Later medieval Latin had alchimia / alchymia "alchemy", alchimicus "alchemical", and alchimista "alchemist". The mineralogist and humanist Georg Agricola (died 1555) was the first to drop the Arabic definite article al-.  In his Latin works from 1530 on he exclusively wrote chymia and chymista in describing activity that we today would characterize as chemical or alchemical. As a humanist, Agricola was intent on purifying words and returning them to their classical roots. He had no intent to make a semantic distinction between chymia and alchymia.
During the later sixteenth century Agricolas new coinage slowly propagated. It seems to have been adopted in most of the vernacular European languages following Conrad Gessners adoption of it in his extremely popular pseudonymous work, Thesaurus Euonymi Philiatri De remediis secretis: Liber physicus, medicus, et partim etiam chymicus (Zurich 1552).  Gessners work was frequently re-published in the second half of the 16th century in Latin and was also published in a number of vernacular European languages, with the word spelled without the al-'.
In the 16th and 17th centuries in Europe the forms alchimia and chimia (and chymia) were synonymous and interchangeable. The semantic distinction between a rational and practical science of chimia and an occult alchimia arose only in the early eighteenth century.
In 16th, 17th and early 18th century English the spellings — both with and without the "al" — were usually with an i or y as in chimic / chymic / alchimic / alchymic. During the later 18th century the spelling was re-fashioned to use a letter e, as in chemic in English. In English after the spelling shifted from chimical to chemical, there was corresponding shift from alchimical to alchemical, which occurred in the early 19th century. In French, Italian, Spanish and Russian today it continues to be spelled with an i as in for example Italian chimica.