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Supercritical CO2 soil remediation and recovery of heavy metals

Supercritical water reaction to recover precious metals

In view of recycling precious metals from discarded electronics we developed a supercritical fluid extraction technology and its equipment.

Supercritical water reaction to recover precious metals

This technology can also be suitable to repairing soil contaminated by crude oil, in which secondary pollution will not occur and the repaired soil can meet soil standardsissued by the Department of agriculture.

The repairing process is cost-effective.

Technical Indicators 

ConfigurationParameter
Extraction vesselVolume: customized according to customer requirements, generally 100ml-1000l
Working pressure: 35MPa-100MPa adjustable
Working temperature: room temperature -85℃ adjustable
3 extraction vessels recommended for production equipment
SeparatorVolume: Configured according to the volume of the extraction kettle, generally 50ml-800l
Working pressure: 10MPa-30MPa adjustable
Working temperature: room temperature -85℃ adjustable
Recommend 3 separation kettles
Working pressure: 16MPa
Gas storage tankVolume: According to the overall plan configuration, generally 10l-6000l
Working pressure: 9.9MPa
PumpCO2 high pressure pump and carrier pump, with adjustable flow and pressure
Production equipment equipped with 2 high-pressure pumps
Heat Exchange SystemWorking pressure: 35MPa-100MPa
Working temperature: adjustable from room temperature to 90℃
Refrigeration system5℃-10℃
Security systemThe high pressure pump and the inlet and outlet of all containers are equipped with overpressure protection device, the safety interlocking device is connected with the PLC control system and the safety valve to ensure the safety of the system

Supercritical CO2 fluid extraction and removal of heavy metals

The heavy metals remaining in crops, Chinese medicinal materials, soil and wastewater mostly exist in ionic form. Because of their insoluble or low solubility in supercritical CO2 fluid, they cannot be removed and recovered by supercritical fluid extraction alone.

For this reason, scientists have developed a new technology of supercritical CO2 fluid chelation extraction to solve the problem of removing heavy metals with green solvent supercritical CO2 fluid extraction.

Basic principle

Chelating agent

By selecting a suitable chelating agent to chelate with metal ions, a chelate compound that can be dissolved in supercritical CO2 fluid is generated, and then the chelate compound is extracted and removed from the pollutants by SCFE method.
It can be seen that the choice of chelating agent is the key to supercritical chelating extraction.

DDC

Diethyldithiocarbamate (DDC) is the most commonly used chelating agent, but the metal chelate formed by it is difficult to dissolve in supercritical CO2 fluids, and is limited to the thermal stability and chemistry of DDC metal chelate Stability, not suitable for supercritical CO2 fluid chelating extraction.

FDDC

DDC was modified to fluoride bis(trifluoroethyl)dithioamino (FDDC), which was successfully used for supercritical CO2 fluid chelation extraction of As3+, etc., and successfully separated the chelated traces by supercritical fluid chromatography Heavy metals.

FDDC chelate has higher stability and solubility in supercritical CO2 fluid than DDC chelate (the difference is 2 to 3 orders of magnitude at 50℃, 100atm), which further proves that fluorine-containing compounds are easily soluble in supercritical CO2 fluid.

Using FDDC as a chelating agent, the supercritical CO2 fluid was used to chelate and extract Cu2+. Under the conditions of 7.93MPa and 35℃, the extraction rate of Cu2+ from the solid substrate was only 47% at 60min, while the extraction of Cu2+ from the aqueous solution took 30min. When the extraction rate can reach 100%.

TBADBDTC

With dibutyl dithiocarbamate tetrabutylamine (TBADBDTC) as a chelating agent, supercritical CO2 fluid chelating extraction of Cd,
Pb and Zn are very effective.

β2 diketone chelating agent

The cost of FDDC and TBADBDTC is too high, which limits their large-scale application.

The fluorinated β2 diketone chelating agent FOD has successfully extracted lanthanides and actinides from filter paper. β-diketone chelating agents, such as acetylacetone (AA), trifluoroacetylacetone (TFA), etc., have high solubility in CO2, and can be used to chelate and extract heavy metal ions.

Fluorinated acetylacetone and acetylacetone

A comparison of Cu2+ and Zn2+ experiments in supercritical CO2 fluid extraction electroplating baths with fluorinated acetylacetone and acetylacetone as chelating agents proves that the extraction rate of fluorinated acetylacetone is higher than that of acetylacetone.

Other chelating agents

Researchers selected a variety of commercial chelating agents for research and found that sulfur-containing organophosphorus chelating agents (Cyanex302, Cyanex302,
301 and D2EHTPA) can also be effectively used to extract heavy metal ions.

Crown3

In order to improve the selectivity of the chelating agent for the extraction of metal ions, tert-butyl dibenzobis-triazole crown ether (Crown3) is used as the chelating agent.
The mixture uses supercritical CO2 fluid with 5% methanol to extract Hg2+ from the coarse sand. The extraction rate is 78%, and the extraction rate is 95% when there is a very small amount of water. Except for Au3+, the extraction rate is 79%, and the extraction rate of other metal ions is less than 4%, indicating that the crown ether chelating agent has a high selectivity.

Dipyridine derivatives

Non-fluorinated dipyridine derivatives and fluorinated dipyridine derivatives are used as chelating agents for supercritical CO2 fluid chelation extraction of heavy metal ions. It was found that non-fluorinated dipyridine derivatives and fluorinated dipyridine derivatives are chelating agents. When mixed, the extraction rate of Ni2+ and Cu2+ is very high, showing superior selectivity.

Influencing factors

Supercritical CO2 fluid pressure

Pressure mainly affects the solubility of chelating agents and metal chelates in the supercritical phase, and high pressure is beneficial to increase the solubility of chelating agents and metal chelates in supercritical CO2 fluid, thereby improving the extraction effect.

The effect of extraction pressure on Cu2+ in supercritical CO2 fluid chelating extraction aqueous solution.
At 35℃, the density of supercritical CO2 fluid is 0.17g/mL (6.2MPa) without extracting Cu2+, 0.22g/mL (6.9 MPa) is 12%, and the extraction rate of 0.37g/mL (7.93MPa) is close to 100%. It can be seen that there is no extraction effect when the pressure is lower than the critical pressure of CO2, and the extraction rate increases with the increase of pressure.

Temperature

The influence of temperature on the extraction of heavy metals by supercritical CO2 fluid chelation is more complicated.

  • On the one hand, increasing the temperature will reduce the density of the supercritical CO2 fluid, which is not conducive to extraction;
  • On the other hand, it will increase the vapor pressure of the chelate, which is conducive to extraction;

The influence of the two will reflect the coupling effect.

In addition, increasing the temperature may also affect the thermal stability of the metal chelate, so the choice of operating temperature requires comprehensive consideration.
Worry.

Using diethylammonium diethyldithiocarbamate (Et2NH2DDC) as a chelating agent, supercritical CO2 fluid is used to extract heavy gold in traditional Chinese medicine
The experimental results of the genus confirmed this.

It is also found that increasing the pressure and increasing the amount of chelating agent is beneficial to the removal of high-content heavy metal ions, but the effect on low-content ions is not obvious, which may be caused by the effect of the chelation reaction.

Cosolvent

Since the supercritical CO2 fluid is non-polar, in order to dissolve the polar chelating agent and its metal chelate, a modifier can be added to the supercritical CO2 fluid to modify or reconcile the polarity of the solvent.

Commonly used modifiers are: water, methanol, ethanol and acetone, among which methanol is the most widely used.

Methanol

Studies have shown that the extraction rate of Cu2+, Zn2+, Co2+ and Cd2+ is significantly improved by supercritical CO2 fluid with 0.5% methanol.

However, the use of modifiers not only increases the difficulty of separation, but also may cause modifier residues. It can be seen that the development of easy-to-separate and environmentally friendly modifiers is also an important research direction.

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