Ferrous Sulfide Precipitation for In-Situ Chromium Reduction

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Solution Overview

Problem

Current methods for remediation of chromite ore processing residue (COPR) contaminated sites are ineffective due to the mobility of soluble remediation compounds and the need for acidic conditions, which damage the COPR structure and are impractical for widespread use.

Innovation Solution

Incorporating ferrous sulfide as a substantially insoluble reducing compound within the pores of COPR, formed by injecting ferrous and sulfide salt solutions, which precipitate and remain effective at high pH levels, reducing hexavalent chromium to trivalent chromium without being washed away by groundwater.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If soluble remediating compounds are used to convert Cr(VI) to Cr(III), then the chromium conversion is effective, but the compounds are washed away by groundwater movement quickly

Engineering Contradiction:

Improvechromium conversion effectivenessVSAvoidduration of remediation compound presence

Core Design Contradiction:

ReliabilityVSDuration of action of stationary object

Solution Approach 1:

The invention changes the solubility parameter of the remediating compound by using ferrous sulfide, which is substantially insoluble in water. This allows the compound to remain in place in the COPR deposit for extended periods while still effectively converting Cr(VI) to Cr(III) through chemical reduction.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention uses a form of the remediating compound (ferrous sulfide) that does not dissolve and wash away like previous soluble compounds. Instead, it remains as a solid phase that slowly reacts with Cr(VI) over time, providing long-term remediation without being lost to groundwater flow.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

2Reliability

If biological reduction techniques are used, then Cr(VI) can be reduced to Cr(III), but large amounts of acid must be added to lower pH, which destroys COPR structure and damages structures

Engineering Contradiction:

Improvechromium reduction effectivenessVSAvoiddamage to COPR structure and ground structures

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The invention changes the pH requirement parameter by using ferrous sulfide, which effectively reduces Cr(VI) to Cr(III) at high pH levels (pH 11-13) native to COPR. This eliminates the need to add large amounts of acid to lower pH, thereby avoiding destruction of COPR particle structure and damage to ground structures.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention converts the high alkalinity (high pH) of COPR, which was previously seen as a problematic condition incompatible with biological remediation, into a beneficial condition that enables ferrous sulfide to function effectively as a remediating compound without requiring pH adjustment.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Productivity

If pump and treat or soil washing is used, then groundwater can be treated, but these methods are ineffective for COPR sites due to slow diffusion of Cr(VI) from particles

Engineering Contradiction:

Improvegroundwater treatment efficiencyVSAvoideffectiveness for COPR sites

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The invention introduces ferrous sulfide as an intermediary substance placed directly within the COPR deposit. This intermediary slowly releases ferrous ions that reduce Cr(VI) to Cr(III) in situ, creating a sustainable remediation system that addresses the slow diffusion limitation by providing a long-term source of reducing agents directly at the contamination source.

Inventive Principle:

Principle #24Intermediary (Mediator)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The ferrous sulfide effectively converts hexavalent chromium to trivalent chromium, remaining in place for extended periods, reducing environmental hazards and eliminating the need for acidic conditions, thus providing a sustainable and efficient cleanup method for COPR sites.

Implementation Method 1

Ferrous sulfide effectively converts hexavalent chromium to trivalent chromium

Methodology Applied

Scientific EffectChemical reduction: Reduction

Implementation Method 2

formed by injecting ferrous and sulfide salt solutions, which precipitate and remain effective at high pH levels

Methodology Applied

Scientific EffectPrecipitation: Precipitation

Data Source

PatentUS7645092B2Treatment of in ground chromium ore processing residue

Publication Date: 2010.01.12 CH2M HILL INC

AI summary

In methods for treatment of in ground chromium ore processing residue contamination, ferrous sulfide is provided as a substantially insoluble material in the residue deposit. The ferrous sulfide accordingly may remain substantially in place, in the pores of the soil or residue, even over long periods of time, regardless of underground water movement or diffusion. As a result, the ferrous sulfide may act continuously to chemically reduce and remove contamination. As hexavalent chromium diffuses from the soil or residue, it is reduced by the ferrous sulfide. The ferrous sulfide may be placed into the underground deposit by injecting a ferrous salt solution into the deposit, waiting for a precipitation to elapse, and then injecting a sulfide solution into the deposit.