Hydrated Zinc Borates and Their Industrial Use

Hydrated Zinc Borates and Their Industrial Use

1. Introduction
Zinc borate ranks among the top ten boron-containing industrial chemicals in global production and use. With its special properties, tens of thousands of tons of zinc borate are used every year for a variety of applications. The zinc diborate family with a total composition of aZnO·bB2O3·cH2O, wherein c> 0, contains at least thirteen unique crystalline compounds; the most important of which is 2ZnO·3B2O3·3H2O or Zn [B3O4(OH)3], Due to early characterization errors, it is commonly referred to commercially as 2ZnO·3B2O3·3.5H2O. The main applications of zinc borate include providing durability to biocomposite building materials and improving the fire and electrical properties of the polymer. Zinc borate is also used as a corrosion inhibitor in coatings, as a flame retardant and preservative, as a flux in ceramic and glazes, as a matrix for scintillation compounds, and as an ingredient in agricultural micronutrients. The chemistry of hydrated zinc diborate is reviewed here, with a focus on industrial scale commercial interests. The use of industrial zinc borate is discussed, including their role as preservatives and as flame retardants in biocomposite building materials. Highlights outstanding issues in the field.

Table 1 lists the crystalline hydrated zinc borate that is well documented in the chemical literature. They are arranged according to their boron-richness, as defined by the B2O3 / ZnO molar ratio or Q value. In this review, the resolved oxide formulas are used interchangeably because they are widely accepted in commercial environments and can also be used to approximate the composition of zinc borate (which has not yet been structurally characterized). Oxide formulations are used almost exclusively in the technical literature related to industrial applications of zinc borate, as well as in regulatory documents.

As the 13 zinc borate listed in Table 1, five are manufactured and sold in truck loading and unloading. These will be briefly described in the next section. Although most crystalline zinc borate has a well-defined composition and structure, some are not fully characterized, including some commercially produced zinc borate. Although this review focuses on hydrated zinc diborate, which has the greatest commercial activity in terms of throughput, it can be mentioned that several crystalline anhydrous zinc borate, including Zn4O(BO2)6, have been structurally characterized ( 4ZnO·3B2O3), Zn3(BO3)2(ZnO·B2O3)[3] and ZnB4O7(ZnO·2B2O3). They can be used in glass manufacturing and electro-optic materials. The chemical properties of metallic and non-metallic borates are reviewed.

 

2, industrial zinc borate
2.1. 2ZnO·3B2O3·3H2O (Q = 1.5) or Zn [B3O4(OH)3]
Today, this recognized commercial compound is produced much more than other zinc borate. In the zinc borate phase, it is generally more practical in most applications due to its relatively high dehydration onset temperature (about 290 ° C). It is also the only zinc borate that has been sterilized by certain brands, including the US EPA and the Canadian PMRA. It is commonly referred to as 2ZnO·3B2O3·3.5H2O in commerce. Its earliest description was first discovered in the 1960s by German academic researchers and industrial researchers in the United States. In the 1960s, industrial chemists developed a practical manufacturing process involving the reaction of zinc oxide with excess boric acid in water, and began commercial production on a multi-ton scale around 1970. This zinc borate is currently produced by a number of manufacturers and sold under various names including Firebrake® ZB, Borogard® ZB, Composibor®, ZB-Shield, ZB 2335, ZB-467 and the like. Lehmann and colleagues described the method of synthesizing 2ZnO·3B2O3·3H2O compounds in 1967 and provided powder X-ray diffraction data that matched modern commercial products. Industrial researchers also discovered the compound in the 1960s, but described it as 2ZnO·3B2O3·3.5H2O. The trade-off between the analyses indicated that the composition ranged from 2ZnO·3B2O3·3.3H2O to 2ZnO·3B2O3·3.7H2O. It was not until 2003 that the single crystal X-ray diffraction study clearly determined its composition as 2ZnO·3B2O3·3H2O and provided the structural formula Zn [B3O4(OH)3]. By that time, the composition 2ZnO·3B2O3·3.5H2O was firmly rooted in the commercial literature and continued to this day. This slightly inaccurate composition can also cause other erroneous nomenclature in commercial applications, such as 4ZnO·6B2O3·7H2O. All of these compositions are the same compounds best described by the formula Zn [B3O4(OH)3]. Initial report by Lehmann et al. It is pointed out that 2ZnO·3B2O3·3H2O is formed by a mixture of boric acid and zinc oxide in a molar ratio of 4:1 at 165 ° C in a closed tube. This hydrothermal synthesis condition is impractical for the manufacture of multi-ton scale products within this range of values. Nies et al. In a 1970 patent, a method for producing zinc borate in water from zinc oxide and a stoichiometric excess of boric acid in the presence of product seeds is disclosed in a patent at a temperature as low as 75 ° C. Manufacturing method. The temperature required to form this phase is about 70 °C. In the past fifty years, although the detailed manufacturing parameters for the production of this zinc borate are still proprietary, many methods for the methods originally described by Nies have been published [12, 20-25]. Although the reaction of zinc oxide with boric acid has been the primary manufacturer's choice for decades, many smaller producers have also prepared this by reacting zinc salts (such as zinc sulfate) with borax and borax. Compound. Boric acid, as shown in the equation. A disadvantage of this process is that it produces by-product salts that must be disposed of, while the Nies process produces only by-product water and recycles the weak reaction liquid to subsequent batches.

 

A commercially important property of Zn [B3O4(OH)3](2ZnO·3B2O3·3H2O) is that its relatively high dehydration onset temperature is about 200 °C. It is 290 ° C compared to other zinc borate phases. Most borate crystallized from water have free water of crystallization or B-OH groups which condense into water at relatively low temperatures. Zinc borate Zn [B3O4(OH)3] is one of the few borate salts which crystallize out of water at industrially practical rates under non-hydrothermal conditions and also exhibits a higher dehydration onset temperature. Only the zinc borate Zn2(BO3)OH (4ZnO·B2O3.H2O) has a higher dehydration initiation temperature (~411 °C). And can be produced in the actual non-hydrothermal process.

Zinc borate Zn [B3O4(OH)3] crystallizes in the monoclinic space group P21 / c. Its structure is based on the linked cyclic B3O4(OH)3 basic component chain, as shown in Figure 1 [15]. The tetrahedral Zn2+ cation is coordinated by three independent polyborate chains of boronic acid B-O-B and B-OH oxygen atoms. A portion of the polyborate chain structure in Zn [B3O4(OH)3] is shown in Figure 2. The structure of this zinc borate is similar to the important industrial borate minerals, such as CaB3O4(OH)3.H2O (2CaO·3B2O3·5H2O), because they all contain polyborate chains [26]. However, higher coordination requirements for calcium ions in the perovskite cause the inclusion of water in the structure.