Oxidation of cyanide in effluents by Caro’s Acid

Abstract

Caro’s Acid (peroxymonosulphuric acid: H₂SO₅) is a powerful liquid oxidant made from hydrogen peroxide that has been adopted for the detoxification of effluents containing cyanides in gold extraction plants in recent years.

The present work reports the findings of a study on the kinetics of aqueous cyanide oxidation with Caro’s Acid. Experiments were conducted in batch mode using synthetic solutions of free cyanide. The experimental methodology employed involved a sequence of two 2³ factorial designs using three factors: initial [CN⁻]: 100–400 mg/L; H₂SO₅:CN⁻ molar ratio: 1–1.5–3–4.5; pH: 9–11; each one conducted at one level of Caro’s Acid strength which is obtained with the H₂SO₄:H₂O₂ molar ratio used in Caro’s Acid preparation of 3:1 and 1:1. The objective was the evaluation of the effect of those factors on the reaction kinetics at room temperature. Statistical analysis showed that the three investigated variables were found to be significant, with the variables which affected the most being the initial [CN⁻] and the H₂SO₅:CN⁻ molar ratio. The highest reaction rates were obtained for the following conditions: H₂SO₅:CN⁻ molar ratio = 4.5:1; pH = 9; and Caro’s Acid strength produced from the mixture of 3 mol of H₂SO₄ with 1 mol of H₂O₂. These conditions led to a reduction of [CN⁻] from an initial value of 400 mg/L to [CN⁻] = 1.0 mg/L after 10 min of batch reaction time at room temperature. An empirical kinetic model incorporating the weight of the contributions and the interrelation of the relevant process variables has been derived as: −d[CN⁻]/dt = k [CN⁻]^1.8 [H₂SO₅]^1.1 [H⁺]^0.06, with k = 3.8 (±2.7) × 10⁻⁶ L/mg min, at 25 °C.

Introduction

Liquid effluents containing cyanides are generated in some industrial operations such as gold and silver extraction, copper–zinc sulfide flotation, iron making, oil refining and metal-plating. Since most aqueous cyanide species are very toxic, these effluents must be treated before discharge into the environment. Typical legal discharge limits are of the order of 1 mg L⁻¹ for total cyanide and 0.2 mg/L for weak acid dissociable complexes (known as WAD cyanide), which include copper and zinc complexes, and free cyanide (HCN and CN⁻).

Two methods are employed for the detoxification of effluents containing cyanides in industrial plants: natural degradation and chemical oxidation.

In natural degradation the cyanide-bearing effluents are discharged into a waste pond where they become exposed to the elements. The main natural degradation mechanisms are: volatilization of HCN caused by neutralization to pH <9.3 of the excess alkalinity by CO₂ absorption from the atmosphere, direct and bio-oxidation of cyanide by dissolved O₂ absorbed from the atmosphere, and solar photolytic decomposition. Natural degradation is a slow process – typically requiring retention times of several days (Smith and Mudder, 1991, Akcil et al., 2003). In most cases the water in the pond will still have [CN] well above the discharge limits. Gold extraction plants that adhere to the International Cyanide Management Code have to ensure that [CN] is kept below 50 mg/L, even in ponds that only hold solutions that are recycled (Mudder and Botz, 2004).

Chemical oxidation in cyanide effluent treatment plants at mineral and/or metallurgical plants nowadays employ mainly hydrogen peroxide or a system based on the combination of sulfur dioxide plus air. Alkaline chlorination, a process that was used in the past, is no longer employed. This is mainly because of the formation of by-products (such as chloramines) in the treated effluent that are persistent and toxic to aquatic life, the risk of release of toxic and volatile CNCl reaction intermediate, and the significant hazard associated with the handling and storing of chlorine.

Although chemical oxidation carries higher operating costs, it is a much faster, more efficient and reliable way of destroying cyanide when compared with natural degradation. Several plants around the world treat their cyanide effluents through a combination of both processes.

With regard to the kinetics of oxidation, both oxidants mainly employed, hydrogen peroxide and the combination of sulfur dioxide (or metabisulphite) plus air (oxygen), generally perform well. For typical initial [CN] values between 200 and 400 mg/L these processes are reasonably fast (it may take from 30 to 120 min) and efficient (yield of residual CN_WAD less than 0.2 mg/L). But with these processes it is essential that the effluent either naturally contains ⩾20 mg/L of [Cu], or the treatment process includes the addition of a Cu²⁺ soluble salt as oxidation catalyst (usually as concentrated CuSO₄ solution), otherwise the oxidation reaction is too slow.

Although cyanide degradation with those oxidants is adequately fast and efficient for various types (compositions) of effluents, there are cases in which a high dosage of the oxidant may be necessary to achieve a higher reaction speed and cyanide removal efficiency level – for example, in the detoxification of slurry effluents (Kitis et al., 2005). In such cases, although it is possible to enhance the oxidation reaction by simply increasing the oxidant dose, other (more powerful) oxidants may be considered in order to attain the legal discharge limits while aiming at minimizing operating costs. In recent years Caro’s Acid – peroxymonosulphuric acid (H₂SO₅) – has been used in industry as a more powerful oxidant for the breakdown of cyanides. As with other oxidants, Caro’s Acid converts cyanide to cyanate. Cyanate then spontaneously hydrolyses yielding ammonium/ammonia and bicarbonate. As it happens in other cyanide oxidation routes, eventually ammonium may slowly become converted to nitrate in the water through bio-oxidation, and/or simply end up diluted in the water receiving body.

A number still relatively small of gold and silver hydrometallurgical plants already employ Caro’s Acid for CN oxidation in effluents; to our knowledge, at least five large scale plants are in operation in South America. For new projects, it is now common to include it in the protocol of experimental trials along with H₂O₂ and SO₂ + air.

The first reports of investigations into the Caro’s Acid oxidation of CN in effluents appeared in the early 1990s in the pioneering works published by Smith and Mudder, 1991, Nugent and Oliver, 1992, Castrantas et al., 1995, Smith and Wilson (2001), Mudder et al., 2001, Botz et al., 2005, and more recently by Breuer et al., 2010. These works describe Caro’s Acid oxidation of cyanide and related species in effluents under typical industrial conditions. Nevertheless, we think that it is still relevant to make an addition to the published knowledge on the actual kinetics of Caro’s Acid oxidation – in particular, the weight and interrelation of the process variables that can affect the speed and efficiency of reaction – as such knowledge may lead to useful improvements in treatment plant design and the optimization of existing operations.

Thus it has been the objective of the present work to conduct a systematic experimental study on the kinetics of oxidation of aqueous free cyanide with Caro’s Acid with a view to attempting to clarify the chemical interrelations of process variables and to build a useful kinetic model. The controlling variables considered at two levels are: initial [CN⁻]; H₂SO₅:CN⁻ molar ratio; and pH. Caro’s Acid was employed at two strengths (3:1 and 1:1) of H₂SO₄:H₂O₂ molar ratio. These control variables, their units and concentration ranges were chosen to correspond to those found by plant, process or design engineers in real cyanide detoxification operations. In such operations the concentration units and the legal limits used for effluent discharge into the environment are mg/L. The pH range of effluents before treatment are limited to pH 9–11, the effluent temperature is normally approximately 25 °C, and there are cost constraints that limit the dose and excess ratios of oxidant to CN⁻.