Brief introduction to the effects of additives on zinc powders

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Chemical titration can be used in measuring the purity of zinc powders, in which EDTA-2Na is used as complexing agent, 5g/L xylenol orange is used as the indicator and 5 percents (mass fraction) of urotropin is used as the buffered solution. Soak zinc powder in the absolute ethyl alcohol, after 15 min of ultrasonic dispersion, observe its appearance and dispersity via KH-1000 stereo microscope and measure its particle size range.

soak zinc powder

Results and discussion: different Zn2+ concentration will have different effects on current efficiency and particle size of zinc powders. In the process of producing metal powders via electrolytic method, it is the key to control cathodic overpotential in the concentration diffusion range as the metal ion diffusion in this section is in fact the control step process, in which the metal ion cannot be supplemented timely in the electro-deposition process and will form metal powders instead of continuous plating on the cathode surface, as a result, Zn2+ concentration is an important indicator in the preparation of zinc powders. According to the databases and statistics, Zn2+ concentration has little effect on the purity of zinc powder; however, the current efficiency will become higher when Zn2+ concentration is higher; meanwhile, the higher Zn2+ concentration will also lead to larger particle size of zinc powder and serious agglomeration of zinc metal powder. This is because Zn2+ concentration in the solution will become higher after being restored and can rapidly supplement the metal iron, thus it can effectively inhibit hydrogen evolution and other side reactions and improve the current efficiency; however, as “zinc crystal nucleus” will grow up constantly with continuous supplement of Zn2+, the particle size of zinc powder will become larger accordingly. It can be seen that when Zn2+ concentration exceeds 40g/L, the increasing extent of current efficiency will decrease greatly and the particle size of zinc powder will also increase, therefore, 40g/L can be regarded as the most appropriate Zn2+ concentration.

It can be seen that the effects of different types of additives can vary on zinc powders. Zinc powders that are not added with additives mostly have dendrite shape, which not only have large particle size, but also will seriously agglomerate. By adding appropriate additives into zinc powders, such as surfactant and polymer protectant, its particle size will be reduced as well as its dispersity will be improved.

It is observed that the particle size of zinc powders almost will not change after adding the anionic surfactant SDS and cationic surface active agent CTAB, on the contrary, H2 is easier to detach from the electrode surface due to their wetting ability, which will lead to the decreasing of current efficiency. PEG and dextrin, generally used as the grain refiner in the electrogalvanizing process, can effectively absorb on the surface of zinc powders and form polymer protective layer that can both prevent the continuous deposit of Zn2+ on zinc powder surface and inhibit the growing of zinc powder; besides, they can improve the dispersity of zinc powders. However, PEG and dextrin are difficult to clean thoroughly as they will firmly absorb on the zinc powder surface, which will lead to the decreasing of zinc powder purity. Therefore, the additive amount of PEG and dextrin should be controlled in a certain range. As PEG is used as the nonionic surfactant in the electrolytic process when H2 is dissolved out, a large number of bubbles will form above the solution, which are difficult to eliminate and will have bad effect on the experiment operation. Based on the above experiment results, dextrin can be used as the additives.