Pure metal iron is silver white, iron powder is black, this is an optical problem, because the surface area of iron powder is small, there is no fixed geometric shape, and the crystal structure of iron block is geometric shape, so iron block absorbs part of the visible light, reflecting the other part of the visible light mirror, showing white; the light that iron powder does not absorb is diffuse reflection, can enter the human eye. Less light, so it's black.
The aggregate of iron particles whose size of iron powder is less than 1 mm. It is the main material of powder metallurgy. According to particle size, it is customarily divided into five grades: coarse powder, medium powder, fine powder, fine powder and superfine powder. The iron powder with particle size of 150-500 micron is coarse powder, medium powder with particle size of 44-150 micron, fine powder with particle size of 10-44 micron, ultra-fine powder with particle size of 0.5-10 micron and ultra-fine powder with particle size of less than 0.5 micron.
Iron powder and ferrous salt are not only food antioxidants, but also food nutritional fortifiers. Iron deficiency is the most common nutritional deficiency in humans. As a natural edible mineral, iron is very important for human growth, development and daily work. Surveys show that about 12% of women aged 20 to 49 in the United States are iron deficient and 5% suffer from anemia. Anemia can damage people's cognitive and immune functions. In order to solve the problem of iron deficiency, food manufacturers usually add iron powder to flour, corn, rice, breakfast cereals and other foods. However, little research has been done on the nutritive bioavailability of iron powder in food around the world.
Iron powder is commonly added in milk powder, and ferrous salt is widely added in soy sauce, vinegar and fruit juice drinks. The effect is very good. It can not only resist oxidation, but also supplement iron to prevent iron deficiency anemia.