Properties and applications of hydroxamic acid derivatives

Hydroxamic acids are a class of organic compounds, which are carboxylic acid derivatives in which the divalent oxygen of the carboxyl group in the carboxylic acid molecule is replaced by an oxime group. Hydroxamic acids are generally light yellow, hard, greasy solids or yellow viscous liquids, with a density of d4 = 0.988 g/cm³ and an ionization constant K value of 2.0 × 10⁻⁹, making them extremely weak organic acids. They form weak acid-strong base salts with sodium (potassium) hydroxide. The pure product is a white, flaky crystalline solid, soluble in water, with its water solubility decreasing as the carbon chain length increases.

Hydroxamic acid is a typical chelating agent with high collecting power and high selectivity.  Its linking groups include alkyl (alkyl hydroxamic acid), benzyl (benzyl hydroxamic acid), salicyl (salicyl hydroxamic acid), naphthylmethyl (naphthylmethyl hydroxamic acid), etc., and the effect varies depending on the linking group.

Due to the presence of oxygen and nitrogen atoms with lone pairs of electrons in its molecular structure, and their close proximity, it can chelate with metal ions to form stable chelates. This special structure has led to the widespread use of hydroxamic acid compounds in the flotation of metal oxide ores, solvent extraction, wastewater treatment, and medicine.

For example, hydroxamic acid is used as an anti-tumor drug in medicine; as a collector in the flotation of various metal oxide ores such as copper oxide ore, hematite, yttrium-containing ore, wolframite, scheelite, ilmenite, niobium-containing ore, cassiterite, and rare earth minerals; in metallurgy, linear hydroxamic acid polymers can be used as flocculants and precipitants; in smelting, hydroxamic acid polymers can be used as extractants for non-ferrous metals such as gallium and germanium, and hydroxamic acid chelating resins can be used as adsorbents.

Alkyl Hydroxamic Acid

Alkyl hydroxamic acid is a dark red viscous liquid, which solidifies into a waxy substance below 15°C. It is slightly soluble in water and soluble in some organic solvents. This product has good collecting properties for various metal oxides, some difficult-to-process metal oxide ores, and some oxidized sulfide ores. Salicylhydroxamic Acid

Salicylhydroxamic acid is a pink to orange-red solid powder, slightly soluble in water, soluble in alkaline solutions, and stable in nature, with a salicylic acid odor. It can form stable chelates with metals such as tin, tungsten, rare earth elements, copper, and iron, while forming unstable chelates with alkaline earth metals and alkali metals. In particular, when salicylhydroxamic acid chelates with cassiterite, it can form not only various forms of outer coordination salts but also inner coordination salts with different structures. Therefore, salicylhydroxamic acid has strong selectivity for tin. This product is usually used in conjunction with P86 in cassiterite beneficiation and has certain foaming properties. This product also has the characteristics of low toxicity (one-sixteenth the toxicity of benzylarsonic acid, thus its application can greatly improve environmental protection issues), low dosage, and strong applicability, making it of high value for promotion and application.

Benzohydroxamic Acid

Benzohydroxamic acid is a pink, flaky solid powder, soluble in hot water and some organic solvents, with a slight benzoic acid odor. It is an effective collector for difficult-to-process minerals such as smithsonite, wolframite, scheelite, and cassiterite. Under specific conditions, benzohydroxamic acid can achieve relatively ideal separation indicators in the flotation of smithsonite; industrial applications show that when benzohydroxamic acid is used in combination with some other reagents in the flotation of wolframite and scheelite, ideal flotation results are achieved with significant improvements in concentrate grade and recovery rate.