Study on new technology of flash roasting of boron mineral powder


In order to produce borate products with boromagite as raw material, iron concentrate is usually selected first, and then the tailings containing boron are processed into powder boron concentrate. In alkali process, boron ore must be roasted to improve the activity of boron trioxide. In this paper, a new process of flash roasting of low grade borite which combines pre-drying and activation roasting is proposed. On the basis of laboratory test and industrial production, the process was evaluated comprehensively, and the corresponding improvement scheme was put forward. The process is efficient and feasible, and the activation rate of boromagite can be increased to more than 85%~90% under certain grade. It plays an important demonstration role for the comprehensive utilization of low grade boromagite in Liaoning Province and even the whole country.
Key words: borite; Flash roasting; Activation rate; Process equipment
1 Foreword
Some boron compounds have the special properties of light weight, flame retardant, heat resistance, high hardness, high strength, wear resistance and catalysis, which play a very important role in modern industry and research field. Boron and boron compounds are widely used in chemical, metallurgical, optical glass, national defense, atomic energy, medicine, rubber and light industry sectors.
At present, our proven reserves of boron mine are about 50 million tons (B2O3) with low grade, half of which is mixed type boron and iron borite ore, the halide type and the salt lake solid boron ore reserves of about 20 million tons, and the easy to process borite mine is mainly "borite ore", which only accounts for 8.98% of the total reserves in the country, less than 3 million tons. According to statistics forecast by relevant departments, the demand of borax and boric acid in the next few years will grow at a speed of 6.25% and 6.50%, respectively, by 2010, the demand of borax, boric acid and other boric chemicals containing boron will be more than 500,000 tons, corresponding boron ore (standard ore) demand is 2.2 million tons, will cause the supply and demand situation of bormagnesium ore becoming more and more severe. There is an urgent need to develop new boron resources to meet the growing industrial demand [1]. Therefore, the reserves in eastern Liaoning alone are 280 million tons, accounting for 57.88% of China's boron resources, "black borite" (namely borite) has become the main target of comprehensive utilization of boron resources.
The main problems of activated roasting of borite are difficult to control and poor activity. The activity of boron in raw ore is about 40%[2], and the activity of boron-iron concentrate after roasting is generally less than 70%. Therefore, it is necessary to design reasonable roasting technology and equipment to utilize boron ore resources efficiently.
Analysis of roasting technology of 2 boron ore powder
The chemical activation rate of raw mineral powder without roasting activation is very low. For example, under the condition that the excess alkali is about 50% and the concentration of lye is about 30%, the alkaline hydrolysis rate of Fengcheng borax mine is only 30%~40%. The alkali hydrolysis rate of cooked mineral powder can reach more than 80%. After roasting, the crystal structure of the mineral changes, the texture is loose and porous, and the chemical activity is increased significantly, so that the B2O3 in the ore is easier to be leachated by the decomposition agent. For this reason, boron ore must be activated and roasted by carbon alkali method, caustic soda method to produce borax or sodium polyborate and carbon ammonia method to produce boric acid. The quality of roasting plays a decisive role in the decomposition rate. At the same time, moderate roasting of ore, hardness reduction, brittle texture, easier to crush. Therefore, in industrial production, we should strive to improve the quality of roasting, prevent raw burning, avoid overburning.
2.1 Roasting mechanism of boromagite
In addition to fiber boromite, boromite also contains serpentine, dolomite, magnesite, calcite, brucite, olivine, talc, chlorite, mica and other associated minerals. The type and quantity of associated minerals vary with the origin of ores [3].
The main physical and chemical changes affecting the activity of boromagite in the roasting process are as follows [4,5] :
When heated to 300~400℃, the oxidation of magnetite to hematite causes the distortion of mineral structure and the increase of microcracks, increases the specific surface area, that is, increases the reaction area during alkalysis, and then increases the activity of B2O3.
4Fe3O4+O2 = 6Fe2O3
At 500~750℃, a large number of micropores appear due to dehydration and decomposition of serpentine, which further increases the specific surface area. Its main reactions are:
Mg3Si4O11·3Mg(OH) 2·H2O = Mg3Si4O11·3Mg(OH) 2+H2O↑
Mg3Si4O11·3Mg(OH)2 = Mg3Si4O11+3MgO+3H2O↑
When the calcination temperature reaches 620~800℃, the fiber boraxite (2MgO·B2O3·H2O), which is not easy to alkaliysis, is transformed into the amite (MgO·B2O3), which is easy to alkaliysis, the activity of B2O3 increases, but the calcination temperature reaches 900℃, and the amite content decreases, and the activity also shows a decreasing trend. Its main reactions are:
2MgO·B2O3·H2O = 2MgO·B2O3+H2O↑
When the roasting temperature rises to more than 800℃ and the roasting time is too long, part of B2O3 will form boromagite (3MgO·Fe3O4·B2O3) which is difficult to alkaloysis, and then the activity of B2O3 will decrease.
After roasting, the bulk density of boron concentrate decreases (about 5%), indicating that the inner porosity of the ore powder increases, which is beneficial to the contact between alkali and B2O3 and improves the activity of B2O3.
Among the above changes, the decomposition of fibrous boraxite is the main chemical reaction in the dehydration of boron ore powder [6].
2.2 Existing boron powder calcination process and equipment in China
In our country, there are about 16 big enterprises that produce boron mine, mainly concentrated in Liaoning, 11 [7]. At present, there is no advanced industrial device for roasting low-grade boron iron ore, and most enterprises discard it, resulting in a large amount of waste of resources and environmental pollution [8]. A few manufacturers use rotary kiln as roasting equipment for boron powder. The products roasted by rotary kiln usually have low activation rate and leaching rate, high coal consumption, poor thermal efficiency, and difficult to make good use of boron ore resources.
Some companies have tried to use the fluidized roasting method, but because the boron ore contains 20% to 30% surface water, its application is greatly hindered. Many domestic experts and researchers are seeking suitable solutions.
3. New process of flash roasting
3.1 Flash roasting process
The boromagite in Fengcheng area of Liaoning Province is a poor complex ore with low iron and low boron. Its mineral composition is as follows: boron trioxide (B2O3) 8%-10%, total iron (TFe) 24%-28%, magnesium oxide (MgO) 25%-28%, silica (SiO2) 18%-21%, calcium oxide (CaO) 1%-2%.
Liaoning Dongda Powder Engineering Technology Co., LTD. (hereinafter referred to as Dongda Powder Company) cooperated with Fenghua Group, carried out a lot of tests and researches on the problems of difficult control and poor activity of activation roasting of low grade boron iron ore, designed a new flash roasting process which combines pre-drying and roasting organic, and the activation rate of finished products after roasting is obviously higher than that of products roasted by traditional rotary kiln. For example, borite with grade of more than 10% can be roasted to an activation rate of more than 90%. The activation rate of borite with a grade of 6%~10% can be roasted to 80~85%. The principle process flow is shown in Figure 1, and the process device is shown in Figure 2.
The specific process is: the use of the feeder to the Sid dryer into the undried boron powder, boron powder is pre-dried in the dryer, the drying heat source is the thermal tail gas produced by the roasting process; After drying, the mixture of material and air enters the gas-solid separator and is separated by gas-solid, the gas part is discharged, and the solid part is sent to the calciner for activation roasting. The heat source of roasting is the hot gas produced by the coal-burning direct fire hot blast furnace (the required temperature is calculated to be about 900~950℃). After roasting, the mixture of material and air is sent to another gas-solid separator for another gas-solid separation. The gas part is sent to the dryer as the drying heat source, and the solid part is cooled and then captured to get the finished product, namely boron mineral powder with high activation rate. After the above process was determined, Dongda Powder Company cooperated with Weathering Group to establish a roasting and drying production line with an annual output of 50,000 tons, and put into operation.

 

Principle process of boron powder flash calcination

Figure 2. Boron powder flash roasting process device

1 - feeder, 2 - Sid dryer, 3 - gas-solid separator, 4 - calcining furnace,
5 - hot blast stove, 6 - finished product catcher, 7 - storage bin, 8 - induced draft fan
3.2 Analysis of industrial production data
In order to minimize the energy consumption of tons of boron mineral products, coal-burning direct fire hot blast furnace is used as the heat source in weathering engineering projects. The thermal energy utilization rate of coal reaches more than 90%. Another way of energy utilization is to directly enter the exhaust gas of boron powder decomposition in calcination furnace into the drying system, so that the exhaust gas temperature is slightly higher than the dew point temperature of SO2. In the laboratory test, the designed roasting temperature is 900~950℃, but in the actual industrial production conditions, it is found that the most suitable roasting temperature is 1000~1050℃, the industrial production results are shown in Table 1.
Table 1 Test results of flash roasting of boron mineral powder

Sample number

Water content of raw material (%)

Boron grade of raw material (%)

Roasting temperature (℃)

Ultimate activation rate (%)

1

20

10~12

1050

92~93

2

20

8~10

1050

87~90

3

20

6~8

1050

80~85

The data measured by the production operation are as follows:
Coal consumption per ton of products: 150~160 kg; Power consumption: about 46 degrees/ton product;
Product loss rate: ≤3%; Hourly yield of boron fine powder: ≥6.25 tons.
In the above system, energy consumption can be reduced and output can be increased significantly if appropriate adjustment is made.
The above industrial production results show that compared with the existing roasting process of low grade borite in China, the activation rate of low grade borite can be improved by this process. There are two reasons for the analysis: First, in this process, due to the dispersal effect of Schede dryer, the borite is processed to a certain fineness before roasting, which increases the specific surface area of borite and is conducive to instantaneous roasting; Second, in the process of pre-drying and activation roasting, the material in the equipment do churning swirl motion, so that the roasting effect is full and uniform.
4 Conclusion
It has been proved that compared with other methods, the activation rate of borite can be greatly improved by this process, which has the following advantages:
(1) Easy operation, short roasting time, the whole process within a few seconds to tens of seconds to complete;
(2) The total energy consumption is low because the hot tail gas produced by roasting is recycled;
(3) The product has high activity. When the grade of boron powder B2O3 is ≥7.5%, the roasting activity can reach 80%~90%;
(4) with waste gas purification device, small pollution to the environment;
(5) The equipment covers an area of small, easy maintenance.
In the actual production process, the following deficiencies of this set of process equipment are also found:
The maximum roasting temperature required by the laboratory test is 900~950℃. The initial machine-fired hot blast stove is designed for heat resistance by referring to this temperature range, but the roasting temperature in the actual production is 1000~1050℃. At this time, the life of the hot blast stove is shortened in the production process and it is easy to fail.
In view of this deficiency, Dongda Powder Company put forward the following corresponding improvement scheme.
Scheme 1: Transform the structure of coal-fired hot blast stove to improve its heat resistance and adapt to the high temperature environment of about 1200℃;
Scheme two: Using the gas combustion generated by the gas producer as the heat source can reduce the failure rate of the machine-fired hot blast stove. But compared with coal-fired hot blast stove, the heat utilization rate of gas hot blast stove is lower (about 10%~15% lower). This is because there is a certain heat loss in the process of converting coal into gas. The heat utilization rate of coal-fired hot blast stove can reach more than 90%, while that of coal-fired gas hot blast stove can only reach 75-80%.
Scheme three: Use a hot blast stove that burns pulverized coal, which can not only produce a high enough temperature, but also better prevent the failure caused by the overheating of the hot blast stove. Pulverized coal with lower ash content can be used to control the dust degree to about 2~3%.
In addition to the above adjustments in the heat source, but also in the optimization of the design can be improved.

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