As the temperature of a solid mixture increases, one part of the mixture may melt while the other parts remain unchanged.
You can easily have a mixture where one portion begins to melt before another. Heterogeneous mixtures have different compositions throughout. Each of these parts has individual physical characteristics. One of those characteristics may be a lower melting temperature. An example might be a mixture of table salt (NaCl) and iron (Fe) filings. As you increase the temperature, the salt will melt at a lower temperature than the iron.