Studies on the precipitation of metals by magnesium hydroxide

Abstract

Industrial waste has historically been treated with a number of alkalis, the purpose of which is to lower the concentration of metals dissolved in the waste below a certain level, imposed by regulatory bodies, for discharge into the nation's water systems. The alkalis most commonly used are sodium hydroxide, sodium carbonate and lime. Magnesium hydroxide is now being considered as an industrially viable alternative to these alkalis. Among the advantages of magnesium hydroxide are cost, manageability and safety considerations, sludge settling times and waste volumes. Redland Magnesia Products (previously Steetley Magnesia Products) have been producing magnesium hydroxide for many years, mostly to produce high temperature resistant bricks, used in kilns and furnaces. In the past few years the company has started to produce "Neutramag"; a concentrated slurry of magnesium hydroxide in water. However, the company has had problems precipitating certain metals from samples of industrial waste streams. The problem is of great concern as it is preventing them selling the product to many potential buyers. This project, and the work therein, was initiated by the company in the hope of understanding the problem at hand and hopefully producing a solution to the problem. Reactions were performed firstly on single metal solutions of zinc and nickel (two of the "problematic" metals) and then on mixtures of the two. Initially magnesium acetate was added to the solution in the expectation that the acetate ions would form some acetic acid in situ and thus form a buffer solution. Although the addition of acetate ions did have an effect on the reaction it was not due to any buffering action. Then different metal acetates were used to see if the effect was general or specific to magnesium acetate. Ammonium acetate gave the best results of any of the acetates tried and so ammonium nitrate was added to the reaction to see if it was the acetate or the ammonium ions that were causing the effect. This reaction was successful, and it was realised that in the reaction conditions, ammonia would be generated, which could act as a ligand to the metals. Hence the idea of the "ligand" effect was generated. Other ligands were added (organic and inorganic) and there was a direct correlation between the rate of reaction and the position of the added ligand in the spectrochemical series, acetate < ammonia < TMEDA < 2,2-bipyridine < PPh(_3). The best ligand was found to be triphenylphosphine. The studies were extended to some real industrial waste samples supplied by Redland, several of which contained large quantities of iron. The addition of PPh(_3) showed another effect on the addition of Neutramag to a dilute iron (II) solution. The precipitate was iron (III), the addition of the ligand having increased the rate at which the iron (II) was air oxidised. Oxidation of iron (II) to iron (III) helps precipitation as the solubility product of the hydrated oxide of iron (III) is much smaller than that of iron (II). Air will oxidise iron (II) although this process can be speeded up by aerating, the solution. This, along with the addition of PPh(_3), was found to increase the rate of reaction significantly. A considerable improvement in clean-up of waste samples, compared with previous results has been achieved. As the supply of industrial waste was small, a generic waste was made up with a constitution close to that of one of the more concentrated iron wastes. It was shown that due to the insolubility of PPh(_3) there is direct reaction of the solid ligand with the metal ions in the solution. Brief attempts were made to try and introduce the ligand into the reaction by different methods in the hope of increasing the rate even further.