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.