Two mainstream continuous basalt fiber preparation technologies

1. Flame Method

The flame method involves a production process where heat is directly delivered to the surface of the Basalt melt within a refractory brick-structured basalt kiln. This heat is typically generated by flames (such as natural gas-oxygen or hot air combustion, or plasma flames) from the top of the furnace. This primary heating method can be supplemented by bottom electrode heating. The entire process covers melting, clarifying, and forming.

Small, standalone flame furnaces, which are currently mainstream in the industry, only use top natural gas combustion heating and lack auxiliary bottom electrodes. However, due to their high energy consumption, high production costs, and low product cost-effectiveness, most companies using this technology are experiencing severe losses and are on the verge of bankruptcy.

The development direction for the flame method is the flame-heated tank furnace, which utilizes a "gas-electric combination" approach with top natural gas-oxygen combustion and auxiliary bottom electrode heating. This "gas-electric combination" method is the absolute mainstream technology for manufacturing Glass Fibers, and these glass fiber kilns operate very maturely and successfully, especially the unit kilns, which have almost become the standard for glass fiber tank furnace drawing kiln designs. Efforts have been made to transfer this technology to continuous basalt fiber manufacturing, but despite limited trials, success has not yet been achieved. In the past two years, some have taken a different approach, changing from pure natural volcanic rock raw materials to formulated raw materials (i.e., incorporating a large proportion of non-volcanic rock). This has led to the successful commissioning and operation of 10,000-ton/year and 3,500-ton/year flame-heated tank furnace production lines.

2. All-Electric Melting Method

The all-electric melting method involves a production process where electrical energy is directly delivered into the high-temperature basalt melt within a refractory brick-structured basalt kiln. This is achieved through electrodes (such as graphite, molybdenum, tin dioxide, etc.) or (and) other physical methods (such as plasma methods). This technology covers melting, clarifying, and forming.

China's continuous basalt fiber all-electric melting method began with the national 863 Program in 2002, which completed a small-scale standalone furnace drawing apparatus using this method. Significant breakthroughs in continuous basalt fiber all-electric melting drawing technology were achieved in 2016, with the completion of a pilot-scale thousand-ton/year all-electric melting tank furnace. This system uses multi-row progressive electrodes, allowing for a melt liquid level depth of up to 1300mm. The product monofilament diameter is concentrated between 9-22μm, and the comprehensive unit power consumption is 3.0-3.5 kWh/kg, demonstrating excellent energy-saving effects. In 2018, a 1200-ton/year all-electric melting tank furnace production line ("one-to-eight," using 400-hole spinnerets) was officially put into operation. It has run stably for over three years, verifying that the kiln life can reach more than three years.

To date, for pure natural volcanic rock raw materials, continuous basalt fiber manufacturing technology is only maintained at the thousand-ton/year tank furnace technology level, and exclusively for the all-electric melting method.

3. Comparison of the Two Technological Routes

The characteristics of high-temperature basalt melt, namely its poor thermal conductivity, high viscosity, and short material properties, are precisely what make the manufacturing of continuous basalt fiber challenging.

• Flame Method

The flame method, a relatively mature technology introduced from the former Soviet Union (now Russia and Ukraine) and adapted for China's specific conditions, has seen widespread use. However, its biggest drawback in industrialization is high production costs and low cost-effectiveness, largely due to inherent physical structural defects in the method itself.

Low Heat Utilization

In this method, natural gas is combusted from the furnace top, with the flame directly heating the basalt melt surface. Over 60% of the heat is reflected by the melt surface and carried away by exhaust gases. Given that high-temperature basalt melt has a thermal conductivity ten times lower than high-temperature glass melt, heat transfer is extremely slow. Small, single-unit furnaces can only maintain a melt depth of about 15cm. While 10,000-ton/year flame-heated basalt batch tank furnaces can reach a melt depth of 50cm with auxiliary bottom electrode heating, the melt forms a dish-like structure within the furnace, leading to a large specific surface area and significant heat dissipation. Heat loss through insulation materials exceeds 10%. Consequently, the actual heat utilization rate is less than 30%.

Low Melting Quality

Due to the shallow melt level in the flame method, the clarification and homogenization sections cannot achieve thorough homogenization, resulting in lower melting quality.

Exhaust Gas Emissions

Natural gas combustion produces exhaust gases such as sulfur and nitrogen oxides.

Greenhouse Gas Emissions

As a fossil fuel, natural gas combustion releases significant amounts of CO2, a greenhouse gas.

High Equipment Investment

Addressing exhaust gas emissions from natural gas combustion necessitates pollution control measures. The low heat utilization also requires waste heat recovery measures. Furthermore, pure oxygen combustion requires oxygen generation equipment. These three factors significantly increase equipment investment. The unit investment for the flame method is approximately 11,000-20,000 RMB per ton.

• All-Electric Melting Method

Compared to the flame method, the all-electric melting method offers notable advantages.

High Melting Quality

The all-electric melting technology is based on the principle that the melt is electrically conductive in a high-temperature molten state, allowing electrical energy to be directly supplied to the melt for internal heating. The vertical arrangement of electrodes facilitates vertical melting. Thousand-ton per year all-electric melting tank furnaces can achieve a melt depth of over 1.2 meters, providing a longer clarification and homogenization section. The high-temperature isothermal zone within the tank is deeper, leading to better melting and homogenization quality for basalt.

Energy Efficiency

Direct internal heating of the melt, vertical melting, deeper tanks, and cold material coverage on the melt surface contribute to high melting rates and high thermal efficiency. Firstly, electrodes directly inserted into the melt ensure full utilization of Joule heat. Secondly, the deep melt level, with its depth approaching the furnace's internal diameter, results in a smaller, near-minimal specific surface area for the melt. This geometric structure significantly reduces heat dissipation compared to the dish-like structure of the flame method. Thirdly, the cold material coverage on the melt surface forms a "cold furnace top," further reducing heat loss.

Low Carbon Footprint

The all-electric melting technology eliminates the carbon emissions associated with natural gas combustion in the flame method. Its carbon emissions are solely determined by the power grid's energy mix. If hydropower or other renewable energy sources are used, zero carbon emissions can be achieved.

Lower Investment

Since the all-electric melting method doesn't involve pure oxygen combustion of natural gas, there's no need to invest in exhaust gas environmental treatment equipment or oxygen generation equipment. Additionally, the cold material coverage on the melt surface means no investment in waste heat recovery equipment is required. The unit investment for the all-electric melting method is therefore lower.

Cost Advantage

Significant energy savings and lower fixed asset depreciation translate into a distinct cost advantage.