An article to understand the technology and characteristics of silica functional materials prepared from industrial solid waste

Electric furnace yellow phosphorous slag, steel and non-ferrous metal smelting process slag, building materials and fly ash from the power industry are common bulk solid wastes.

Depending on the sources and production methods of solid waste, other different mineral phase components may also be contained, and sometimes rare earth elements, alumina and other high-value non-ferrous metal elements may also be present

1.

White carbon black is a white amorphous nano-silica substance with excellent porosity, high dispersibility, thermal stability and good electrical insulation.

At present, the use of solid waste to prepare white carbon black mainly adopts the precipitation process, which can be subdivided into the acid dissolution method and the alkali fusion (dissolution) method

Alkali melting (dissolving) method uses solid waste and alkali to react in molten or aqueous solution to prepare water glass, and then pass CO2 or react with hydrochloric acid or sulfuric acid solution to precipitate silica in flocculent form, filtered and washed , After drying, a white carbon black product is obtained

In addition to silica, fly ash also contains high-value aluminum oxide.

In summary, the acid solution method is mainly aimed at the preparation of silica from impurities containing acid-soluble metal oxides or solid wastes rich in silica in rare earth oxides with high value.

2.
Preparation of silica aerogel from solid waste

Since the silica airgel having a large specific surface area, large pore volume characteristics, the low bulk density and low thermal conductivity and the like, can be used in the catalyst art transparent insulating material, a film, a light emitting radio devices, drug delivery systems and the like
.
Generally, a sol-gel process is used to prepare aerogel materials from silica in solid waste through three steps of synthesis, aging and drying
.

When using solid waste to prepare silica aerogel, firstly, the solid waste and alkali are reacted to prepare water glass of different modulus, and then an acid solution is added to the water glass to make it gel directly, or the water glass is subjected to cations.
After the resin is exchanged, a silicic acid gel is prepared
.
The aging stage is to treat the moisture of the formed gel pores.
This process lasts for several hours or several days at room temperature, or is carried out at 50-60°C, which can significantly shorten the time of this stage
.

The aging process of the gel is usually carried out with molecular alcohols such as methanol and ethanol.
When higher alcohol homologues (such as n-propanol, n-butanol, isobutanol or n-pentanol) are used, the synthesized gas condenses Glue has better parameters
.
Supercritical drying or atmospheric drying process is often used in the drying stage.
The supercritical drying process can avoid capillary stress and subsequent drying shrinkage, thereby preparing an aerogel that maintains the original shape and structure of the wet gel
.
However, due to the harsh conditions of high temperature and high pressure and the flammability of organic solvents, supercritical drying is very dangerous.
In practice, atmospheric drying is used.
The atmospheric drying equipment is simple and cheap, and has a broader prospect
.

3.
Preparation of zeolite from solid waste

Due to its porous structure, zeolite can hold a variety of cations, has one or more properties such as adsorption, ion exchange and catalysis, and is widely used in the following areas:

(1) Industrial adsorbent in the process of separation and purification;

(2) Preparation of catalysts for advanced materials;

(3) Membrane materials and additives for membrane processing;

(4) Ion exchange materials;

(5) chemical sensors
.

Zeolite is generally synthesized by hydrothermal treatment of aluminosilicate gel formed from alumina source and silica source.
However, due to the high cost of synthesizing zeolite from alumina source and silica source, it is necessary to find cheap materials for synthesizing zeolite from solid waste.
Become a research hotspot
.
There are many types of commonly used industrial solid waste, such as fly ash, lithium slag, scrap metal slag and cupola slag
.

There are two main methods of using solid waste to synthesize zeolite: one method is the traditional synthesis method, that is, the solid waste is hydrothermally synthesized after alkali fusion; the other method is a new process of microwave or ultrasonic assisted synthesis
.
The traditional synthesis method is to perform alkali fusion treatment on the solid waste that can provide silica and alumina, and then conduct hydrothermal synthesis
.
Alkali melting treatment refers to the use of sodium carbonate or sodium hydroxide as an activator to decompose alumina and silica in solid waste and convert them into soluble sodium silicate and sodium aluminate, and then prepare zeolite under hydrothermal synthesis conditions
.

The hydrothermal synthesis process can be divided into three steps: the dissolution of Si4+ and Al3+ in the solid waste in the alkaline solution; the dissolved silicate ions and aluminate ions react in the alkaline solution to form aluminosilicate gel; aluminosilicate The salt gel crystallizes to form zeolite crystals
.
The use of microwave and ultrasonic radiation to catalyze and strengthen the chemical reaction characteristics, the introduction of microwave or ultrasonic assisted synthesis technology in the process of preparing zeolite from solid waste, that is, the use of microwave or ultrasonic radiation to catalyze the alkali melting process can significantly reduce the time and energy consumption of zeolite synthesis , And improve the crystallinity of zeolite
.