Technical feasibility study on the chromium recovery from electroplating effluents

Diplomarbeit von Daniel Wiemken

Introduction:

Rapid industrialisation and growth in population over the past two hundred years exert an increasing pressure on natural resources and the environment. Billions of tons of controlled and scheduled waste are generated every year by the industrial sector worldwide which is often either pre-treated on-site or at a licensed contractor prior to disposal in landfills. This practice if continued is leading to resource depletion and creates a potentially harmful legacy for future generations. In order to move towards a more sustainable development as outlined in the Bruntland Report, waste reduction, reuse and recycling coupled with pollution prevention measures play an important part to slow down if not reverse this practice.

Heavy metals such as cadmium, mercury, lead and chromium are not degradable or renewable like biomass hence if they are to be used in future processes reuse and recycling are the only options. At present, heavy metals are used in the chemical industry sector for applications ranging from batteries to catalysts and surface coatings, and can be found at various concentrations in gaseous, liquid or solid waste.

Chromium is of particular interest owing to its legislative status and unique chemistry. Chromium exists in nature primarily in one of two oxidation states. There are other chemical oxidation states of chromium, which include 0, II, IV, and V, but they are considered transitory compared to more stable Cr(III) and Cr(VI) species. Hexavalent chromium is a strong oxidizer which can react with DNA causing mutation, while the trivalent, organically complex form is a dietary supplement to help with proper glucose metabolism, weight loss and muscle tone. Unlike many other metals, Cr(VI) can combine with oxygen to form water-soluble, negatively charged anions known as yellow chromate (CrO42-) or orange dichromate (Cr2O72-), which adsorb to positively charged sites in contrast to cationic metal species. Therefore, hexavalent chromium species are not strongly bonded in many soils under alkaline to slightly acidic conditions, for example. Thus, they can be very mobile in subsurface environment while other metals precipitated out and exert toxic effects on biological systems.

Various well-established methods may be used to treat industrial effluents and contaminated water such as reduction and precipitation, reverse osmosis, evaporation, ion exchange and adsorption. While these processes are able to remove the pollutants from the waste stream to meet stringent discharge limits, they may produce concentrated waste and be less suitable for efficient and selective material recovery. Solvent extraction, on the other hand, is one of the most effective conventional methods extensively used in separation science, with potential to selectively recover metals from industrial effluents that can also be operated in a closed loop. Various reviews dealing with solvent extraction of chromium and other heavy metals are available. Several ion-association forming systems such as triisooctylamine, tetrabutylammonium bromide (Aliquat 100), trioctylmethylammonium chloride (Aliquat 336), trioctylamine (Alamine 336), triphenylsulphonium and triphenylphosphonium and have been studied to extract chromium in the anionic form (Figure 1 Chemical structure and formula of various extractants used in chromium recovery processes).

In this study Alamine 336, a tertiary amine, will be used for hexavalent chromium extraction because of the lack of publicly accessible information. By inspecting eq. 1 and eq. 2 Alamine 336 should be able to extract anionic species of hexavalent chromium. According to supplier information, the Alamine series of reagents exhibit the following general extraction chemistry: (eq. 1 and eq. 2).

The first equation illustrates amine salt formation with R representing a variety of hydrocarbon chains, and the second equation represents true ion exchange. The extent to which B- will exchange for A- is a function of the relative affinity of the two anions for the organic cation and the relative stability of the anions in the aqueous medium. Typically, extraction favours larger lowly charged anions over smaller more highly charged anions.

A number of extractants have been used for the recovery of Cr (VI) such as aqueous solution of (NH4)2SO4, benzene, chloroform, dichloroethane, dichloromethane, hexane, kerosene, methyl isobutyl, toluene, tri-n-butylphosphate, and xylene.

Most of the solvents reported are acceptable for use in lab-scale extraction processes but are not suitable for industrial scale extraction processes due to economics, health, safety and environmental concerns. Of all the solvents cited above kerosene appears to be the preferred solvent of choice.

Kerosene, a mixture of various hydrocarbons obtained by distillation of petroleum at boiling point 175 to 330°C, is also used as an aircraft gas turbine and jet fuel by both commercial airlines and the military service, as heating oil, and as a spray oil to combat insects on citrus plants. With regards to its use as solvent in commercial extraction operations the lighter aromatic compounds in kerosene such as benzenes, toluene and xylenes are still a cause of concern due to their toxicity, volatility and water solubility.

In order to render the solvent extraction process safer and more environmentally friendly a renewable and harmless solvent for metal recovery would therefore be highly desirable. In this study the extraction performance of refined palm oil and kerosene as solvent is investigated and compared using Alamine 336 on Cr(VI) extraction from acidic synthetic wastewater solutions. In addition, experiments are extended to best matching synthetic multi element solutions and finally extractions were carried out in diluted effluent. Software based speciation modelling in dependence of altered pH, SO42- and Cr(VI) concentration also was part of this survey.

Table of Contents:

Text Sample:

Chapter 3.5, Solvent extraction:

The term solvent extraction refers to the distribution of a solute between two immiscible liquid phases in contact with each other, i.e., a two-phase distribution of a solute. It can be described as a technique, resting on a strong scientific foundation. Scientists and engineers are concerned with the extent and dynamics for the distribution of different solutes – organic or inorganic – and its use scientifically and industrially for separation of solute mixtures.

As applied to refining operations, particular uranium, solvent extraction commenced in the 1940ies. These plants were small by present plant sizes. Shortly after the first solvent extraction plant was installed to treat hydrometallurgical solutions at the mine site for the recovery of uranium. The obtained success in this first generation of uranium operations eventually led to the application of solvent extraction to copper operations in 1969. Afterwards hundreds of SX plants have been installed for the recovery of many metals. With only a couple of exceptions, all plants were mixer settler in design. Considerable research on the chemistry and engineering of the solvent extraction process was achieved throughout the succeeding years. Based on process improvements many reagents have been developed as well as numerous contactor systems, constructed from upgraded materials and furthermore equipped with sensing and control devices became marketable in order to enforce the mass transfer.

Generally SX is primarily used when the separation could not accomplished by distillation (e.g. cause of azeotropic formation or thermal sensitivity) or is not economical beneficial.

However the main applications of SX in treatment of wastes are in the recovery of recyclable fractions from waste streams to provide regenerated process water. This reduces the effect on the environment by minimizing the amount of effluent for downstream treatment besides the reduction in fresh water consumption. The most important liquid effluents where SX could be applied are from various metal finishing operations: plating, pickling, etching, and the rinse waters arising from the cleaning of work pieces. In the treatment of those wastes, the objectives must be:

Purification of a metal from unwanted contaminants either by extracting the desired metal from the impurities or by extracting the impurities from the desired metal.

Concentration of the metals in order to reduce downstream processing costs and to meet regulatory discharge limits.

Conversion into a metal form which eventually promotes the recovery.

In any given solvent extraction process, one, two or all three objectives may be accomplished.

Selecting a proper reagent and operating under optimised process conditions should cover these objectives usually.

A variable number of mixer-settler stages, adjustable temperatures and flow rates as well as the ability to use wash stages allows the optimization of process conditions as well as design flexibility.

3.5.1. Principles of solvent extraction (SX):

SX always consists of a minimum of two stages, even the intensive mixing of aqueous and organic phase and the respective, preferably completed, separation of both fluids phases.

During the mixing the mass transfer takes place, i.e. the desired extract moves from the first solvent (feed or aqueous phase) into the second solvent (extracting agent or organic phase). Assumed that the solvent is not (or in small parts) soluble in the feed solution. The principle of solvent extraction is illustrated in Figure 6. To achieve an excellent mass transfer, and there with a high extraction performance, a preferably high surface area between the liquids is necessary. To obtain a great surface area, one of the liquids is dispersed in small drops with an agitator. However, it must be avoided, that an emulsion occurs the separation of which is difficult or even not possible. During the separation (settling) the dispersed fluid has to coalescence to a homogeneous phase and must be able to separate by sufficient different densities from the other liquid. The output is significantly influenced by the speed of separation (Figure 6 Mass-transfer in liquid-liquid extraction (Wilichowski, 2008)).

The assumption for evaluating a separation by extraction and design of the extraction process is the knowledge of the distribution equilibrium, which define the distribution of the extract between the two phases.