Biomass Cashew Nuts Shell (CNS)

A potential bio-resource for the production of bio-sourced chemicals, materials and fuels

Abstract

Exploitation of agricultural waste as green starting materials to produce various valuable products is attracting the attention of academic, industrial and other practitioners. Cashew nut shell (CNS) and its liquid extract (CNSL) in particular have been identified as agro-wastes rich in valuable and functional renewable products. The unique structural features of the CNSL constituents offer the possibility for different modifications to suit various applications. This review article provides recent developments in CNS and CNSL as green sources for use in the production of biorenewable chemicals, materials and energy. Extraction methods and applications of CNS, CNSL and isolates are discussed. Furthermore, a literature survey of the current status and efforts made on the utilization of these agricultural and food wastes for different applications is well outlined.

Properties and Composition

Cashew Nut Shell liquid (CNSL) is a dark viscous oil with a characteristic smell, very much unlike other vegetable oils. It is an oily natural resin with a yellowish sheen. It is opaque and when applied as a thin film, it is reddish-brown. Industrial-grade CNSL is reddish-brown. It is immiscible with water but is miscible with most organic solvents. CNSL has germicidal and fungicidal properties.

Naturally occurring cashew nut shell liquid contains mainly four components – cardanol, cardol, anacardic acid and 6-methyl cardol.
Commercial-grade CNSL contains hardly any anacardic acid due to decarboxylation during the roasting process, which converts anacardic acid to cardanol. This roasting process also results in polymerization, which accounts for 20 to 25% of polymeric substances in CNSL. Raw CNSL is corrosive but becomes less so by decarboxylation and removal of hydrogen sulfide (H2S) during the refining process.
The components of CNSL are themselves mixtures of four constituents differing in side-chain unsaturation, namely, saturated, monoene, diene and triene. The structures of the side chains of varying degrees of unsaturation occurring in the four major components of CNSL are identical.

The chemical structures of various components enable CNSL to take part in a large number of chemical reactions. The phenolic nature of these constituents, along with varying degrees of unsaturation in the side chains, make it a highly polymerizable substance amenable to a variety of polymerization reactions. Of overriding importance, however, is the condensation reaction with active methylenes that formaldehyde exemplifies. This leads to a phenol-formaldehyde type condensation.
CNSL undergoes several types of chemical reactions, such as, decarboxylation, hydrogenation, polymerization, sulfonation, nitration, halogenations, etherification, esterification, epoxidation, etc. Reactions of CNSL are of commercial importance because several useful industrial products can be produced starting from this natural chemical.

Manufacturing

Cashew nut shell liquid constitutes about 20 to 25% of the weight of the cashew nut. A number of different methods are employed to extract CNSL from cashew nuts. The popular methods of manufacturing CNSL are given below in brief. Many a times, a particular region follows a particular manufacturing technique by convention rather than by deliberate choice.

1. Hot oil bath process – The raw cashew nuts with shells are passed through a bath of hot CNSL at 180 to 200 deg. C, resulting in the bursting of the shell and releasing the CNSL. However, only 50% of the CNSL present in the shell is extracted by this process. Several improvements to this process are also used.

An additional 20% of the remaining CNSL could be extracted by passing the spent shells from the above process through an expeller and the rest by a solvent extraction technique. The CNSL obtained through the expeller can be upgraded by acid washing, centrifugation, and heating treatment.

2. Roasting process – Shelled cashew nuts are exposed to sudden high temperatures (400 to 700 deg. C) from the ambient. Shells are charred in the process forcing the CNSL out of the shell. Variations of this process using step-wise heating to higher temperatures and using vacuum and electrical or infrared heating are also used. Mostly, this technique is used in conjunction with an oil expeller which expels the oil from shells to an extent of 90%.

Several other methods using superheated steam, extraction with solvents like benzene and toluene, supercritical carbon dioxide, etc. have also been reported.

The raw or crude CNSL obtained using the above extraction methods is refined to produce commercial grades.

During refining, raw CNSL is treated with hydrocarbon sulphates and sulphuric acid to remove unwanted materials like sulphides, nitrogenous materials and minerals. Aqueous solutions of acids such as hydrochloric acid, sulphuric acid, acetic acid, chloroacetic acid or phosphoric acid, or acid sulphates such as sodium hydrogen sulphate are used for refining crude CNSL. This refining process results in the evolution of hydrogen sulphide, and reduces the chemical components or agents in CNSL responsible for skin irritation.

The refined CNSL can be readily distilled under a vacuum. Alternatively, it can be hydrogenated to give refined CNSL with good colour stability. Deodorization may be effected by steam treatment of sulphuric acid-treated CNSL by distillation.

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