Flotation Reagents: The Role and Application of Thioglycolic Acid

Classification and Function of Flotation Reagents

In the flotation process, adjusting agents are used to improve selectivity, enhance the performance of collectors and frothers, reduce the mutual contamination of useful mineral components, and optimize pulp conditions.

Flotation reagents can be classified into several types based on their functions:

Thiol Compounds as Inhibitors: A Key Role in Flotation

Thiol compounds are important inhibitors in the froth flotation process.

They improve the selectivity of flotation by preventing the adsorption of non-target minerals on collectors or by forming a hydrophilic membrane on the mineral surface.

Types and Characteristics of Thiol Compounds

Thiol compounds used as inhibitors are typically short-chain compounds with carbon chain lengths ranging from C1 to C5. Common thiol compounds include:

• Thioglycolic Acid (or its sodium salt)
• Thioethanol
• Y-Mercaptopropanol and its derivatives

These compounds are primarily used to inhibit the flotation of sulfide minerals such as sulfide copper and sulfide iron minerals.

Synthesis Methods of Thioglycolic Acid

Thioglycolic Acid (TGA) can be synthesized through several methods, including:

1. Sodium Sulfide Method: Using sodium sulfide, sodium chloroacetate, and hydrochloric acid as raw materials, with the addition of sodium chloride. The mass ratio of the raw materials is Na₂S:CICH₂COONa:HCl = 2.2:1.0:0.7. Reaction conditions include a temperature of 75°C, reaction time of 2 hours, and pressure of 0.4 MPa, resulting in a conversion rate of 65.6%. Sodium hydrosulfide (NaHS) can also be used as a substitute for Na₂S. After the reaction, the product can be precipitated by acidification.
2. Thio-Sulfate Sodium Method
3. Thiourea Method
4. Trisulfide Carbonate Sodium Method
5. Alkyl Xanthate Method
6. Electrochemical Reduction Method

Chemical and Storage Properties of Thioglycolic Acid

Physical Properties

Thioglycolic Acid is a colorless, transparent liquid with a pungent odor. It is miscible with water, ether, alcohol, and benzene.

Its density is 1.3253 g/cm³, with a melting point of -16.5°C and a boiling point of 60°C (decomposes at 133.3 Pa).

Chemical Properties

1. Acidity: Thioglycolic Acid solution is acidic, with an acidity stronger than acetic acid. Both the -COOH and -SH groups in its molecular structure can undergo acid dissociation. The first and second dissociation constants (pKa values) are 3.553.92 and 9.2010.56, respectively.
2. Oxidation: Thioglycolic Acid (especially in alkaline solutions) is easily oxidized by air to form dibasic acid or disulfide salts. Trace amounts of copper, manganese, or iron ions can accelerate the oxidation reaction. Weak oxidizing agents (e.g., iodine) can oxidize thioglycolic acid, while strong oxidizing agents (e.g., nitric acid) can oxidize it to HO₃SCH₂COOH.
3. Self-Polymerization: Pure thioglycolic acid can self-polymerize at room temperature. A 98% pure thioglycolic acid solution can lose 3%~4% of its content after one month of storage. To slow down the polymerization reaction, 15% water is typically added.

Safety and Storage

• Corrosivity: Thioglycolic Acid salts are corrosive. Protective measures are required during handling. If skin or eyes come into contact with the substance, rinse thoroughly with water and apply medication afterward.
• Toxicity: Thioglycolic Acid has moderate toxicity, with an LD₅₀ of 250300 mg/kg for poultry and 120150 mg/kg for rats. At lower concentrations, it does not affect plant growth, and due to its susceptibility to oxidation in air, it does not accumulate toxic effects in the environment. Its toxicity is lower than that of sodium sulfide, making it an ideal substitute for cyanides, sodium sulfide, and sodium hydrosulfide in flotation.

Applications and Effects of Thioglycolic Acid

Mechanism as an Inhibitor

Thioglycolic Acid contains two functional groups (-SH and -COOH):

• The -SH group can interact with the surface of sulfide copper and pyrite minerals and adsorb onto the mineral surface.
• The -COOH group does not have collector properties due to its short carbon chain, but it is strongly hydrophilic and can form a water membrane on the mineral surface.

Thus, thioglycolic acid is an effective inhibitor for sulfide copper and pyrite minerals.

Case Studies: Industrial Applications

1. Patent by American Cyanamide Company
   In 1948, the American Cyanamide Company patented thioglycolic acid under the trade names “Aero 666” (thioglycolic acid) and “Aero 667” (50% thioglycolic acid sodium solution). These products were used in a large copper concentrator to inhibit sulfide copper minerals during the flotation process.
2. Industrial Trial by Jinchuan Company
   From September 1984 to October 1985, the Xi’an Institute of Metallurgy conducted an industrial trial at a Jinchuan Company concentrator, using sodium thioglycolate as a replacement for NaCN and Na₂S. The results showed:

• Sodium thioglycolate had a significant inhibitory effect on chalcopyrite and could also inhibit gangue minerals such as silicate.
• Its dosage was only 50% of that of sodium cyanide, but the recovery rates were similar.

Based on the success of both laboratory and industrial trials, Jinchuan Company fully implemented the use of sodium thioglycolate as an inhibitor in copper flotation in 1994.

3. Inhibitory Effects at Different pH Values
   Research on the inhibitory effects of thioglycolic acid on chalcopyrite and sphalerite at different pH values revealed that thioglycolic acid has a strong inhibitory effect on chalcopyrite but almost no effect on sphalerite. At a pH of 10.5, thioglycolic acid can effectively separate chalcopyrite from sphalerite.

Multiphase Reaction Mechanism

Raghavan et al. studied the interaction between 98% pure thioglycolic acid and chalcopyrite (-0.1mm + 0.074mm particle size). The results showed:

• Thioglycolic acid is rapidly adsorbed onto the chalcopyrite surface and undergoes extensive oxidation to form disulfide compounds.
• The reaction mechanism involves the adsorption of thioglycolic acid molecules at the interface, followed by a reaction between the adsorbed molecules and those in solution to form disulfide compounds. The reaction can be expressed as:
  2HSCH₂COOH (adsorbed) + 2HSCH₂COOH (solution) + O₂ → 2HOOCCHS-SCH₂COOH + 2H₂O

Both thioglycolic acid and its disulfide derivatives are hydrophilic and can form a water membrane on the mineral surface, leading to inhibition.