Purification of graphite by mixed acid method

With the wide application of high purity graphite in high-precision industry, further purification of graphite has become the focus of scholars. Graphite was purified from flotation concentrate by mixed acid method. The acid leaching reaction time and solid-liquid ratio experiments were carried out. The graphite raw material and water-solid-liquid ratio was 1:2. The reaction time was 6 hours and the reaction temperature was 80℃. Under the optimum mixed acid dosage, 99.5% high-purity graphite was obtained.
Graphite, as an important strategic mineral resource, has attracted more and more attention since the 21st century. Experts have predicted that the 21st century will be the century of carbon. With the exhaustion of high-quality graphite resources and the increasing demand for graphite purity in the process of scientific and technological development, the research of extracting graphite with higher purity from traditional process has become the focus. In recent years, acid-base method, hydrofluoric acid method, chlorination roasting method and high temperature method are widely used in graphite purification process. These methods have achieved certain results, and each has its own advantages and disadvantages. Nowadays, graphite purification is developing toward high efficiency, energy saving and environmental protection. In view of the high toxicity and corrosiveness of hydrofluoric acid, the orthogonal experiment of mixed acid method is used to determine the optimum proportion of mixed acid to purify graphite and reduce the amount of hydrofluoric acid, so as to obtain qualified high-purity graphite products.
1 raw ore properties
The raw ore was taken from the flotation concentrate of a certain graphite concentrator in Jilin Province. The sample preparation steps were as follows: sampling raw materials, preparing experimental samples, analyzing samples and reserve samples after mixing and shrinking the samples.
1.1 phase composition of raw materials
The phase composition of raw materials was determined by X- powder diffraction (XRD). It can be seen from the XRD map of the original ore that the main minerals in the raw materials are graphite and saponite minerals. According to the semi-quantitative method of XRD, the contents of the two minerals are approximately 92% of graphite and 8% of saponite.
XRD results show that the raw material used in this experiment is relatively simple mineral composition, the main impurity mineral is saponite, and saponite as clay mineral has fine particle size, easy to adhere to the surface of graphite, poor physical and chemical activity, which is difficult to prepare high-purity graphite.
1.2 analysis of major chemical elements in raw materials
The fixed carbon content in the raw material measured by calcination method is 91.39%, of which the ash content is 7.20%, the volatile content is 1.41%, and the fixed carbon content is basically consistent with the XRD analysis of the raw material. Therefore, the fixed carbon content in the raw material can be determined to be 91%-92%. The chemical elements are determined by calcination method, and the ash obtained is analyzed by chemical multi-element analysis. The results show that the main metal elements in the raw materials are Si, Al, Fe, Mg and so on, which is basically consistent with the XRD analysis of the impurity minerals in the raw materials as saponite.
2 experimental method
2.1 purification process of graphite
Graphite raw materials are weighed and mixed with a certain amount of water evenly. The measured acid is added, stirred and heated, deacidified after a period of reaction, washed to neutral, dehydrated and dried, and high purity graphite products are obtained.
2.2 effect of acid type and dosage on graphite purification
According to the previous experiments and practical production experience, the purification temperature is 80 C, hydrochloric acid 100 g / kg, sulfuric acid 400 g / kg, nitric acid 400 g / kg, hydrofluoric acid 100 g / kg, the fixed carbon content of graphite after acid leaching is more than 99%. The amount of nitric acid can be reduced appropriately, and the amount of hydrochloric acid 100 g / kg and sulfuric acid 300 g / kg can be determined. The amount of hydrofluoric acid is determined by further experiments.
The dosage of hydrofluoric acid is the most important factor in the experiment of preparing high-purity graphite by acid leaching. The dosage of hydrofluoric acid should be optimized. The experimental conditions were as follows: hydrochloric acid dosage 100 g/kg, sulfuric acid dosage 300 g/kg, reaction time 8 h, reaction temperature 80℃. The relationship between hydrofluoric acid dosage and graphite fixed carbon content was investigated.
The results showed that the fixed carbon content in the product could reach 99% when the dosage of hydrofluoric acid was 100-200 g/kg, but only 98.78% when the dosage reached 250 g/kg. The dosage of hydrofluoric acid was 150 g/kg considering the factors of guaranteeing the effect of experiment and saving the cost of medicine.
2.3 effect of acid leaching time on graphite fixed carbon content
In order to determine the effect of acid leaching time on fixed carbon content, the experiment of acid leaching time was carried out.
The experimental conditions were as follows: hydrochloric acid dosage 100 g/kg, sulfuric acid dosage 300 g/kg, hydrofluoric acid 150 g/kg, reaction temperature 80℃, solid-liquid ratio 1:1. From the experimental results, with the increase of acid leaching time, the fixed carbon content of the product slightly increased with the increase of reaction time in the first few hours, the increase of fixed carbon content was not significant when the acid leaching time reached 6h. The acid leaching time was determined to be 6h considering the experimental results, production cost and energy consumption.
2.4 effect of solid-liquid ratio on Purification of graphite
The main purpose of solid-liquid ratio experiment is to determine the initial ratio of graphite raw material to water volume during acid leaching. Based on past experience and existing production technology, the following parameters are determined for experiments. It can be seen from the experimental results that when the solid-liquid ratio is 1:1 and 1:2, the fixed carbon content in the product is slightly different. When the solid-liquid ratio is 1:2, the fixed carbon content in the product is slightly higher. In order to ensure the product quality, the solid-liquid ratio of graphite raw material to water is 1:2.
The raw material mineral components are relatively simple. The main mineral components are graphite and saponite. The semi-quantitative calcination method by XRD gives about 92% graphite and 8% saponite. The fixed carbon content in the raw material was 91.39%, of which ash content was 7.20%, volatile content was 1.41%. The main metal elements were Si, Al, Fe, Mg, etc.
The mixed acid method was used to prepare high purity graphite. The optimum conditions of the mixed acid method were obtained through the experiments of the kinds and dosage of acid, the reaction time of acid leaching and the ratio of solid to liquid. The optimum conditions were as follows: hydrochloric acid 100 g/kg, sulfuric acid 300 g/kg, hydrofluoric acid 150 g/kg, the ratio of graphite to water 1:2, the reaction time 6 h. High-purity graphite with fixed carbon content of 99.5% can be obtained by one-step acid purification process at reaction temperature of 80℃.
Author: Gu Shunqi