1. Introduction

The special properties of boron and its compounds make it an important application in various industries as a raw material[1,2]. The raw materials of boron are mainly derived from solid boron ore and brine ore. During exploitation of salt lake brine, high-efficiency separation of boron can lead to high-quality boric acid products, while minimizing influence of boron on the quality of valuable elements such as lithium and magnesium[3]. For the recovery of boric acid, several methods which have been studied are boron precipitation[4,5], adsorption[6-8], membrane filtration[9,10], electrodialysis[11,12], reverse osmosis[13,14], solvent extraction[15-17]. However, most of these processes are unsuitable for SL industrial application due to the low boron recovery rate and the high concentration of brine components. On the other hand, low capital cost and relatively simple operation technique make solvent extraction a promising process of large-scale application for recovering boric acid from SL brine. Thus, solvent extraction process has been widely employed to recover boron from SL brine.
In general, unitary alcohols, diols and some polyols have excellent properties for boron extraction[18,19]. Among these extractants, diols have demonstrated a high extraction efficiency over a wider pH range since diols can form relatively stable five-membered or six-membered cyclic borate compounds with boron[20], especially 1,3-diols are considered to be the most effective. Narbutt et al.[21] used 2-ethyl-1,3-hexanediol and 2-ethyl-2-butyl-1,3-propanediol to extract boron from radioactive corrosion and fission products dissolved in aqueous solutions. Karakaplan et al.[22] synthesized 9 different 1,3-diols and compared the boron extraction effects of the synthesized extractants, in which 2,2,4-trimethyl-1,3-pentanediol and 2,2,5-trimethyl-1,3-hexanediol had a high extraction effect. The results showed that the extractant had good extraction performance for boric acid, with a single-stage extraction rate of >95%. Hano et al.[23] studied that 2-ethyl-1,3-hexanediol (EHD) dissolved in kerosene is considered to be the best extractant to remove and recover boron from hot spring water. When the pH value was 1~8, the extraction rate reached 90%, but it decreased when the pH was greater than 8. However, due to the high solubility of EHD in water, EHD is not considered suitable for practical application in boron removal from water. The extraction of boron using 2-butyl-2-ethyl-1,3-propanediol and 2-ethylhexanol at different boron and extractant concentrations was investigated by Kwon et al.[24]. The results of equilibrium concentration of the extractants on distribution of boron indicates that the mechanism of boron extraction by these extraction systems is the same.
The SL in the Qaidam Basin of China is rich in boron resources. However, at present, the extraction process of boron from SL brine in Qaidam, Qinghai province, China is to acidify the brine first, and then extract boric acid by monoalcohol extraction system[25]. The acidification process will precipitate a large amount of boric acid, resulting in an increase in production costs and a low recovery rate of boric acid. Therefore, a new process needs to be found to remove the brine acidification process.
In this contribution, a 1,3-diol, 2,2,4-trimethyl-1,3-pentanediol (TMPD) was introduced to examine the possibility of extracting boron from SL brine without acidification. The optimal extraction parameters were determined by performing extraction experiments under the influence of different factors. The entire extraction process was carried out by the separation funnels, and the organic materials were recycled and reused. Furthermore, we investigated the mechanism of the complexes formation by means of Raman spectroscopy, IR spectroscopy and the slope ratio method.