore take for granted that any material entering the Recovery 2.0 system is the less desirable residual materials that can not be easily recycled by manual or mechanical means.
This leads us to a bottoms up approach to base our recovery methods on a lowest common denominator default for the ultimate treatment of disposition of residues.


Acid Leaching
When you encounter elements that are difficult or impossible to segregate with manual or mechanical methods, you need to use alternative processes such as Pyrometallurgy or hydrometallurgy. The extraction of metals from ore concentrates, metal residues and complex metals matrix materials may be achieved by an Acid Leaching process. By utilizing any combination of acids to dissolve the metal complexes into ionic solutions the segregation and purification is possible.
A wide variety of acids are commonly encountered that are generated from industrial waste streams, but the most common are Hydrochloric (HCl), Sulfuric (H2SO4), Nitric (HNO3) Acids.

Controlled in-vessel leaching, dissolving metals into an acid solution is a highly exothermic reaction. Harnessing energy from this redox reaction may be achieved in several ways. One method is with the use of a Harvesting Module that captures the heat energy in a Thermal Energy Generator.

Further advancements in material sciences may allow for the conversion of the Leaching system into a duel purpose metal refining and energy harvesting Electrolytic cells.
The movement of the metal saturated ionic solutions between the leaching reactor, storage buffers and the displacement reactors provides the opportunity for the location of a highly tuned, low velocity Magneto Hydro Dynamic (MHD) harvester.

Acid Regeneration
The treatment and processing of Acid Wastes or material generated from Acid Leaching operations are contained in a closed loop system to insure the regeneration on a Mass Balance Equilibrium basis.

A common example of closed loop acid recycling is by capturing the fumes generated by nitric acid leaching and sparging them through a water column creating a dilute nitric acid. This regenerated acid may be stored and cycled back into the leaching reactor as required, conserving the need for a fresh supply of acid and water.

Metal Recovery
Purified Metal or other desired products may be recovered from the ionic acid solutions. The typical recovery methods used are precipitation, direct displacement and Electrolysis

The recovery of metal residues and mixed metal complexes may be desirable as items for processing with hydrometallurgy methods. A wet chemical approach may be an efficient option to achieve the recovery of those items that are difficult or impossible to segregate manually or mechanically.

Highly mixed materials such as Recovered Electronic Components accumulated from Depopulating Circuit Boards contain a vast number of combinations of different types of precious and base metals. In order to efficiently recover the spectrum metals contained within these electronics mixes, following a systematic sequence may be the best approach to ensure the extraction of all the values contained.

Recent advancements in technology have allowed for the real-time identification of materials as they flow through the recovery process.

Hydrometallurgy Pathways


an example of a
Recovery Cycle
of metals from acid solutions

Silver Recovery

Copper Recovery

Tin Recovery

Steel Recovery

hydrometallurgy

Silver Recovery
Scrap materials such as ornamental silver plated items or small electronic switches or contacts that are permanently affixed to copper buss bars, or lead free solders that contain small quantities of silver, are difficult or impossible to manually or mechanically segregate.
Dissolving these types of scrap materials in a solution such as Nitric acid is one approach that may be used to achieve silver separation and recovery.

Once the acid solution is saturated with metals, the ionic silver nitrate may be displaced out of the solution and recovered as metallic silver with the addition of excess metallic copper. Real Time Analytics will allow the immediate determination of the quantity of copper required to complete the displacement reaction.

An alternative method to precipitate the silver nitrate out of solution is with the addition of hydrochloric acid. The silver will be converted into a silver chloride which then may be oxidized and the silver oxide may then be reduced into metallic silver. This is a multi-stage process that leaves you with multiple acid complexes to deal with.

Copper Recovery
Continuing on with the nitric acid solution, once the silver has been removed, the remaining copper nitrate may be reduced by the direct displacement with iron. The quantity of iron required may be determined with Real Time Analytics.

Tin Recovery
Tin (Sn) may be recovered from circuit board trimmings and depopulated circuit board scrap, tin platted coppers and lead free solder scraps.
In nitric acid, Tin reacts to form a Tin Oxide (SnO2) precipitate. The recovery of Tin is a source for the Strategic Use of Tin.

Steel Recovery
The Iron that remains as a Ferric nitrate saturated solution may be directly thermally reduced. The reduced iron may be recovered as the nitrogen dioxide is distilled off, contained, captured and regenerated into a fresh batch of nitric acid.


Electrochemical Cells       Oxidation/Reduction & Displacement
Molten Media Extraction
recovery2.0

Temperature Gradient       Seebeck effect
Reactivity Series
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